The least discussed and most overlooked aspect of tuning on slide type carburetors is the fit of the needle shank in the needle jet. This fit of the cylindrical shank of the needle in the needle jet which affects throttle openings around 1/8 to 1/5 throttle also happens to be the most difficult aspect of mixture control to change on most slide type carburetors.
72cc Jin Ching Scooter and a Honda Needle
24mm "27mm" PZ27 Carburetors
Lean Vs Rich Vs Flooded
Energy Density Changes Over the Years
The big needle jet problem I was having was with the little 16mm Walboro carburetor on my 72cc Jin Ching Chinese pit bike. The stock needle had been one of those crappy concave designs that just don't work. With an extreme concave shaped needle the 72cc Jin Ching pit bike had a huge rich spot at around 1/2 throttle that was extremely problematic. It just wouldn't run with that stock needle. Replacing the needle in the 16mm Walboro with the rather similar needle out of my old Honda ATC 70 had gotten the 72cc Jin Ching scooter running and working fairly well, but it wasn't perfect. There were a couple of problems with the Honda ATC 70 needle in the 16mm Walboro carburetor. One was that the Honda ATC 70 needle was a bit too short, which caused the mixture to richen up more dramatically around 3/4 throttle. This was however not much of a real problem as the mixture control was still fairly good everywhere from 1/2 throttle to wide open throttle under all conditions. Another problem was that the leanest needle clip position often seemed excessively lean at 1/4 to about 2/3 throttle openings where the second needle clip position tended to seem too rich across the board. It was a big change from the first to second needle clip positions, but this is a common problem with small slide type carburetors. The clip grooves are just too far apart on most small carburetors. The shank of the Honda ATC 70 needle was also smaller than the shank of the stock Walboro needle.
This last problem turned out to be the most pervasive and most severe. The Honda ATC 70 needle had about a 0.001" smaller shank diameter than the extreme concave stock Walboro needle. This seemed like a small difference, but with a 0.080" needle in a 16mm carburetor small changes in shank diameter resulted in large changes in mixture. The smaller shank diameter caused an overly rich mixture at around 1/8 to 1/5 throttle openings. This overly rich mixture down low wasn't all that much of a functional problem, but it was annoying and eventually seemed to lead to other problems. An advantage of that overly rich mixture down at 1/8 to 1/5 throttle openings was that the 72cc Jin Ching scooter was usually able to fire up cold and pull out right away without the choke on. For a bike that got used a lot just to run a very short distance out to a shop building and back it was a good feature to be able to jump on and go with zero warm up time. The problem with the extremely overly rich mixture down low was that the engine was dirty and inefficient once warmed up, and sometimes would even load up and stall if the throttle was repeatedly and pervasively blipped to 1/5 openings while idling.
Under normal operation I didn't use 1/8 to 1/5 throttle openings much, so it didn't seem like all that big of a problem. Mostly I just twisted right past 1/5 throttle up to around 1/4 to 1/2 throttle for normal operation so it was easy to ignore the overly rich mixture at 1/8 to 1/5 throttle. The big problem though was when the gasoline sometimes turned to weak low energy density garbage and the 72cc Jin Ching scooter would then only run at 1/5 to 1/8 throttle openings. When that happened I had to go up to the second needle clip position to even out the mixture control, and then when the gasoline went back to normal it was just overly rich everywhere. Going up to the second needle clip position evened out the mixture control on weak low energy density gasoline, but then the rich spot at around 1/5 throttle got even worse on normal gasoline.
Eventually I realized that something had to be done about that mismatched Honda needle. What I did was simply to lay some plumbers solder around the shank of the Honda ATC 70 needle. I melted solder onto the cylindrical part of the needle, and then while the solder was still molten I wiped it off with a paper towel. The result was a light "tinning" of the needle, similar to the plating on electrical terminals. When I measured the diameter I found that I had added nearly exactly 0.001" back to the diameter. The diameter of the new plated needle was a bit irregular, but mostly it was measuring right at 0.0798" with a 0-1 micrometer. With the same micrometer the original Walboro needle measures 0.0797" on the shank. A near perfect match, and attained on the first try with a rather sloppy technique.
The overly rich spot was totally gone, even with the needle clip in the first groove. Instead of a extremely rich mixture at 1/8 to 1/5 throttle followed by a rather lean mixture at 1/4 to 1/2 throttle it was just seeming like an even mixture all the way across with the needle in the first groove. I also tried the second needle clip position, and this also worked, although there was a quite noticeable richening of the mixture from 1/5 to 1/3 throttle openings. I was amazed at how easy it had been to repair that problem. It is not all that good of a repair though as the solder on the needle will wear faster than the original brass needle. For now though the 16mm Walboro carburetor is fully functional.
On the day that I tin plated the shank of the needle I did quite a bit of testing. When I first put it together I rode it around, and it worked great with the needle clip on the first groove. The throttle response across all smaller throttle openings was seeming very good, and torque was about as strong as I have ever seen from the 72cc Jin Ching motor. With the needle clip in the first groove it did richen up noticeably up from 1/2 throttle to wide open throttle, and twisting up onto the main jet did deliver slightly more torque at all midrange and elevated engine speeds. The afternoon ambient temperature was 60 degrees Fahrenheit on that first test ride.
I waited three hours and then fired the 72cc Jin Ching scooter up for another test ride. The early evening ambient temperature was down to 53 degrees Fahrenheit, and again the 72cc motor ran very well with the needle clip in the leanest position. I did notice that in the first few seconds after firing the engine up cold in that colder ambient temperature there was a bit of a bog at 1/2 throttle when I shifted to second gear. Within about five seconds of firing the engine up though it was running consistently and pulling nicely in every gear at all throttle openings.
Then I waited another three hours and fired the 72cc Jin Ching scooter up at an ambient temperature of just 44 degrees Fahrenheit, too cold for riding really. And too dark also, but the headlight on the Jin Ching scooter works rather well at low to moderate speeds. By waiting just a few seconds longer to shift to second gear the engine ran great without any stumbling or bogging even in the very chilly evening air. Within a very short 10 seconds or so after first firing up cold the engine was running consistently and pulling nicely. Again throttle response was good at all engine speeds and all throttle openings. At small cracks of the throttle the engine ran well, and opening the throttle farther delivered smoothly increasing torque with a good feel and good modulation over a wide range of low to midrange engine speeds. This was all with the needle clip in the first groove.
Then the next morning at an ambient temperature of 55 degrees Fahrenheit the 72cc Jin Ching scooter wouldn't run. Or at least it wouldn't run acceptably with the needle clip in the first position. When I first fired it up that next morning I noticed right away that it was weak feeling, but there wasn't much stumbling. When I pulled out without letting it warm up there was a lot of bogging, even in first gear. The real problem though was that this bogging didn't go away once the engine warmed up. I rode around quite a bit to get the engine fully warmed up, and it just wouldn't run correctly. It had a weak chugging lean feeling at all smaller throttle openings and at all lower and midrange engine speeds. Twisting up onto the richer main jet delivered more torque, but it still seemed unusually weak.
I tried going up to the second needle clip position, and this certainly did deliver more torque at all engine speeds. It still felt weak though. Not lean anymore, just weak and sluggish. Obviously not the same gasoline.
On subsequent days the 72cc Jin Ching scooter seemed different each time, but it was consistently better than it had been before plating the needle back up to the stock shank diameter. Then when I went back down to the first needle clip position it again ran well with nice modulatable torque at all throttle openings.
The 16mm Walboro carburetor with the J23020 marked Honda ATC70 needle might be a bit on the lean side in the first needle clip position, but not by much. What is clear is that one needle clip position makes for a big change on this little carburetor. The spacing between the clip positions is 1mm, which is huge for a 16mm carburetor.
With the huge success in modifying the Honda ATC 70 needle to work better with the 16mm Walboro carburetor I decided to tackle the mixture control problems of the 24mm "27mm" PZ27 carburetors that I have on the Lifan 125 and Lifan 150 motors. The problem with the 24mm "27mm" PZ27 carburetors was that they were seeming too lean from 1/8 to 1/5 throttle openings. They had come with very small unmarked pilot jets to match this lean mixture on the shank of the needles, but that just didn't work. The small unmarked pilot jets were even more problematic than the excessively lean mixture at 1/8 to 1/5 throttle openings. With more reasonable pilot jet sizes the 24mm "27mm" PZ27 carburetors had worked fairly well, but the lean spots at 1/8 to 1/5 throttle openings were sometimes very annoying. Particularly with the stupid 9 degrees of all-at-once spark advance at 3,200RPM on the stock Lifan 125 ignition system the lean spot at 1/8 to 1/5 throttle openings was sometimes very problematic. The combination of a very lean mixture at 1/5 throttle opening and the extremely excessive 9 degrees of all-at-once spark advance at 3,200RPM was nothing but disaster.
Anytime the gasoline was weaker and lower energy density than normal the Lifan 125 simply wouldn't run at 2,500 to 3,200RPM. And even on normal gasoline there was often a slight chugging feeling around 3,000RPM and just bellow. Once warmed up a bit on normal gasoline the Lifan 125 often appeared to run without difficulty everywhere from 2,000 to 3,200RPM, but there were way too many times when it just kept stumbling at 2,500 to 3,200RPM mile after mile. Particularly when the Lifan 125 Husqvarna was fired up just for a short little jaunt this chugging weakness at 2,500 to 3,200RPM tended to be very annoying.
The 24mm "27mm" HongDa PZ27 carburetor with it's HD27B needle on the Lifan 150 Husqvarna didn't appear to be quite as lean at 1/8 to 1/5 throttle openings. With a real advance curve from 2,000 to 3,500RPM the Lifan 150 also tended to run much better down low. The combination of an advance curve and not quite as extreme lean mixtures at 1/8 to 1/5 throttle openings meant that the Lifan 150 Husqvarna didn't have any particular problem with chugging weakness at 2,500 to 3,200RPM. Still though I did notice that the mixture was similarly very lean at 1/8 to 1/5 throttle openings. Perhaps not quite as bad as on the 24mm "27mm" HS Japan PZ27 carburetor on the Lifan 125 Husqvarna, but there was still a lean spot. Particularly when I went up to a richer needle clip position and a bigger 98 size main jet the Lifan 150 Husqvarna seemed excessively lean at 1/8 to 1/5 throttle openings. Even with the needle clip in the second groove on the HD27B needle and a 95 size main jet though the Lifan 150 did seem to richen up too much from 1/5 to 1/3 throttle openings. On normal gasoline this didn't cause any severe problems, but it was a bit annoying sometimes.
