The first problem was hard starting and stalling when cold, then the 10.2:1 big cam SEM ignition 610 was lean stumbling once fully warmed up. This got worse, and eventually caused some very severe problems. The root of the problem turned out to be a stock size marked pilot jet that had been drilled out several sizes larger than stock.
Repairing the Revalved Forks
The 10.2:1 Big Cam SEM 610 Running Leaner
Air Box Repair
Pilot Jet
Gasoline Search
Too Loud
40mm DellOrto Jetting Realities
At first the period correct 1991 Husqvarna WMX 610 was too uncomfortable to ride with the badly modified suspension. I called the phone number on the stickers on the White Power forks and shock, but the phone company said that the number had been "disconnected or is no longer in service". So much for that race shop, I guess other people had similar ideas about that shop's particular brand of suspension valving.
I had replaced the stiff "68" shock spring with a stock "56" 1991 Husqvarna WXE 350 shock spring, and that had helped some with delivering a smoother ride. The race shop modified suspension valving seemed to particularly disagree with the stiffer spring rate, so going back to the stock spring was certainly an improvement even if that stock spring is a bit on the soft side for my 200+ pound heft. I had thought before that the stock springs were marked "58-280", but upon closer inspection it turns out that they are marked "56-280". Especially the nearly new stock spring I got off of that 1991 WXE 350 shock was clearly marked "56-280", not "58-280". When I looked at my other springs they all appear to be marked "56-280" also.
The stock shock spring was an improvement, but the race shop modified valving still didn't work well. The big problem was that it seemed very harsh across most types of bumps. I tried going out from the #2 compression clicker settings I had been running to the #1 compression clicker settings on the forks and the shock, and this also helped. The bike felt even more bouncy and out of control at higher speeds, but the comfort improved noticeably. Overall it did seem like an improvement going all the way out on the compression clickers When I hit a rocky trail with lots of small to medium sized sharp bumps though it was still just horrendous. Much worse harshness and discomfort than the stock 1991 Husqvarna WMX 610 White Power suspension. A lot slower than the stock 1991 Husqvarna WMX 610 White Power suspension also. Harsh, bouncy and uncomfortable and the bike was dancing around on the rocks so much that it was hard to keep it in control.
As a next step I replaced the fork seals with ARI-035 seals and filled the forks with 2.5W BelRay fork oil. With the compression clickers back to the #2 setting and the rebound clickers in at the #4 setting the action of the forks was much improved. The compliance over small bumps was much better, and the front end felt more stable and more in control at high speed than it had with the 5W oil and the compression clickers all the way out. The 2.5W oil was certainly an improvement on the race shop modified valving in the 1991 Husqvarna WMX 610 White Power 40mm USD forks. The front end still felt stiff and harsh though, stiffer than the stock 1991 Husqvarna WMX 610 White Power 40mm USD forks with 5W oil. I tried going out from the #4 rebound setting to the stock #3 rebound clicker setting, and this was a big improvement. The rebound felt excessively loose and active, but the compliance over bumps improved considerably.
Finally with the 2.5W oil and the clickers in the same positions that I run them on the stock 1991 WMX 610 White Power 40mm USD forks with 5W oil the race shop modified valving was sort of usable. Finally the extreme levels of excess stiffness and harshness over sharp bumps was gone, and finally the front end of the period correct 1991 Husqvarna WMX 610 was feeling similar to my other 1991 Husqvarna WMX 610 bikes. The race shop modified 1991 Husqvarna WMX 610 White Power shock still wasn't great, but with the compression clicker all the way out it was sort of working. With the compression clicker all the way out like that the modified shock was feeling sort of similar to the stock shocks with the compression clicker in one click for more hold up. The modified shock was delivering some substantial hold up even with the compression clicker all the way out, and although it wasn't feeling quite as competent as the stock 1991 Husqvarna WMX 610 White Power shock it was feeling almost usable under some conditions. With more usable suspension I started riding my period correct 1991 Husqvarna WMX 610 more, but other problems began to show up.
The actual very first jetting problem with my period correct 1991 Husqvarna WMX 610 was that the 10.2:1 big cam stock SEM ignition 610 motor was running flooding rich when I first bought the bike in the spring of 2017. That problem turned out to be a 190 marked main jet that had been drilled out to bigger than the 200 size and the needle clip in the 3rd groove on the stock K32 needle in the stock 40mm DellOrto carburetor. Installing a 176 size main jet and setting the needle clip to the 1st (leanest) groove got the 10.2:1 big cam SEM 610 motor running strong, but the good times didn't last. At first the only real problem was just that the engine was very harsh and rather weak at all lower engine speeds bellow about 6,000RPM running the stock 33 degree BTDC spark timing. Then that normal gasoline disappeared and the big cam SEM 610 motor was just surging horribly all over the place. The horrendous surging was bad enough, but then the motor was also totally loosing power above about 4,000 feet of elevation. Then the 10.2:1 big cam SEM 610 motor started running too lean. The jetting had been fine with the 176 main jet and the needle clip on the leanest position, which is the same jetting I have been running on my other Husqvarna 610 motors for a few years now.
The first problem with the 10.2:1 big cam SEM 610 motor running lean was just that it was feeling and sounding lean and power output was lower than it had been before with the same 176 size main jet. With the huge 33 degree BTDC stock spark timing and the big camshaft the 10.2:1 motor remained sufficiently crisp to get going and make power, there just wasn't as much power to be had on the weak watered down gasoline. At first it just sounded horrible and felt very lean, but power output was only down moderately from where it had been before. Then the top end power actually started to disappear. Even down at 1,000 feet of elevation it was reluctant to rev out, and needed to be fully warmed up on successive big pulls to make top end power. Mostly it just sounded horrible. It was very loud, but a hollow sort of a lean sound.
Along with the lean operation had come hard starting where the engine would fire up and stall several times before continuing to run. Interestingly though the engine did continue to fire with the kick starter. Often it would start and stall over and over again many times when first fired up in the morning, but it did continue to fire with the kick starter with a stock 0.027" spark plug gap. This was back in the summer in rather warm conditions, but the lean operation was still causing hard starting. The very lean operation was also causing some lean stumbling even when the engine was fully warmed up. At first this was just a bit of instability now and again at small throttle openings around 2,000 to 3,000RPM, but it did get worse.
Then the really big problem showed up in the fall of 2017. Popping out the intake. Yeah, that's bad. It's what has been referred to as a "flame out", and it is one of the worst things a dirt bike can suffer from. At first this popping out the intake was only very occasional, and mostly only as the engine was still warming up after I first rode off. The popping out the intake did however get worse. Much worse to the point where the bike was hardly ridable. It was still only at small throttle openings around 2,000 to 3,000RPM that the 10.2:1 big cam SEM 610 motor was popping out the intake, but it was getting pretty bad.
Then one day in late December 2017 the popping out the intake was so severe that the bike wouldn't cruise along at constant throttle openings at low speeds. If I opened the throttle and accelerated it ran without stumbling and without popping out the intake, but cruising along at constant small throttle openings it just wouldn't play nice. It was the lean stumbling that was causing the popping out the intake. The 10.2:1 big cam SEM 610 motor would cruise along for a while at small throttle openings, but then it would start lean stumbling. As soon as the lean stumbling would set in it would then give a big pop out the intake and just go dead for a second. It kept running without stalling, but it was a bit of work to keep it going.
I rode up to higher elevation, and the lean stumbling subsided as I climbed higher. It was still lean stumbling a bit even up to 6,000 feet of elevation, but the popping out the intake nearly totally went away. Then when I descended back down to lower elevation the popping out the intake came back bad at about 4,000 feet of elevation. Heading down a very loose rocky steep descent I blipped the throttle open to accelerate on a short flat section, and then the throttle wouldn't close! I rocked the throttle tube open and closed several times, but the bike just kept accelerating. Before I could even get the clutch pulled in the bike had blasted me right off the trail and into the bushes. Even running lean and crappy a Husqvarna 610 gives quite a burst of acceleration when the throttle is cracked open.