It was the Lifan 125 though that got attention first. With weak low energy density watered down gasoline coming out of the pumps the Lifan 125 Husqvarna just never seemed to run well, and the lean mixture at 1/8 to 1/5 throttle openings was the glaring problem.
One day somewhat out of exasperation and anger I ripped the top off of the HS Japan carburetor and hacked a chunk off of the needle shank with the file on my pocket knife. This was a very crude sort of a modification, and I didn't have much of any idea of how much material to take off. All I had to go by was that adding 0.001" to the diameter of the needle shank on the 72cc Jin Ching scooter had made a rather large difference.
The PZ27 needle has a 0.0983" diameter shank, which is very large for a 24mm carburetor. I cut two flat spots on opposite sides of the needle, each about 0.001" deep. I thought this would be a rather small change since it was taking 0.001" off of only a small portion of the total diameter.
As it turned out though this appeared to make a dramatic difference. It was a very small change in jetting, but the mixture at 1/8 to 1/5 throttle openings had been extremely lean on the low energy density watered down garbage gasoline. When the mixture is so lean that an engine won't run at all then very small changes in jetting can make a dramatic difference in engine operation.
The Lifan 125 Husqvarna went from extreme chugging and near zero torque at 2,500 to 3,200RPM to actually running and making torque at all engine speeds with that very small adjustment to the needle jet size.
With the shank of the needle shaved down that small bit the mixture control was much more even. Even with the needle clip in the first groove there had been a rather dramatic richening of the mixture from 1/5 to 1/3 throttle openings on the stock 24mm "27mm" HS Japan PZ27 carburetor. With the PZ27 needle shaved down just that very small amount the mixture control seemed nearly perfectly even everywhere from 1/8 throttle to 1/2 throttle openings. There was still a small richening of the mixture from 1/4 to 1/2 throttle, but it was slight and not really noticeable under most conditions. Just right.
With the needle clip in the leanest position the smallest main jet size that works in the 24mm "27mm" HS Japan PZ27 carburetor is a 95 size. Even going down to a 95 marked jet that measures a slightly smaller 94 or 94.5 size was causing a noticeable leaning out of the mixture from 3/4 to wide open throttle. The jet size that appears perfectly matched to the fist needle clip position is a 95 marked main jet that measures about 95.5 or 96 size. I made this very small jet size change with test rides on the same day. First I rode the Lifan 125 Husqvarna with the smaller main jet that had been in it. It was making torque at 1/4 to 2/3 throttle openings, but then it was extremely reluctant to get going on the top end at wide open throttle. Opening the throttle all the way caused a drop in torque across all midrange engine speeds, and the only way to get top end power was to get the engine fully warmed up and then roll the throttle on just right.
This reluctance to get going on the top end is of course mostly due to the extremely low compression ratio and huge 25 degree BTDC spark timing on the two inch bore engine. It is not so much the large amount of spark advance that directly causes the reluctance to get going on the top end, but rather the very low compression ratio that the huge 25 degree BTDC spark timing requires. With so much spark advance on a two inch bore engine the compression ratio needs to be rather low, and it is the low compression ratio that causes the reluctance to get going on the top end. With a rather small camshaft and rather small valves the cylinder filling is dropping off up at high engine speeds. With the low compression ratio the engine gets into a situation where it simply can't muster enough pressure to pop off up at those high engine speeds, even with that giant 25 degree BTDC spark timing.
I am fully aware of this tendency for the low compression ratio Lifan 125 to be reluctant to get going on the top end, but this is not the same thing as lean jetting on weak low energy density gasoline. With the 94.5 size main jet the 24mm "27mm" HS Japan PZ27 carburetor was leaning out at wide open throttle even with the needle clip at the leanest position. The direct evidence of this was the drop in torque even down at 4,500 to 7,000RPM midrange engine speeds with the throttle opened wide.
I went up to a 95 marked genuine Keihin main jet that was noticeably larger. To make the comparison I used a #63 drill stock and a #62 drill stock. There was noticeably more side to side wiggle of the 0.0368" diameter #63 drill stock in the larger jet than in the smaller 94.5 size jet that came out of the engine. The bigger 0.0376" #62 drill stock wouldn't go into the smaller 94.5 size main jet at all, but fit easily into the larger jet. The 0.0376" #62 drill stock was able to slip into the larger 95 marked jet, but there was a bit of resistance and no side to side wiggle. The 95 marked genuine Keihin main jet is bigger than the 95.5 size, but probably not more than the 96 size. The difference in jet size is very small, but the difference in performance was quite noticeable.
With the very slightly larger 95.5 size main jet the Lifan 125 Husqvarna was able to take wide open throttle at all engine speeds without difficulty, and it was able to more easily get going on the top end with noticeably more power output. It seemed like an unexpectedly large difference considering the very small main jet size change. The smaller 95 marked main jet might have been as small as the 94 size, and the larger 95 marked main jet might have been as big as the 96 size. It could have been nearly one jet size difference, although I suspect it was more like a half jet size difference. And they were both un-modified 95 marked jets.
That first day when I shaved the PZ7 marked needle and switched to the slightly larger 95.5 main jet the Lifan 125 Husqvarna ran rather well. That small jetting change made a very dramatic improvement in power and performance on the Lifan 125 Husqvarna. With that weak low energy density gasoline it still felt like it was way over on the lean side of what can work, but was working and delivering fairly instant and very reliable power and torque over a wide range of engine speeds. Everywhere from about 4,000RPM to 10,000RPM it was pulling just about as strong as ever, and even all the way down to 2,000RPM it was torquing along smoothly and reasonably strong without any noticeable stumbling or even chugging weakness.
Then the gasoline turned to crap overnight and the Lifan 125 wouldn't run the next morning. Firing up cold it was stumbling and cutting out all over the place at all throttle openings and all lower and midrange engine speeds. The chugging weakness at 2,500 to 3,000RPM was back very bad, and low end torque was practically non-existent. The top end power just wasn't there either. It was extremely reluctant to get going on the top end, and then once pulling power was dramatically lower than it had been the day before.
The big difference was that the stumbling and cutting out no longer was isolated to 1/8 to 1/4 throttle openings. With the jetting evened out and dramatically lower energy density gasoline in the tank it was stumbling, cutting out and doing all sorts of horrible things at pretty much all throttle openings. Very disappointing on the gasoline front, but the jetting adjustment was a success.
To get the Lifan 125 going again I drained the tank and took some gasoline out of one of my other bikes. Instant pour in jetting improvement. Again it seemed way over on the lean side like the day before, but it was just barely able to work and deliver fairly good power and performance at all engine speeds and all throttle openings.
This precise jetting of the 24mm "27mm" HS Japan PZ27 carburetor on the Lifan 125 Husqvarna had worked out so well that I decided to work over the 24mm "27mm" HongDa PZ27 carburetor on the Lifan 150 Husqvarna also. The first thing I wanted to do was set the main jet size the same on both 24mm "27mm" PZ27 carburetors. I didn't have another 95.5 or 96 size main jet, but this happened to be a size that could easily be set with the 0.0376" diameter #62 drill. I ran the shank end of the 0.0376" diameter drill stock through a 95 size jet. I didn't even use the drill bit end, I just twisted the drill stock end through until it fit similarly to the jet in the Lifan 125 Husqvarna. When I was done turning the drill stock by hand in the jet I measured the part I had been using. It was 0.0375" in diameter, so the new jet is larger than 95 size but probably not bigger than 95.5 size. A real 95 size jet. This seemed a bit smaller than the 95 marked un-modified genuine Keihin main jet in the Lifan 125 Husqvarna, but not by much. They are certainly the same jet size.
I then turned to the HD27B needle, which measured 0.0988" on the shank. Obviously the two PZ27 carburetors don't have exactly the same size needle jet in them. The PZ27 marked needle measured 0.0983" on the shank and the HD27B needle measured 0.0988" on the shank, and the HD27B needle had been appearing to deliver a slightly richer mixture at 1/8 to 1/5 throttle openings. I wanted to make just a very small change to the HD27B needle, but somehow I ended up cutting too much off. I shaved down opposite sides of the shank of the needle as I had done on the Lifan 125, and I was trying to take off less material. Somehow though I ended up making the same 0.001" deep cuts as I had made on the PZ27 needle in the Lifan 125 Husqvarna, and this resulted in more of a change than I had wanted.
When I rode the Lifan 150 Husqvarna with the needle clip in the second groove there was a noticeable dip from 1/3 to 2/3 throttle openings. The mixture seemed to lean out slightly from 1/5 to 1/3 throttle openings at lower midrange engine speeds. I also noticed that the 95.5 size main jet was much richer than the second groove needle clip position. At all midrange and top end engine speeds there was a rather dramatic increase in torque as the throttle was opened up from 2/3 to wide open. Even down at rather low engine speeds around 5,000RPM opening the throttle all the way delivered a dramatic increase in torque. The engine actually sounded lean at 2/3 throttle and then sounded more normal at wide open throttle.
The problem seemed to be that the needle clip position was too lean. I had noticed before that the first needle clip position on the PZ27 marked needle in the 24mm "27mm" HS Japan PZ27 carburetor is actually richer than the second needle clip position on the HD27B needle in the 24mm "27mm" HongDa PZ27 carburetor. I had noticed this difference first in riding both bikes around on the same gasoline, and then when I compared the needles I found the difference to be in the dimension from the lower clip groove down to the start of the taper. It was about a 0.060" difference, which with 1mm between the grooves is one and a half needle clip positions.
To get the mixture at 1/4 to 3/4 throttle openings as close as possible on the Lifan 125 Husqvarna and the Lifan 150 Husqvarna I needed a half needle clip position. To attain this half needle clip position I made a spacer to go on the needle. I used a small ring terminal electrical connector which I snipped off and shaved down. The terminal was 0.030" thick, but I filed that down to 0.016". This was too much of a change in thickness. When I tried the Lifan 150 with the 0.016" spacer it was still too lean at 1/4 to 2/3 throttle openings. I made up another spacer at the 0.024" thickness, and this worked much better. That is only a quarter of a needle clip position change, which is miniscule. When such a small jetting change has a large effect it is a sure sign that it was far too lean.
It really does seem like I took too much off of the HD27B needle shank, but not by much. With the 0.024" spacer delivering the second and a half needle clip position on the shaved down HD27B needle the 24mm "27mm" HongDa PZ27 carburetor appears to have mostly perfectly even mixture control all the way across from 1/8 throttle opening up to wide open throttle on the 95.5 size main jet. This is good, but not perfect. On the small two inch bore engine with a fixed advance curve it would be better for the mixture to stay just a bit leaner at all small throttle openings bellow about 1/3 throttle. As it is the mixture is just as rich, if not slightly richer, at 1/5 throttle as it is at 1/3 throttle. Very close to perfect, but somewhat backwards none the less.
It turned out though that this was extremely unusually low energy density watered down garbage gasoline. When I took a quart of it out of the Lifan 150 Husqvarna and put it in the empty tank on the Lifan 125 Husqvarna the Lifan 125 wouldn't run. Even with the revised slightly richer jetting the Lifan 125 was stumbling horribly at all throttle openings when first fired up cold and wouldn't reliably make torque even once fully warmed up. Once warmed up it was able to make slightly weak torque across the midrange engine speeds but there was still some stumbling here and there and for the most part it just wouldn't get going and make top end power. When I aggressively heated the engine up with repeated big pulls I did finally get it going with some top end power, but it was weak and very lean feeling. The extremely low energy density watered down gasoline just wouldn't work at all.
Next I got another quart of gasoline out of one of my other bikes, and again it was weak and low energy density but a bit more towards normal and it did sort of work in the richened Lifan 125 Husqvarna. The stumbling was gone and the power and torque were more instant and much more reliable. It still felt way on the lean side, but it was just barely able to work. The top end power was much more reliably available, and somewhat stronger also.
I then took what was left of this quart of gasoline out of the Lifan 125 Husqvarna and put it in the Lifan 150 Husqvarna, first draining the old partially flammable liquid out of the tank on the Lifan 150 Husqvarna. The Lifan 150 Husqvarna ran stronger, pulled harder and revved out more willingly on the top end.
When I swapped the same gasoline back and forth between the Lifan 150 Husqvarna and the Lifan 125 Husqvarna I found that the 24mm "27mm" HongDa PZ27 carburetor with the 0.024" spacer under the second needle clip position was seeming slightly richer at 1/4 to 3/4 throttle openings than the 24mm "27mm" HS Japan PZ27 carburetor with the clip on the first groove on the PZ27 marked needle. When I went back to the 0.016" thick needle clip spacer on the HD27B needle both the Lifan 125 Husqvarna and the Lifan 150 Husqvarna seemed to have the same mixture at 1/4 to 3/4 throttle openings on the same gasoline. The transition up onto the main jet felt the same on both carburetors also, just the slightest little bit of richening of the mixture twisting up onto the main jet. Mostly it felt like even mixture control though, with pretty much the same mixture at all throttle openings from 2/3 up to wide open throttle on both of the 24mm "27mm" PZ27 carburetors.
It seems clear that the HD27B needle is more than one needle clip position leaner than the PZ27 needle, but not quite all the way to one and a half needle clip positions leaner. The difference appears to be very close to 1.4 needle clip positions.
This is however just on appearances. The reality is that the Lifan 125 and the Lifan 150 engines have quite different compression ratios. The Lifan 150 with a 5% larger bore diameter tends to run a bit less spark advance than the Lifan 125. That indicates fairly clearly that the Lifan 150 has a substantially higher compression ratio than the Lifan 125. A larger bore AND less spark advance running on the same gasoline with the same jetting certainly means a considerably higher compression ratio.
This compression ratio difference tends to skew comparison results, especially when it is such an extremely low compression ratio that is just on the edge of being so low that it won't work at all. It is quite common for the Lifan 125 to get into a situation where it is reluctant to make top end power, and this can very easily be misinterpreted as too small of a main jet size or excessively weak gasoline.
It is also true that mixture control can be very importance in trying to get a low compression ratio engine to make top end power. An overly rich mixture displaces more intake air, and that can easily prevent top end power. On the flip side an excessively lean mixture simply doesn't produce as much combustion. With a very low compression ratio there is often a rather narrow window of main jet sizes that will deliver top end power. If the main jet is off too far to either side then it is very easy for a low compression ratio engine to suffer dramatic loss of top end power.
What it comes down to is that the higher compression ratio of the Lifan 150 is closer to correct and therefore more forgiving. The Lifan 150 always seems better able to handle overly rich mixtures, and even overly lean mixtures don't necessarily cause as dramatic a loss of top end power as on the Lifan 125. The Lifan 125 is right on the edge of being able to deliver top end power, even on gasoline for rather low compression ratio engines. The Lifan 150 on the other hand has a substantially higher compression ratio that is better able to handle higher pressure types of gasoline. Both the Lifan 125 and The Lifan 150 do however appear to have compression ratios dramatically lower than their advertised compression ratios. They are not high compression ratio engines by any stretch of the imagination.
With the success modifying needle jets on the smaller bikes I started thinking about the DellOrto carburetors on my 1991 Husqvarna WMX 610. I looked up what sizes of replacement needle jets were available new, and there seemed to be some strange holes in the model lineup. In DellOrto parlance the needle jet is called an "Atomizer", and they screw into the bottom of the carburetor behind the main jet holder. They are actually pretty easy to change on the DellOrto carburetors, and replacements of various sizes are available. What I noticed though was that there were some holes in the lineup strategically placed around what I needed to do. The Atomizers are available in 0.01mm increments from the 264 size up to the 266 size, but going the other way the lineup jumps down to 262. The 263 size is strangely missing. This was disappointing and a bit confusing. The 1991 Husqvarna 610 motors are listed as coming with a 265 size Atomizer on the K32 needle. The later Husqvarna 610 motors are listed as coming with a 264 Atomizer on the same K32 needle. The confusing thing was that it was on the 1997 motor with a stock 264 size Atomizer that I had been experiencing difficulties with mixtures at 1/8 to 1/5 throttle that were too rich to work with the leanest needle clip position. Something seemed wrong.
I pulled the bottom of the carburetor on my 12.2:1 hot rod 610 motor off and I pulled out the main jet and the Atomizer. It was the same 264 marked Atomizer that was in there back in the spring of 2015 when I put the rebuilt 1997 motor together in my beat up old 1991 Husqvarna WMX 610 chassis. The challenge was to figure out what size it really was. My smallest small hole gauge only goes down to 0.125", so I hadn't been able to even attempt a precise measurement. Instead of trying to find a smaller small hole gauge set I had just decided to assume that because it appeared to be close to the stock size that it was exactly at the stock size.
This year in 2017 though I was having a lot more difficulty with carburetors, so I felt I needed to precisely verify the size. What I did was cut my 0.125" small hole gauge down so that it will go in a 0.1" hole. I did this with a bench grinder and a piece of 400 grit silicon carbide paper to finish it by hand. The modified hole gauge turned out irregular as expected, but it was the best I could do with what I had on hand.
To start with I measured a 270 marked needle jet out of another carburetor. By opening the hole gauge up until it just had some substantial drag in the hole and then measuring across the largest diameter with a micrometer I repeatedly got 0.1061" (2.695mm). This seemed very close to the marked 270 size, so my confidence in the funked together tool improved.
When I measured the 264 marked Atomizer out of my 1997 40mm DellOrto I kept getting 0.1037"(2.634mm). This seemed to be in fairly good agreement with the 2.695mm measurement of the 270 marked Atomizer. It really seemed like it was probably a 264 size needle jet in the 1997 carburetor. I did find a problem though. At the extreme end of the needle jet, about 1/16" from the tip, the size measured a much wider 2.66mm. It was only at the extreme end that the needle jet was tapered, but it tapered out quite dramatically. This seemed like a large problem, and might have been part of what was causing the excessively rich mixture at around 1/5 throttle opening with the needle clip on the first groove.
I determined that I needed to order a replacement AB 264 DellOrto Atomizer, but in the mean time I thought of ways to repair the one I had. As far as I know it is only the English company that owns DellOrto now that stocks the parts. I haven't been able to find them in stock anywhere in America, so it takes a while and costs more to get parts. What I came up with was to drive the end of the brass Atomizer into a tapered hole in a chunk of aluminum. Amazingly this worked nearly perfectly. I tapped the Atomizer down into the tapered hole as much as I thought was likely to make some change in the diameter and then I measured it again. Amazingly the extreme end of the hole then measured essentially exactly the same diameter as farther up in the hole. Instead of that rather large taper there was just a slight bulge at about 1mm in from the extreme end. At the extreme end and in at least 1/32" the measurement was the same 0.1037" as 1/8" up into the hole. Right at about 1mm up into the hole though there was a slight bulge out to perhaps about the 265 size. I figured this would be functionally very close to a new 264 size Atomizer.
While I had the main jet out I reset it to the approximately 175 or 176 size that had worked so well when I first put the rebuilt 1997 hot rod 610 motor together in the spring of 2015. When I headed out for a test ride it was very weak and low energy density gasoline so I couldn't tell much about what was going on down low. With a 16 degree BTDC static timing setting the 12.2:1 hot rod 610 motor was running along smoothly and consistently at all very small throttle openings at all lower engine speeds from 2,000 to 3,000RPM. There was some torque from 3,500 to 5,000RPM, but it was slow flame front travel speed gasoline and it wouldn't make power and wouldn't rev past 6,000RPM at all. The torque was most instant at 3,300RPM to 4,000RPM, and then there was hesitation from 4,000 to 5,500RPM. Very severe hesitation and a near total lack of power from 5,000 to 5,500RPM. Obviously slow flame front travel speed gasoline. There was also some harshness at rather small throttle openings at 2,500 to 3,200RPM, again obviously slow flame front travel speed gasoline. It was however also very weak and very low energy density and the big 610 motor basically wouldn't run. I was however fairly sure that the mixture control was more even from 1/8 to 1/3 throttle openings. Only time will tell if I happen to get some more of that higher energy density gasoline to see if the repair really fully removed the overly rich spot at 1/5 throttle openings, but at this point I feel confident that it was a successful repair.