I crashed fairly hard, but nothing was broken. Nothing on my body and nothing on the bike. Just some scrapes and small cuts to my forearms and knees and similar scrapes and gauges to the plastic on the bike. When I realized that I was mostly unhurt as the pain and shock subsided I turned to the bike, which was low idling in gear with the rear wheel off the ground. The aluminum bars were torqued around in the clamps, but not bent and the hand guards had saved the brake lever from being bent or broken. The bike was still in one piece and I was mostly unscathed, but very confused.
As I rode off down the trail I realized what had happened. The popping out the intake had damaged the foam air filter element, and eventually a piece had broken off and jammed the throttle slide open a bit.
This period correct 10.2:1 big cam 1991 Husqvarna WMX 610 had two giant 3 inch holes drilled through the air box cover, and I had felt the popping out the intake on the inside of my right hand leg. When I looked at the air filter element through the giant holes I could see that it was badly damaged, with broken pieces hanging off.
The first thing I needed to do was install a new air filter element, as the old one was in sore condition. That had been the old air filter element that I had been running back in 2005, so it was a dozen years old. I figured that a new foam element would be stronger, and wouldn't beak apart as easily even if the engine continued to pop out the intake. While I had the air box apart though I decided to install a stock type intake screen. Someone had removed the intake screen from the air boot on this period correct 10.2:1 big cam 1991 Husqvarna WMX 610 before I bought it, which wouldn't necessarily have been any kind of a problem. The Husqvarna 610 motors don't normally pop out the intake, so the screen is a bit redundant. The 1997 Husqvarna 610 motor I have been running for nearly three years doesn't have an intake screen, and it never pops out the intake even though it has the rather large 1994 camshaft. When I had that 1994 camshaft installed straight up with split overlap at top dead center I did sometimes notice a slight bit of instability at low engine speeds and small throttle openings, but it wasn't usually a problem. With the 1994 camshaft advanced a few degrees of crankshaft rotation in the stock setting for the 1997 motor even that small bit of instability is never present. Bigger camshafts with more overlap do tend towards causing some instability at small throttle openings and lower engine speeds, but the stock Husqvarna camshafts aren't all that big and problems don't usually show up.
The camshaft in my 10.2:1 big cam SEM 610 motor is however even bigger than the 1994 camshaft, and it is installed straight up. This wasn't a problem in itself, even without an intake screen. It was only when the watered down gasoline caused lean stumbling that popping out the intake actually occurred. It took a bigger than stock camshaft, the removal of the intake screen and also extremely watered down gasoline to actually get the Husqvarna 610 motor to pop out the intake. None of those three problems could alone cause popping out the intake, but the combination of all three was disaster.
After installing the new air filter element, a stock intake screen in the air boot and an air box cover with no holes drilled in it the popping out the intake was gone. The engine was still lean stumbling very bad over a wide range of smaller throttle openings, but it wasn't popping out the intake. That's why they call it a "backfire screen" it actually prevents popping out the intake.
So that got the period correct 10.2:1 big cam SEM ignition 610 motor working again, at least that one day. The next day it wouldn't start. The 10.2:1 big cam SEM 610 motor fired when I kicked it, but low and weak and then it wouldn't fire at all on subsequent kicks. When I rolled the bike down a little hill the engine fired up, but it ran horribly with tons of lean stumbling all over the place. It was lean stumbling not only at small throttle openings, but at larger throttle openings also. It would barely run. It did make some power through the midrange, but lean feeling and horrendously loud. It seemed to be recovering from the lean stumbling at lower engine speeds and I was able to ride the bike up a hill. Then when I tried to rev the engine out wide open it was lean stumbling bad and this lean stumbling at higher engine speeds seemed to foul the spark plug. The 10.2:1 big cam SEM ignition 610 motor stalled and wouldn't fire at all with the kick starter. When I rolled it down a hill it fired a little bit, but wouldn't start up and run. I was up a series of little hills, so I kept trying to roll start it. Eventually after about four tries it fired up and ran horribly with tons of lean stumbling all over the place. I made it back, but then the engine died and it wouldn't restart even when rolled down a hill.
The problem was obviously with very weak watered down gasoline that just wouldn't run, but I had a clue about something else that might be wrong. I had noticed that the lean stumbling wasn't as bad at the first little crack of the throttle on the pilot jet. The lean stumbling was much worse at more substantial small throttle openings around 1/8 to 1/4 throttle opening. The first thing I did after the engine stalled and wouldn't restart was to pull the bowel off of the carburetor and take the pilot jet out. What I found was a pilot jet marked 62 but drilled out to about the 64 or 65 size. A 0.0248" diameter #72 drill stock just dropped right through with no resistance. There was some substantial side to side wiggle of the 0.0248" drill stock, but a 0.0256" diameter #71 drill stock wouldn't start in at all. That's bigger than the 63 size, but not over the 65 size. Considering the rather substantial side to side wiggle of the 0.0248" #72 drill stock in the hole it seems clear that this pilot jet was larger than the 64 size. Probably about 64.5 or 65 size, which is quite large compared to the stock 62 size that is stock in the 40mm DellOrtos on the Husqvarna 610 motors. And that stock 62 size pilot jet is already on the rich side for the 40mm DellOrto, so a 64 or 65 size pilot jet is way out there.
Again it seems to have been a case of a drastically oversized drilled out jet somehow attracting watered down low energy density gasoline. I had found the likely cause of the problem, but I didn't have a stock 62 size pilot jet to install in place of the drilled out 64 or 65 drilled out disaster. Repairing a pilot jet was a rather difficult task. It's one thing to drill a small pilot jet out with a twist drill, but making the hole smaller is different sort of task.
I drilled the 65 marked pilot jet out of my 1991 Husqvarna WXE 350 out larger and soldered up the hole. That's the first step, filling in the hole. That is easy enough, it just requires drilling out much larger so that the solder will flow through. The difficulty was in drilling such a small hole. The cheap little drills I had didn't seem to cut well, even in the plumbers solder. Using a #75 drill I finally got a hole punched through, and then I was able to finish up the hole by hand with the #73 drill. I twisted the 0.0240" diameter #73 drill stock in the hole by hand until it just barely went all the way through. This seemed like very close to a 61 size pilot jet. Perhaps a slight bit smaller at around 60.7, but not anywhere near all the way down at the 60 size. Substantially a 61 size pilot jet, not a 60 or a 62 size pilot jet.
The jetting was now close to correct, but I still didn't have any gasoline and the 10.2:1 big cam SEM 610 motor wouldn't fire up. I got it to fire a little bit on the old gasoline, but it wouldn't start up and run. Looking for gasoline I took my 12.2:1 hot rod 610 motor out for a ride. When it first started up it ran well, with good throttle response at small throttle openings and hardly any popping out the exhaust on deceleration. When I shut it off it fired right back up very easily on the first kick. Torque was building nicely from 3,500 to 4,500RPM, and it seemed to be running quite well with no cutting out and no surging. That was only on the gasoline in the carburetor bowel though. Once running on the gasoline from the tank the throttle response at small throttle openings deteriorated and the engine felt very lean and unwilling to run. Popping out the exhaust on deceleration also increased enormously. It went from essentially no popping out the exhaust to horrendous loud popping out the exhaust every time I closed the throttle. The idle quality also deteriorated. It had been idling smooth and consistent on the gasoline in the carburetor bowel, but once running on the gasoline from the tank the idle was very unstable. It was still running and pulling, and there still wasn't any surging. Interestingly the torque was still increasing rather nicely around 4,000 and 5,000RPM and it was still revving out to 8,000RPM without cutting out, but it would only run at wide throttle openings up on the fat 176 size main jet. It felt horrendously lean, but it was running and pulling without surging and without cutting out. This seemed like more of that 60% race gas and 40% ethanol expensive garbage. It could make power in the big 3" stroke length engine without surging, but the energy density was horrendously low for any sort of normal jetting. On the lower energy density gasoline the 12.2:1 hot rod 610 motor was then also hard to start. It did fire up with the kick starter, but not willingly. It took a lot of frustrating kicks to get it going. When I got back from a little test ride I checked the static timing setting on the points ignition 12.2:1 hot rod 610 motor, which I found to be at the same 19 degrees BTDC where I have been running it in recent months.