I also pulled one of my stock 1991 Husqvarna WMX 610 40mm DellOrtos apart to make the same needle jet measurement. It was a stock AB 265 marked Atomizer, but when I measured it I kept getting 0.1039" (2.640mm) and no larger. It seemed like the AB 265 DellOrto Atomizer might not actually be a full 0.01mm larger in diameter than the AB 264 DellOrto Atomizer. It is hard to be sure though as my funked together small hole gauge is really very irregular and difficult to use. There also might be more wear on the 264 marked Atomizer, even far up into the hole. It is just very hard to be sure when the measurements have to be that precise with such funky equipment.
Something to keep in mind is that mixture control on carburetors is never perfectly consistent. Small openings such as pilot jets and needle jets don't respond to changes in the mechanical properties of the fuel the same as larger main jets do. Even just a change in fuel temperature results in some small changes in mechanical properties, and changes from one type of gasoline to another type of gasoline would tend to result in even more dramatic changes in mechanical properties. This means that the relative mixture ratios of pilot jets to needle jets to main jets won't always stay exactly the same. Fairly even mixture control across a wide range of operating conditions certainly is possible, but there will be some small changes. Carburetors simply are not perfectly consistent.
This brings up a particular problem with small slide type carburetors; they have extremely oversized needles. When a 0.1" diameter needle is used in a 24mm carburetor or a 0.08" diameter needle is used in a 16mm carburetor the result is a very tight fit between the needle shank and the needle jet. With such dramatically oversized needle very small changes in the fit of the needle shank into the needle jet result in large changes in the jetting. The result is that any small amount of wear of the needle or needle jet results in a more dramatic change in jetting than would be the case with a properly sized needle.
The very close fit of an oversized needle in a small carburetor also results in the opening between the needle shank and the needle jet being extremely thin relative to the main and pilot jets. This thin opening then responds much more dramatically to small changes in the mechanical properties of the fuel. The specific problem is that the very thin opening between the shank of the needle and the needle jet has a much larger surface area to opening volume ratio than for the main jet. The oversize needle dramatically exaggerates changes in jetting due to small changes in the mechanical properties of the fuel.
What it comes down to is that oversized needles in small carburetors result in more difficult tuning and less consistent and less reliable operation. Any carburetor ends up being less than perfectly consistent, but a dramatically oversized needle in a small slide type carburetor is asking for trouble.
Somewhat ironically the modifications I made to the PZ27 and HD27B needles in the 24mm "27mm" PZ27 carburetors actually tend to increase consistency and reliability. By cutting larger isolated passages on the sides of the needle the ratio of surface area to opening volume decreases, tending to at least somewhat mitigate the problems associated with an oversized needle in a small slide type carburetor.
People always talk about lean versus rich mixture ratios, but mixture control is not as simple as just two states. The reality is that there is a range of mixture ratios that will work for any particular type of gasoline. Above or below this range of mixture ratios the engine simply won't run, or will run extremely poorly. With a slightly excessively lean mixture power output is low and problems with missing, stumbling and cutting out show up. At this lean end there is a certain threshold mixture ratio, and this threshold minimum mixture ratio is different for different engines, different ambient temperatures and different types of gasoline and obviously engine speed and engine load make a substantial difference here also. A mixture ratio leaner than this threshold value causes missing, stumbling and cutting out. Bellow this threshold value an engine essentially isn't running at all, even if it doesn't immediately come to a complete stop.
At mixture ratios just barely rich enough to prevent missing, stumbling and cutting out torque and efficiency may still be dramatically low depending on the engine parameters and operating conditions. Generally it is very high mean piston speeds that cause efficiency to plummet with very lean mixture ratios even if there isn't any missing, stumbling or cutting out. To support a very high mean piston speed with the best possible efficiency the engine has to be running at high power output, and this means burning as much fuel as can be made use of. At very low mean piston speeds though a rather lean mixture is generally good for efficiency, so long as it isn't so lean that misfiring occurs. Misfiring obviously causes a dramatic increase in unburned hydrocarbon emissions and a corresponding decrease in efficiency. In between mean piston speeds work best with in-between mixture ratios, or at least mixture ratios rich enough to produce substantial torque.
If an engine is running extremely overly rich black deposits build up on the spark plug, exhaust system and on the inside of the combustion chamber. In extreme cases gasoline engines might even blow small wisps of black smoke, a sure sign of extremely overly rich conditions. Generally there is a maximum practical mixture ratio for any engine and any operating conditions. This richest practical mixture is where a small excess of gasoline is blown out the exhaust unburned or partially unburned. This is the richest practical mixture for absolute maximum power production, and obviously not the correct mixture ratio for most purposes. If more fuel is dumped in beyond this richest practical mixture then the exhaust just gets blacker without any substantial increase in power output. Generally peak power output, or very close to peak power output can be obtained over a rather wide range of mixture ratios. At the richest end the exhaust gets very black and efficiency plummets. At the leanest end of this range for near maximum power output efficiency is generally at it's highest. If the mean piston speed is very high or if the weights of the pistons and the rods is excessive then this leaner end of the range for near maximum power output might still result in a rather black exhaust and a substantial amount of unburned or partially burned fuel being blown out the exhaust.
Generally much leaner mixtures can be used and are entirely practical. For most good running engines under most conditions leaning the mixture out substantially results in higher efficiency and much cleaner operation. Just how far an engine can be leaned out with good results depends on the engine parameters and operating conditions. If mean piston speeds are still substantially high then leaning out the mixture beyond where the exhaust is no longer black results in both dramatic reductions in torque as well as substantial reductions in efficiency. The point here is that supporting high mean piston speeds requires burning as much fuel as is practical without blowing unburned fuel out the exhaust.
The ideal mixture ratio is close to the stochiometric ratio, and usually a bit on the lean side. This means that there is a slight excess of oxygen in the combustion chamber so that very nearly all of the fuel can burn. This is generally quite a bit leaner than where maximum power is produced. As much as about a 10 or 15% lower fuel flow rate compared to the fuel flow rate for maximum or near maximum power output. The implication then is that at maximum power output up to about 10% of the fuel is blown out the exhaust unburned, or more likely partially burned.
Mixture ratios leaner than this ideal mixture ratio don't really accomplish much in terms of peak efficiency. In fact peak efficiency would always be attained at the ideal mixture ratio where very nearly all the fuel burns with a minimum excess of oxygen or even somewhat richer than this ideal mixture ratio in some engines. Particularly in engines that operate at high mean piston speeds or in engines that have extremely excessively heavy pistons and rods peak efficiency would tend to come at mixture ratios substantially richer than this ideal mixture ratio.
Just because peak efficiency is never found at leaner mixture ratios than the ideal mixture ratio where very nearly all of the fuel burns does not however mean that leaner mixture ratios are useless. In fact very lean mixture ratios can be useful under some circumstances.
Leaning the mixture out past the ideal mixture ratio where very nearly all the fuel burns can be useful for a variety of specific reasons. One is to get that last miniscule little bit of fuel to burn to get unburned hydrocarbon emissions down to an absolute minimum. A more practical use for extremely lean mixture ratios is to support light loads at very low mean piston speeds. To support low mean piston speeds a leaner mixture works well because it is essentially like a lower temperature of combustion potential fuel. Burning less fuel makes less heat which works better for low mean piston speeds. Efficiency at very low mean piston speeds is still very low, but a lean mixture allows an engine to do as well as possible way down there. Burning extra fuel at very low mean piston speeds doesn't accomplish anything other than using more fuel. At very low mean piston speeds the extra combustion is not well able to be converted into useful output, so efficiency plummets even further.
Lean mixtures are useful for engines that need to operate over wide ranges of speeds and wide ranges of loads, but the mixture can only be leaned out so far before missing, stumbling and/or cutting out sets in. Generally speaking, on any engine the mixture ratio can be leaned out somewhat below the ideal mixture where very nearly all the fuel burns. Just how far the mixture can be leaned out and how practical it is to lean the mixture ratio out depends on the engine parameters and the operating conditions.
Somewhat contrary to popular belief late compression ignition mode operation can actually support leaner mixtures than full flame front travel mode, but this can be tricky to attain without sophisticated controls. The irony is that late compression ignition mode can make more torque with richer mixtures than full flame front travel mode, so it seems somewhat counter intuitive to say that late compression ignition mode is also able to support leaner mixtures. It is true though.
At extremely lean mixture ratios the actual flame front travel speed slows down substantially and the engine gets into a situation where it will barely run. If an opened throttle and substantial spark advance is able to attain late compression ignition on this very lean mixture though then the engine can run consistently and cleanly. From a practicality perspective though this is difficult to obtain because the extremely lean mixture means a lot less heat in the combustion chamber so the engine is unlikely to actually get into late compression ignition mode. With a well tuned load dependant advance it is however possible to get some late compression ignition at very lean mixture ratios. Whether this is practical or not depends on how well the sophisticated engine management system is tuned for this specific type of very light load operation. Again, somewhat contrary to popular belief, very short stroke lengths are most likely to work with this type of light load and extremely lean operation. Even with a very lean mixture late compression ignition still works best up at somewhat elevated engine speeds. Getting a sufficiently low mean piston speed for very light load lean operation also requires a rather short stroke length to keep the engine speed up close to where late compression ignition has some chance of working well.
For most gasoline engines it is not practical to lean the mixture out beyond the point that works for full flame front travel mode operation. This means that there is a rather abrupt minimum mixture ratio cut off. A leaner mixture than this cut off simply won't work.
Obviously just where this minimum mixture ratio cut off occurs depends on the engine parameters and the operating conditions: Smaller bore sizes allow leaner minimum mixture ratios, any type of load dependant spark advance mechanism tends to allow leaner minimum mixture ratios, and obviously extremely cold ambient conditions or cold starting an engine requires richer mixtures even at very low speeds and very light loads (low idle).