I took a quart of this lower energy density gasoline out of the bike with the 12.2:1 hot rod 610 motor and I put it in the empty tank on my period correct 1991 Husqvarna WMX 610 with the 10.2:1 big cam SEM 610 motor. I drained the carburetor bowel and let it refill with the gasoline from the 12.2:1 hot rod 610 motor. The 10.2:1 big cam SEM 610 motor still wouldn't start. I got it to fire a little bit, but it wouldn't start up and run. I tried a new spark plug, but that didn't get it to fire up either. I had to get a tow to spin the engine up to 4,000RPM to get it to start up and run, and even at that it took quite a bit of spinning over before it finally fired up and ran. Once running the 10.2:1 big cam SEM 610 motor ran horribly, with worse lean stumbling than ever. Again I could get it to pull some midrange power sometimes, but it was very unreliable and would barely run. There was just horrible lean stumbling all over the place. Lean stumbling at all engine speeds and lean stumbling at all throttle openings. It was even lean stumbling wide open on the big 176 main jet. Just horrible lean stumbling all over the place. It was able to low idle for a few seconds, but then it stalled and wouldn't restart. I got a tow again, and this time it fired up a bit more quickly but still ran horribly with lean stumbling at all throttle openings.
What was so very interesting was that there hadn't been any lean stumbling from the 12.2:1 hot rod 610 motor on the same gasoline. Same gasoline, same jetting, same 98mm bore by 76.5mm stroke length 577cc Husqvarna 610 motor. Same everything, but the 12.2:1 hot rod 610 motor wasn't lean stumbling at all and the 10.2:1 big cam SEM 610 motor was lean stumbling so bad that it would hardly run and wouldn't start. Both motors had 176 size main jets and both motors had the needle clip in the first groove on the stock K32 needle. If anything the 12.2:1 hot rod 610 motor would be a bit leaner down low with the smaller 60 size pilot jet. Everywhere above the first little crack of the throttle though both 40mm DellOrto carburetors were jetted identically. In this case the difference wasn't in the jetting, as strange as that may sound. The difference was in the compression ratio and the spark timing.
What was going on was that the weak extremely watered down 30% or 40% ethanol garbage gasoline was just on the edge of causing lean stumbling with that jetting. With the gasoline and jetting right on the cusp of causing lean stumbling it was the compression ratio and the spark timing that was making the difference. When more highly compressed the molecules were closer together. When less compressed in the lower compression ratio engine the molecules were farther apart, and that slight difference was enough to cause lean stumbling on this very severely watered down garbage gasoline.
The higher compression ratio pushes the molecules of gasoline and the molecules of oxygen closer together in the combustion chamber, but the later 19 degree BTDC spark timing also causes the spark to fire into a denser intake charge. The early 33 degree BTDC spark timing on the 10.2:1 big cam SEM 610 motor was firing before the piston had come up as far, so it was firing into a lower density intake charge. Both the lower compression ratio and the earlier spark timing caused the spark plug to fire into a less compressed intake charge, and that made the difference between no lean stumbling and very severe lean stumbling to the point where the engine would barely run.
So that was very interesting to figure out why the 12.2:1 hot rod 610 motor hadn't been lean stumbling on that same gasoline that the 10.2:1 big cam 610 motor had been stumbling severely on, but I still didn't have any gasoline.