For most mechanically controlled gasoline engines there is only a moderate range of lean mixture ratios between the minimum lean mixture ratio and the ideal mixture ratio where very nearly all the fuel can burn. This moderate range of lean mixture ratios can also seem like a bad joke to anyone who has messed with tuning gasoline engines. Sometimes this moderate range of mixture ratios that works can get so narrow as to completely cease to exist. Generally speaking the larger the bore diameter the narrower this moderate range of lean mixture ratios becomes. Four inch bore automotive engines are notorious for requiring such rich mixtures to idle and run at small throttle openings when cold that they are fairly well drowning in gasoline once warmed up and under normal operation. To say that tuning a four inch bore engine is impossible is a gross exaggeration, but it is a rather large bore diameter and difficulties are not uncommon. Obviously a choke or other mechanism for richening the mixture during cold starting and during a warm up period helps a lot on large bore engines.
What it comes down to is that gasoline engines with large displacements per cylinder generally have little use for extreme lean mixture ratios bellow the ideal mixture ratio where very nearly all the fuel can burn. Big gasoline engines tend to run best close to, or on the rich side of, the ideal mixture ratio under nearly all conditions. It may still be possible to get a big engine to idle down low at very lean mixture ratios with the use of sophisticated engine management systems, but it is not necessarily easy and the benefits are less substantial than for smaller engines.
For good running practical engines of moderate bore and stroke dimensions a rather wide range of mixture ratios are attainable. This total range of attainable mixture ratios can be broken down into three segments. Overly rich to the point of reduced torque and dramatically reduced efficiency but still running, the efficient and powerful mixture ratios from the ideal mixture ratio where very nearly all the fuel burns up to the somewhat richer mixture where maximum or near maximum power is produced, and thirdly the lean mixture ratios bellow the ideal mixture ratio and down to the cutoff point where an engine stops working because of missing, stumbling or cutting out. These three segments could be called overly rich, ideal torque production and functionally lean. A fourth segment is overly lean, but this is outside of the total range of attainable mixture ratios because an engine won't run at all, or runs so poorly as to be mostly useless at these excessively lean mixture ratios. The mixture ratios so rich to the point that the engine won't run at all are also outside of the range of attainable mixture ratios, and this is called flooded.
Most carburetors from the past 80 years have been jetted to run predominantly in the overly rich segment on normal gasoline with occasionally forays into flooded. The easiest explanation for this huge problem is simply that overly rich but still running has been a better selling point than overly lean and not running at all.
Understanding the reality of the historic predominance of extremely rich jetted carburetors also requires understanding something about the carburetors that came before this era. Carburetors from the first three decades of automotive gasoline power typically had both a choke and a "dial-a-jet" on the main jet circuit. The choke was used to control mixture for starting and warming up at small throttle openings, and the dial-a-jet knob was used to tune the main jet for power and performance. A great feature for an expert level engine tuner forced to use a highly variable gasoline supply. Not such a good feature for a novice just trying to get to work.
Ultimately it was the novices trying to get to work that had the most robust financial influence over the automotive industry, and the mixture control knobs slowly disappeared in the 1930's.
The butterfly valve type carburetors are running on the main jet over a very wide range of throttle openings from way down at about 1/3 throttle to wide open throttle. This means that the dial-a-jet main jet circuit influences the mixture across the entire range of normal operable throttle openings. The obvious problem though is that butterfly valve type carburetors are extremely difficult to tune. In fact butterfly valve type carburetors don't respond to jetting changes anything like a slide type carburetor. It is generally possible to go up on the main jet size on butterfly valve type carburetors to deliver a richer mixture for slightly higher peak power output, but this isn't really tuning the carburetor. It is just selecting a feature that is already build into the design and setup of the carburetor. A properly designed and setup butterfly valve type carburetor has a mostly even mixture control across all small to medium throttle openings. Going up to a larger main jet dumps more fuel in at all larger throttle openings, but the change is most dramatic at wide open throttle. This works fine if stock setups are for an even mixture control and clean and efficient operation at wide open throttle. For hot rodding a slightly larger main jet can then be used to deliver maximum power output with a bit of exhaust blackening. This is however the ONLY adjustment that is available on most butterfly valve type carburetors. Adjustable idle circuits have also been very common, but they are usually best left at the stock setting for normal gasoline. Adjustable intermediate circuits have also been available on many non-automotive butterfly valve type carburetors, and these offer an additional level of mixture tuning in approximately the 1/8 to 1/4 throttle opening range. The combination of the idle mixture screw and the intermediate circuit screw allows for fairly flexible tuning in the first 1/4 throttle opening range, but that is all. The operational range of throttle openings has very little easy adjustability on a butterfly valve type carburetor. Changing the main jet changes the mixture across all larger throttle openings, but there is no easy way to adjust mixture response at 2/3 throttle versus at wide open throttle. When the main jet size is changed it changes the mixture response curve versus throttle openings, but if this new mixture response curve versus throttle openings ends up wrong there isn't any easy way to adjust it.
What it comes down to is that a butterfly valve carburetor has to be designed and setup to work with a particular main jet size. Many carefully designed butterfly valve type carburetors have been setup to work fairly well with a small range of main jet sizes, but this range of adjustment is only for one purpose: Converting the carburetor to work as a high output racing carburetor.
If a butterfly valve type carburetor ends up a bit too lean at around 1/3 to 1/2 throttle openings there is very little that can easily be done to correct the problem. Going up to a larger main jet to richen the mixture slightly at 1/2 throttle results in a dramatically richer mixture at 3/4 throttle and wide open throttle.
Butterfly valve type carburetors shouldn't be adjustable. That is to say butterfly valve type carburetors shouldn't need to be adjusted on a daily basis. Butterfly valve type carburetors shouldn't need to be adjusted at all for normal operation. Changing the setup on a butterfly valve type carburetor to actually change the mixture response versus throttle opening curve requires changing either the Venturi shape or the fuel delivery apparatus shape. These are major modifications that certainly are not done with the twist of a knob.
A dial-a-jet main jet adjustment system that is setup to deliver the best possible match between the mixture response curves and expected operating conditions on normal types of gasoline can sort of work, but it is extremely tricky. There are myriad things that can very easily go wrong. Basically it was a very bad idea in the first place. Butterfly valve type carburetors just don't respond well to jetting changes, and they shouldn't be adjustable other than swapping out main jets to deliver maximum power in hot rod applications.
The dial-a-jet main jet circuits on early automotive and tractor gasoline engines were only popular because the gasoline supply was highly variable. Once someone actually figured out how to tune a gasoline engine they needed a means of adjustment if the gasoline supply didn't remain constant or close to constant.
An expert tuner can get any butterfly valve type carburetor to run on any type of flammable liquid that will run in any gasoline engine, but it won't necessarily run well. If a very low energy density mixture of ethanol and gasoline is used on a butterfly valve type carburetor then the mixture is probably going to be horrendously lean bellow about 1/2 or 2/3 throttle openings. If an intermediate circuit adjustment screw is also available then the fueling rate can be turned up way down low, but there is still a huge lean spot in the middle. The only way to use a carburetor like this is to get it high idling on either the intermediate circuit or with the choke partially closed and then jump right to near wide open throttle to make power. A very choppy power delivery to be sure.
The alternative would be a giant main jet so that the mixture would be rich enough to run at 1/2 throttle on alcohol. Then the mixture tends to become hugely rich at wide open throttle. With a higher compression ratio and a big camshaft this hugely rich mixture at wide open throttle can make power on alcohol, but it is going to be a fire breathing monster. Like giant flames out the exhaust.
Going the other way if a butterfly valve type carburetor is setup for alcohol then it is extremely difficult, but not impossible, to get it to run on gasoline. Going down on the main jet size enough so that it will make power on gasoline is possible, but then it tends to be horrendously flooding rich everywhere bellow about 2/3 throttle opening. Again if an intermediate circuit adjustment screw is available then the fueling can be turned down at small throttle openings, but a huge flooding rich spot remains in the middle. An alcohol carburetor setup like this to sort of run on gasoline also has to jump directly between high idle and making power wide open. A very choppy power delivery to be sure, and arguably more problematic even than the choppy power delivery of trying to setup a gasoline carburetor to run on alcohol.
The flooding rich spot is more problematic for two reasons. One is that if the engine floods and stalls with a weak spark magneto ignition system then it may require spark plug cleaning before it will fire up again. Even with a strong points ignition spark a flooded engine with an overly rich carburetor may need to have the spark plugs removed just to crank all the raw fuel out of the cylinders before it will start up again.
The other reason that a flooding rich spot may be more problematic is that torque output is going to be higher on gasoline than on alcohol. If the entire vehicle is designed and setup around running on alcohol then an abrupt hit of full wide open throttle gasoline power could easily be difficult to use. Especially in city traffic.
Switching back and forth between 100% ethanol and 100% gasoline is an extreme case, but anything more than small changes in the energy density or mechanical properties of gasoline tends to be very problematic for butterfly valve type carburetors. Butterfly valve type carburetors simply don't respond well to large jetting changes.
Understanding why this is relevant to the very large number of precisely designed carburetors manufactured in the past 70 years again requires some historical perspective. In the early years of automotive power it was not entirely clear what the fuel would be. Many people seem to have been avid proponents of limited use of combustion engines running entirely on rather small quantities of renewable bio-fuels. In other words it may have seemed entirely reasonable back in 1908 to think that most automobiles would run on 100% ethanol.
What happened though was that automotive power was a very compelling force in society and demand skyrocketed. Large increasing demand for automotive power, industrial power and electrical power necessitated heavy use of petroleum. At that time oil and gas were the modern abundant clean alternatives to coal. Switching to oil and gas heating and eliminating most forms of coal burning motive power resulted in huge improvements in air quality and public health. Petroleum was the way of the future, the easy fix and the panacea that satisfied everyone. Or nearly everyone.
For quite some time throughout the first few decades of the 20th century bio-fuels proponents appear to have held large sway with engine manufacturers and fuel suppliers. Large quantities of ethanol probably showed up in the gasoline supply at various times. The result though was just a bunch of very severe problems. The world had already adopted petroleum use and bio-fuels were no longer a viable alternative for meeting the large and increasing demand for combustion fuels.
What appears to have happened is that expectations of the energy density of gasoline became skewed. Engines were delivered from the factory jetted to run mixtures of ethanol and gasoline, and adjustable dial-a-jet main jet circuits on automotive engines allowed such extremely rich mixtures that very low energy density fuel seemed normal.
An extremely overly rich jetted engine runs better on lower energy density combustion fuels. When the majority of commercially available engines are delivered extremely overly rich jetted then a demand for lower energy density combustion fuels is inevitably created.
It is not entirely clear how this all played out, but the end result appears to have been that fuel suppliers were forced to supply unusually low energy density petroleum derived combustion fuels. In other words synthesizing ethanol or methanol or other low energy density specialty fuels from petroleum.
By the time consistent non-adjustable carburetors and consistent gasoline supplies began to predominate in the 1930's the bar had already been set very low. The non-adjustable carburetors were setup with extremely rich jetting and the problems continued.
The extremely low energy density specialty fuels were more expensive and worked more poorly, so normal cheap types of gasoline eventually predominated. The overly rich jetted carburetor standard continued though. The common type of carburetor setup became rich to the point of flooding at all smaller throttle openings and then a leaner mixture that would make power wide open. This leaner mixture at wide open throttle was not the same on all carburetors. It varied from quite a bit on the lean side to where power output was somewhat low up to an overly rich mixture that actually reduced power output. What was consistent was the overly rich mixture down low. Most carburetors were setup very similarly to deliver a precisely metered extremely rich mixture that was just on the verge of flooding under extreme conditions. This became normal, and it eventually led to another round of low energy density specialty fuels starting in the 1970's.
The impetus for low energy density specialty fuels in the late 20th century was driven by pollution control legislation. People demanded that cars stop belching noxious exhaust and black soot, and this required leaner jetting. For the most part though the carburetor problems were only partially remedied in the 1970's. Carburetors did get slightly leaner setups, but the energy density of gasoline also plummeted.
By the early 1990's when I started working on carburetors the gasoline supply was mostly very consistent and it was easy to see how carburetors of different eras performed. Old 1950's and 1960's carburetors were generally extremely rich down low, to the point that using the choke for more than a few seconds was almost unheard of even in freezing conditions. Newer carburetors on emissions controlled vehicles from the 1970's and 1980's were cleaner, more efficient, easier to tune and generally much better for any purpose.
Overly rich setups continued to be a large problem though. The extreme flooding level of overly rich jetting was mostly gone by the 1990's, but the new carburetors and EFI systems still tended to dump huge quantities of gasoline into the engine under all conditions.
And that is still the situation now 30 years later. Carburetor and EFI setups are so rich that lower energy density specialty fuels actually work better under most conditions. With more precise computer controlled engine management systems perfectly flat even mixture control is possible. When this perfectly metered even mixture control is somewhat overly rich for best overall performance and cleanest operation though a demand is created for lower energy density specialty fuels. Oversized engines also contribute to the problem. When automotive engines are sized several times too large for the application then they run most of the time under very light loads. Under light loads in full flame front travel mode an overly rich mixture results in quite dramatically worse performance, higher fuel consumption and dirtier exhaust emissions.
When the feedback type engine management systems are designed to maintain overly rich mixtures then the only way to actually get the leaner and cleaner mixture is with extremely low energy density specialty fuels. If the feedback type engine management system regulates the fuel delivery rate to maintain an overly rich mixture then the only way to get a leaner and cleaner mixture is to use such an extremely low energy density specialty fuel that the fueling rate for a rich mixture would be outside of the normal automatic range of adjustment.
The demand for leaner and cleaner mixtures in oversized computer controlled engines is twofold. One is emissions testing. Stricter requirements for very low unburned hydrocarbon emissions precludes extremely rich mixtures at idle. The other demand is more immediate though. Just about anybody can feel the difference between an overly rich engine and a functionally lean engine in full flame front travel mode. The overly rich mixture tends to feel a bit sluggish even if it makes just as much torque. With an overly rich mixture changes in throttle position at low power output have a slightly delayed effect. An engine running in full flame front travel mode also generally sounds crisper and healthier with a functional lean mixture or ideal mixture than with an overly rich mixture. And finally, even on computer controlled vehicles an overly rich mixture is much more likely to cause a noticeable abrupt hit in power delivery on the transition from full flame front travel mode to late compression ignition mode. The overly rich mixture can't make quite as much power in full flame front travel mode at elevated engine speeds, and then the overly rich mixture makes maximum torque once the engine enters late compression ignition mode.
The situation from the late 1990's through about 2015 was that computer controlled automotive engines consistently ran quite rich. The result was that 1.8 to 2.4 liter four cylinder engines with five speed manual transmissions in 3,000 pound medium size four door sedans were getting abysmal 25 to 30 miles per gallon in normal mixed driving at 25 to 50mph speeds on small highways and lager flowing rural and urban thoroughfares. Fuel mileage even with a small 1.8 liter engine might be expected to plummet down to 30 or even 25mpg in slow stop and go city traffic conditions or at extremely high speeds above 75mph on the freeway, but 25mpg under normal conditions just wasn't credible. It didn't make sense to most people. The cars didn't seem like they would be able to use that much gasoline. And it turns out that there was something wrong.
There were of course a lot of other things wrong with the 1980's and 1990's "economy" cars. Chief among them was too much displacement and extremely excessive stroke lengths. These big engines were also paired with inappropriately low gearing. An oversize engine does even worse when it is spinning too fast for the light loads it is running under. Even with the prevalent 3.25 to 3.5 inch automotive stroke lengths, the huge two liter displacements and low gearing that spun the big engines up to 2,500RPM just to go 50 or 55mph though 35mpg under ideal conditions was still entirely practical. With more appropriate gearing 40mpg was possible at moderate 45 to 60mph cruising speeds with the same long stroke length two liter port injected four cylinder engines in rather roomy four door sedans. And some common models were actually able to turn in 40mpg with two liter port injected engines. The reasons that many of the five speed economy cars were actually getting 25 to 30mpg instead of 35mpg under those ideal cruising conditions was due mostly to problems with overly rich air/fuel mixtures.
The mixture ratios were far too rich at small to medium throttle openings. The engines were sucking down 10 or 15% extra fuel that went straight to waste heat and higher exhaust emissions. Somewhat low energy density gasoline apparently also remained fairly common. Just ask any dirt bike rider what jetting he had to use. Pretty darn fat is the answer. Even up at 3,000 to 5,000 feet of elevation where the best heavily forested riding areas are located seemingly irrationally large main jet sizes and correspondingly rich needle clip positions were routinely required to deliver trouble free power.
In any discussion of fuel mileage though it is important to point out that there are other severe problems with the cars themselves. The angled cylindrical roller bearings used both in transmissions and on ring and pinion sets and as wheel bearings have a lot of friction which sucks down extra power. The extremely heavy automotive pistons and connecting rods also suck down a lot of extra power, and overly low gearing makes this problem even worse. Especially under light loads at lower speeds these inefficiencies are very significant and contribute to seemingly irrationally high fuel consumption. And of course it also has to be pointed out that cruising down a gear was a rather common practice throughout the 1980's and 1990's. Obviously a dramatically oversized engine that is already geared too low is going to do even worse when it's not in high gear.
Myself I didn't see much of the unusually low energy density gasoline throughout the 1990's and into the first decade of the 21st century, but I do have some indirect evidence of it's existence. One of course is the main jet sizes found in dirt bikes. They have often been very large. So large that power output actually increases with a smaller main jet on normal gasoline. The other direct evidence is a few cars that I saw run overly lean with what should have been normal jetting.
Lower energy density gasoline did show up sometimes. For the most part though gasoline straight from the gas stations was always very consistent. I could always count on a tank of gasoline going into any engine delivering pretty much exactly the same mixture response. Gasoline engines didn't always run exactly the same under all conditions. I noticed sever problems with intermittent loss of power on most gasoline engines in the 1990's, but the mixture ratios at small throttle openings always remained pretty much exactly the same on most engines.
In the 1990's and into the first decade of the 21st century the emissions controlled carbureted engines from the 1970's and 1980's always seemed to have an ideal power production mixture at low idle and at the first small crack of the throttle that then richened up to a somewhat excessively rich mixture at 1/5 to 1/2 throttle openings. Mostly they were all like that, and they always seemed pretty much exactly the same.
Things did go wrong with those carburetors sometimes. Stuck or broken chokes, power valves stuck open, broken or out of adjustment accelerator pumps, and even out of adjustment float levels. All of these problems showed up from time to time, but the mixture control at small throttle openings remained very consistent on the 1970's and 1980's emissions controlled carburetors.
The energy density changes in the past few years are easy to see in the response of my 1991 Husqvarna WMX 610. Back in the late 1990's and through about 2005 the stock jetted 40mm DellOrto always seemed a slight bit on the rich side. With the stock 180 main jet and the stock K32 needle on the second clip position jetting was plenty rich, usually noticeably overly rich. The engine nearly always started without the choke, although cold conditions sometimes benefited from a priming kick with the choke on. On the few rare occasions that the Husqvarna 610 did fire up with the choke on it normally flooded and stalled within one second. Only sometimes was I able to reach down and flip the lever off quickly enough to prevent it from flooding and stalling. With the strong spark of the points ignition it did however usually fire right back up. With the stock CDI ignition the engine firing up with the choke on always required spark plug cleaning or a new spark plug to get it going again. During this time from 1998 through 2005 the exhaust got a bit blackened, but not extremely. It was just a light smattering of carbon build up in the exhaust system and the spark plug was also usually a dark brown or somewhat black. The jetting was too rich, but it did deliver big power. The only thing that stopped the power was intermittent severe hesitation. When it wasn't hesitating it pulled hard, but the intermittent hesitation easily caused a total loss of torque at 3,500 to 5,000RPM or a total loss of power above about 6,500RPM. Often the loss of torque at 3,500 to 5,000RPM and the loss of power above 6,500RPM came at exactly the same time as I gained elevation. The result was either broad strong power from 4,000 to about 8,000RPM or a brutally narrow powerband from 5,000 to 6,500RPM.
Then in 2015 the same stock 10:1 engine with the same jetting ran pretty much exactly the same. The big difference was that kicking back when kick starting was absent, so I was able to run any spark timing value required from 22 to 29 degrees BTDC to get rid of the hesitation. What I also noticed in early 2015 though was that the exhaust was alternately sometimes getting more severely black and sometimes not getting black at all. The engine was also occasionally firing up with the choke on and continuing to run for several seconds without flooding and stalling. The gasoline was obviously not the same all the time anymore. The fact that I had to go up and down dramatically on the spark timing on a daily basis also strongly indicated that the gasoline was not remaining anywhere near consistent in early 2015. For the most part though the gasoline remained just as powerful in early 2015 as it had been. The biggest problem in early 2015 was that when I was forced to go up to 27 and 29 degree BTDC spark timing to eliminate hesitation the engine got extremely loud and harsh at all engine speeds with a narrow top end only power band from about 5,500 to 7,000RPM. It still ran and pulled with 29 degree BTDC spark timing, it was just a very narrow and slightly weaker power band with nothing but huge crashing smashing harshness and severely diminished torque down lower.
Then in late spring of 2015 I switched to slightly leaner jetting and a higher compression ratio with the rebuilt 1997 Husqvarna 610 motor. The higher 11:1 compression ratio eliminated the intermittent problem of the engine getting harsh and losing power, and the smaller 175 size main jet eliminated the excessive blackening of the exhaust. On the same gasoline that was delivering extremely harsh and loud operation with a brutally narrow top end only power band on the stock 10:1 engine the leaner jetted 11:1 engine pulled huge torque everywhere from 3,500 to 8,500RPM. Even with the leaner 175 main jet it was more power than I had ever seen from a Husqvarna 610 motor, but the bigger valves, bigger camshaft and lighter piston also contributed to increased top end power. The dramatically stronger and also dramatically smoother torque from 3,500 to 6,000RPM on the exact same gasoline was all in the compression ratio. Swapping the same gasoline back and forth between the two bikes the 10:1 engine was very harsh with weak torque at 3,500 to about 5,500RPM and the 11:1 engine was smooth and pulled about 50% more torque at 3,500 to 5,000RPM. On lower pressure gasoline the 10:1 engine made essentially just as much torque at 3,500 to 5,000RPM, but top end power from 7,500 to 8,500RPM was never as high on the stock engine as on the hot rod Husqvarna 610 motor.
Then in early summer of 2015 the gasoline turned to dramatically slower flame front travel speed gasoline than I had ever seen before. First it showed up overnight in my gas cans and the tanks on my bikes, but then in June of 2016 the dramatically slow flame front travel speed gasoline started coming out of the pumps all the time also. On the slower flame front travel speed gasoline both of the Husqvarna 610 motors suffered severely from a lack of revs. They would just crap out at around 5,500RPM and wouldn't rev higher. This was a more severe problem on the stock motor. On the slow flame front travel speed gasoline it just wouldn't go. On that stock motor I usually just gave up and parked the bike. It wasn't my main bike anyway. The next day, or the next time I went to the gas station it would be back to normal faster flame front travel speed gasoline and performance was back. On the hot rod 11:1 motor though the slow flame front travel speed gasoline was persistent. If it was in the tank one day, then it was still there the next day. If the slow flame front travel speed gasoline came out of the pumps, then the next time it was slow flame front travel speed gasoline out of the pumps again. For weeks it was nothing but slow flame front travel speed gasoline in the 11:1 hot rod 610 motor.
Then the slow flame front travel speed gasoline was gone. In it's place though in August of 2015 was high pressure race gas that wouldn't work in the 11:1 motor. Even down at 1,000 feet of elevation it was just huge harshness and a narrow power band. The same problem I had been having with the 10:1 stock 610 motor on and off throughout the spring of 2015. In the 11:1 hot rod motor though the high pressure gasoline was pervasive. It wasn't the same every time, but time after time it was just such extreme high pressure gasoline that the 11:1 motor required 29 degree BTDC spark timing and the torque disappeared into a sea of smashing harshness. At it's worst the extremely high pressure gasoline caused a quite dramatic loss of top end power in the 11:1 motor. It was just a brutally narrow power band from about 6,000 to 7,500RPM with very little above and nothing but harshness bellow.
That extreme high pressure race gas that wouldn't work in the 11:1 motor only came out of the pumps for a short period of time, but what followed was even worse. Up to August of 2015 the gasoline had always seemed to have either just as high of an energy density and power production potential as traditional gasoline from the 1990's and first decade of the 21st century, or an even higher energy density and more power production potential.
Then in the fall of 2015 the gasoline started getting progressively weaker while also remaining rather high pressure stuff for very high compression ratio engines. The spark timing persistently stayed up very early around 26 to 31 degrees BTDC on the 11:1 engine, but torque and power output vacillated severely with the dramatically changing combustion properties. Energy density was sometimes obviously lower, the flame front travel speed was sometimes obviously rather low and it seems that the temperature of combustion potential of the gasoline was also sometimes lower. The worst thing about this switch to weaker and highly variable gasoline was that the engine was more difficult to tune. It always seemed to need way too much spark advance, and performance dropped off dramatically with slight changes in spark timing. The spark timing had to be exactly correct for the engine to work at all, and even at that performance was generally lower than it had been. And the gasoline just kept getting weaker and weaker while the spark timing stubbornly remained way up close to 30 degrees BTDC.
Then I went up to the 12.2:1 compression ratio in late 2015 and this got the hot rod 610 motor to work much better. Right away I was able to go down to 24 degrees BTDC on the 12.2:1 motor which delivered broader and more reliable torque on the still rather weak and highly variable gasoline. Sometimes I still had to go up to 27 degrees BTDC, but for the most part the 12.2:1 compression ratio broke the cycle of excessive spark advance. I never had to go past 27 degrees BTDC on the 12.2:1 motor, and even 27 degrees BTDC was a vast improvement over the 28 to 31 degrees I had been being forced to run with the 11:1 compression ratio.
The gasoline kept getting weaker though. Slow flame front travel speeds were again common in early 2016, but the 12.2:1 motor was much better able to handle slow flame front travel speed gasoline. The combination of the higher 12.2:1 compression ratio and the straight up cam timing that I ran for a time dispensed with the slow flame front travel speed problem very handily. Even on extremely slow flame front travel speed gasoline the 12.2:1 motor could be made to rev out fairly easily.
The gasoline kept getting worse though. The next problem was a large and dramatic drop in the temperature of combustion potential of the gasoline. On this dramatically lower temperature of combustion potential gasoline the three inch stroke length engine started surging and whining a huge amount. Previously the three inch stroke engine had essentially never suffered from surging or whining under any conditions. Then all of a sudden it was nothing but huge amounts of surging and whining all over the place accompanied by a dramatic loss of top end power above about 7,000RPM. The surging and whining dramatically low temperature of combustion potential gasoline could still be made to rev out, but the power above 7,000RPM was always severely low.
The extremely dramatically lower temperature of combustion potential gasoline only stuck around for a few short weeks, but the temperature of combustion potential of the gasoline continued to be variable and generally much lower than it had ever been before. Throughout the spring of 2016 the combustion properties of the gasoline continued to swing around dramatically and frequently, and the energy density, flame front travel speed and temperature of combustion potential were variously somewhat low to quite dramatically low.
The next big change was a large and dramatic drop in the energy density of the gasoline in the Summer of 2016. This new gasoline was obviously severely watered down with ethanol, methanol or some other extremely low energy density specialty fuel. Up to early summer of 2016 the energy density of the gasoline had sometimes dipped down noticeably, but generally the energy density remained fairly close to what it had traditionally been in the 1990's and the first decade of the 21st century. Then in the summer of 2016 the energy density of the gasoline plummeted dramatically to much lower levels than I had ever previously seen. And that was the situation for nearly a year. Dramatically low energy density watered down garbage frequently coming out of the pumps. An untenable situation for all mechanically controlled gasoline engines. An extremely low energy density and highly variable energy density gasoline supply is computer only fuel. The only type of engine that will reliably run on extremely low energy density and highly variable energy density gasoline is a "Flex-Fuel" computer controlled engine. A computer controlled engine that automatically makes large adjustments to the fueling rate and spark timing as required to maintain torque output on any mixture of gasoline and low energy density specialty fuels.
Up to the summer of 2016 the energy density of the gasoline supply was only somewhat variable. Throughout late 2015 and early 2016 the energy density of the gasoline was certainly variable, but the changes were generally rather small. One very important thing related to energy density does stand out from this 2015 and 2016 period of highly variable combustion properties. The outstandingly highest energy density gasoline was also the most dramatically different in some way. The very slow flame front travel speed gasoline usually appeared to have a rather high energy density, which might be expected. What is very unexpected though was that the most dramatically lower temperature of combustion potential gasoline that caused the most extreme levels of surging and whining and the most severe loss of top end power in the three inch stroke length engine also appeared to have a rather high energy density. On the days that the 12.2:1 hot rod 610 motor was surging and whining like crazy and lacked top end power the torque was still fairly strong over some narrow range of engine speeds around 5,000 to 6,500RPM if the spark timing was set just right. What I also noticed was that on these days when extremely dramatically low temperature of combustion potential gasoline caused extreme levels of surging and whining and tricky difficult tuning the exhaust also got more blackened than it ever had before. I would have expected a low temperature of combustion gasoline to also have a lower energy density, but this was not the case with the gasoline that I saw. The lowest temperature of combustion potential gasoline I ever saw also seemed to have about the highest energy density of any gasoline I had ever seen. Very strange.
The race gas style extreme high pressure gasoline also often had a rather high energy density. The moderately high pressure pump gas that constantly plagued the 10:1 engines requiring huge 28 to 35 degree BTDC spark timing in 2014 and 2015 was more variable in energy density. Usually a fairly high energy density, but not necessarily an extremely high energy density. The extreme high pressure race gas coming out of the pumps in the summer of 2015 that required 29 degree BTDC spark timing and caused extreme levels of harshness in the 11:1 hot rod Husqvarna 610 motor seemed to have a consistently rather high energy density. Despite the huge levels of harshness there was still some power, and it seemed to be requiring an awful lot of combustion to get that power through the harshness. What the extreme high pressure race gas style pump gas didn't do though was blacken the exhaust as much as other high energy density gasoline types. The race gas style pump gas seemed to produce an awful lot of combustion, but the exhaust emissions remained mostly free of black soot. One explanation is that the race gas style pump gas made better use of the available oxygen or was itself heavily oxygenated. I have heard of heavily oxygenated race gas being used with rich jetting to deliver big power without blackening the exhaust. It also stands to reason that the most powerful specialty race gas would be something that makes good use of the available oxygen. Another possibility though is that the higher temperatures and pressures associated with the high pressure race gas simply reduces build up of visible black carbon deposits on the exhaust system. While also dramatically increasing nasty high pressure pollutants such as oxides of nitrogen it should be noted.
The latest twist now in the spring of 2017 is that very slow flame front travel speed gasoline has again been repeatedly comming out of the pumps. The new twist though is that this latest round of slow flame front travel speed gasoline also has both a rather low energy density and a rather low temperature of combustion potential. The slow flame front travel speed gasoline of 2015 generally had both a considerably higher energy dnesity and a considerably higher temperature of combustion potential than this lattest garbage. In 2015 the slow flame front travel speed gasoline ran quite well in three inch stroke length engines. Back in the summer of 2015 when I increased the spark advance on slow flame front travel speed gasoline to get the big 98mm bore 610 motor to make power to 8,000RPM the only problem was that the motor got overly crisp and unusably harsh at all lower engine speeds bellow 4,000RPM. On this lower temperature of combustion potential slow flame front travel speed gasoline now in the spring of 2017 increasing the spark advance to reduce cutting out at high engine speeds causes surging at 4,500 to 6,000RPM without much harshness down low. There isn't much harshness at 3,000 to 4,000RPM, but there also isn't much torque down there. There isn't much torque down there at any spark timing setting on the weaker lower temperature of combustion potential gasoline.
This weak slow flame front travel speed gasoline has been comming out of the pumps both as 91 (RON+MON)/2 octane rating premium gasoline and also as 87 (RON+MON)/2 octane rating regular gasoline at a six percent lower price point. This latest slow flame front travel speed gasoline requires ratter jetting, won't make as much power and also causes quite a bit of surging in three inch stroke length engines. Basically it is worse gasoline in every concieveable way. And the pirce per gallon has been going up also. The highe price is probably due to the higher cost of the ethanol relative to the gasoline. Getting the energy density of slow flame front travel speed regular down so extremely low would seem to require very large quantities of ethanol. And indeed this latest garbage gasoline has indeed been causing rather noticeable ethanol exhaust smell straight from the pumps. On gasoline straight from the gas station I have been noticing quite a lot of mildly acrid ethanol exhaust smell. Along with the ethanol exhaust smell has been large amounts of popping out the exhaust on deceleration and unusually unstable low idle quality.
On the same gasoline the 85mm bore 9.7:1 386 stroker motor revs to 8,000RPM without cutting out, but then it also goes very flat shortly after 8,000RPM. There was also a lot of surging at 5,000 to 6,500RPM. Even more surging than the same gasoline in the slightly longer stroke length 610 motor. It is very weak low energy density gasoline that just won't make as much power in any engine. It's not like it won't make power at all though. Torque was sort of close to reasonable at 4,500 to 6,000RPM in the 12.2:1 hot rod 610 motor. Power in the 9.7:1 386 stroker motor was also sort of close to reasonable at 6,000 to 8,000RPM. On both bikes though torque was dramatically low down lower around 3,000 to 4,000RPM, and top end power in both bikes was also dramatically low. It is just weak gasoline that won't make power. When I put a quart of this gasoline in the Lifan 150 Husqvarna the much smaller bore engine ran well and revved out effortlessly, but again both power and torque were somewhat low. It is just very weak low temperature of combustion potential, low energy density gasoline that also has a very slow flame front travel speed. This was 91 (RON+MON)/2 octane rating gasoline straight from the gas station that I ran in all three motors on the same afternoon at the exact same 77 degree Fahrenheit ambient temperatuer.
A sure sign of the very slow flame front travel speed was that I had both the 9.7:1 386 stroker motor and the 12.2:1 hot rod 610 motor running on the same gasoline with the same 21.5 degree BTDC static timing setting on both engines. Only very slow flame front travel speed gasoline would allow the dramatically higher compression ratio big bore engine to run the same amount of spark advance as very low compression ratio 9.7:1 386 stroker motor. On very slow flame front travel speed gasoline the difference between the 98mm bore of the 610 motor and the 85mm bore of the 386 stroker motor is a very significant difference. This was 91 (RON+MON)/2 octane rating gasoline straight from the gas station that I ran in both motors on the same afternoon at the exact same 77 degree Fahrenheit ambient temperatuer. On faster flame front travel speed gasoline the huge compression ratio difference dwarfs the bore size difference and the 12.2:1 hot rod 610 motor typically runs about a 3 to 6 degree later static timing setting than the 9.7:1 386 stroker motor.
Another interesting comparison was the low idle quality on this ethanol rich slow flame front travel speed gasoline straight from the gas station. The low idle was extremely low and very unstable on the 12.2:1 hot rod 610 motor with static timing settings everywhere from 16 degrees BTDC to 21.5 degrees BTDC. The 9.7: 386 stroker motor running on the same gasoline on the same afternoon also had a very unstable low idle, but it was up much higher close to where it usually low idles. When I turned the throttle stop in on the 12.2:1 610 motor it seemed unresponsive. It required a large twist of the throttle stop to get a small increase in low idle speed, and the low idle quality was still extremly unstable. It was like this both at 16 degrees BTDC and at 21.5 degrees BTDC. The low idle was just extremely unstable on this highly ethanol laden gasoline. It's probably more than 25% ethanol. Certainly a lot more than 10% ethanol. Just getting the slightest hint of ethanol exhaust smell typically requires quite a bit of ethanol. When I used to mix 10% ethanol into normal gasoline for the overly rich jetted stock 1991 Husqvarna 610 back 15 years ago I never noticed any ethanol exhaust smell. I also tried 20% ethanol once, but performance dropped off noticeably so I stuck with just 10% ethanol. That one time I ran 20% ethanol though I don't remember any noticeable ethanol exhaust smell.
When I tried 100% ethanol recently in the 12.2:1 hot rod 610 motor with the needle clip at the fourth groove and the choke left on there was hardly more ethanol exhaust smell than from what has now been comming out of the pumps. When I was done with the short 100% ethanol test I just filled the tank back up with gasoline without draining the aproximately half gallon of remaining ethanol, which resulted in about a 25% ethanol mixture. I went back to the first needle clip position on this 25% ethanol mixture and the bike was running and starting and there was only a slight bit of ethanol exhaust smell. Performance was however very poor with 25% ethanol in the gasoline so I drained the tank after two short rides.
Something else that is of interest is how the 25% ethanol affected the temperature and pressure requirement for late compression ignition. Or rather how it didn't make any difference. This was true back 15 years ago also. Adding 10% ethanol didn't do much of anything since the carburetor was jetted ovely rich. Adding 20% ethanol noticeably increased hesitation and reduced power and torque.
It was the same thing recently. When I added gasoline back in on top of the 100% ethanol the mixture was still requiring a fairly high temperature and pressure point for late compression ignition. The only noticeable difference was that the gasoline was weaker and leaner with 25% ethanol in it.
What happened with that 25% ethanol mixture the next day though is quite bazar. The next day it was a dramatically lower pressure fuel that poped off on late compression ignition at dramatically lower temperature and pressure points. The dramatically lower pressure gasoline had also taken on a nasty sharp smell. That strange smell somewhere between pinsol and lysol. The same smell that I had noticed from the thick green liquid in the carburetor bowel on the 1997 Husqvarna TE 610 motor when I first got it in the spring of 2015. That 610 motor was put together at a 7:1 compression ratio when I got it, and the needle clip was at the 4th groove with a main jet drilled out to the 195 size. It seemed obvious that someone had been trying specialty low pressure fuels to try to get that very low 7:1 compression ratio to run, and the thick green goo in the carburetor smelled strongly of something bad.
When I was getting the bianual smog certifiate for my car last year I noticed that same strong nasty pinsol and lysol sort of a smell comming from a container outside. When I asked the guys at the shop what it was they said it was a tank of "bad gas" they were letting evaporate to get rid of it.
I have heard of "stale gas" having a strong bad smell, but it was something that I had never experianced. In my experience gasoline just evaporates leaving little or no residue. That was true all through the past three decades. Gasoline just slowly evaporated leaving very little behind. Many times I have left carburetors sit for months or years until the gasoline evaporated, and usually I was ablet to just add gasoline and use the carburetor without taking it appart. I have seen all sorts of old gaskets and seals dry up and leak after a carburetor is left dry, but the gasoline itself seemed to just evaporate.
I have however also bought quite a few old used carburetors that had huge amounts of thick white chalky residue in the bowel that had to be cleaned out before the carburetor could be used. People used to talk about carburetor cleaner that would desolve deposites like that, but I was never able to find anything like that. The thick chaulky residue just had to be mechanically scraped out to get the carburetor working again. If any chunks of the chaulky residue are left in the bowel they can easily clog idle passages, so it takes a very thourough cleaning to press a contaminated carburetor like that back into service. I had always wondered where all that gunk had come from, but usually it was very old carburetors from the 1950's and 1960's that were gunked up like that.
It seems now that there might be some connection between the overly fat jetting on most of those old 1950's and 1960's carburetors and the thick white deposites that were common in the bowels on the same types and vintages of carburetors. Weaker and/or lower energy density gasoline has to have lower temperatuer and pressure points for late compression ignition to run in an engine with the same compression ratio and the same spark timing. Dramatically reducing the energy density of gasoline with the addition of large quantities of ethanol would generally tend to require the use of pressure lowering additives to run at the same low compression ratios in the same engines as normal gasoline.