Next I tried my bone stock 10.2:1 Czech Republic CDI 610 motor, and it was the same story of missing gasoline. The 10.2:1 Czech Republic CDI 610 motor fired up and ran well on the gasoline in the carburetor bowel. It was able to low idle right away, and when I rode off there was nice throttle response at small throttle openings and the torque built substantially from 3,000 to 5,000RPM. There was essentially no surging, and the torque was fairly good across the middle at 3,500 and 4,500RPM. Power was strong from 6,000 to 8,000RPM without any cutting out, and the power actually seemed to be increasing a bit from 7,200 to 7,900RPM. Fairly strong power for the bone stock 610 motor. That was on the gasoline in the carburetor bowel. After riding around for a few miles severe surging set in and torque dropped off sharply bellow 5,000RPM. There was still some top end power, but it felt very narrow and unpredictable. The bone stock 10.2:1 Czech Republic CD 610 motor was still able to rev to 7,900RPM, but it felt flat and lifeless up at higher engine speeds. Power seemed to be dropping off above 7,000RPM and it felt unwilling to rev up. Worst of all was the surging. Very severe surging across a wide range of engine speeds that made the power delivery feel unpredictable and hard to control. And down at 3,000 and 4,000RPM there simply wasn't as much torque as there had been minutes earlier on the more powerful gasoline.
Less torque, narrower power, much worse surging and on top of it all the throttle response at small throttle openings deteriorated. It seemed to be needing wide throttle openings up on the big 176 size main jet to make power, and down at 1/4 and 1/3 throttle openings it felt very weak and unresponsive even though there wasn't any lean stumbling at all.
I tried two quarts of this gasoline in the 10.2:1 big cam SEM 610 motor, and it still wouldn't start. It wouldn't start with the kick starter, and it wouldn't' start when rolled down a hill. I got it to fire a little bit, but it would only run for a very short half second at most before dying.
On another day I tried my 11:1 but otherwise stock SEM ignition 610 motor, and I was glad to find that it fired right up fairly easily on the third kick and ran without difficulty at an ambient temperature of 60 degrees Fahrenheit. It ran, but it was surging very bad. Just tons of surging over a very wide range of engine speeds, and then top end power was unimpressive and a bit reluctant. The energy density of the gasoline didn't seem all that low as throttle response was still fairly good at all smaller throttle openings across the lowest engine speeds and there was hardly any popping out the exhaust on deceleration. It was just a low temperature of combustion potential of this gasoilne that was causing the bad surging, somewhat of a lack of torque at 2,500 to 4,500RPM and a lack of top end power. At least it ran without lean stumbling and without cutting out and it was able to restart very easily and reliably on the first or second kick every time.
I took two quarts of this gasoline out of the 1990 Husqvarna WMX 610 with the 11:1 SEM ignition 610 motor and I put it in the empty tank on my period correct 1991 Husqvarna WMX 610 with the 10.2:1 big cam SEM ignition 610 motor. Again I drained and rinsed the carburetor bowel, and again the 10.2:1 big cam SEM 610 motor would fire but it wouldn't start up and run. When roll started the 10.2:1 big cam SEM ignition 610 motor did fire and accelerate, but only for a short half second before stalling. That was it, just one little pull and as soon as the engine stumbled the plug fouled and the engine died. At this point it was seeming like the spark plugs I had were hopelessly fouled, and the gasoline wasn't working either.
Then I picked up two gallons of 91 (RON+MON)/2 octane rating premium gasoline from a busy gas station and took it straight home with no stops. I drained the tank on the period correct 1991 Husqvarna WMX 610, and I even rinsed the tank through with a few ounces of the fresh premium gasoline. I drained the carburetor bowel and rinsed it through with a little bit of the fresh premium. When I rolled the bike it fired up and seemed like it was going to run. It did run and pull for a few hundred feet, but then it died and wouldn't restart. I got it to fire a few more times, but it only ran for a brief half second before stalling.
Before I went to the gas station that day I had also tried to buy a new spark plug for the 10.2:1 big cam SEM 610 motor, but this didn't work out either. The local dirt bike shop didn't have any NGK C7E or C8E spark plugs. Instead they sold me a CR7E resistor plug insisting that it would work just as well. Even after I explained that my bike had a 5KOhm resistor cap and took a normal C7E spark plug the two guys at the counter continued to insist that the CR7E resistor plug was a fully compatible replacement for a C7E spark plug. Wrong! I got the engine to fire and run for a second on the old C7E, but it wouldn't fire at all on the CR7E. Not even one little pop.
On a powerful ignition system the combined 5KOhm resistance of the resistor cap and the 5KOhm resistance of the resistor plug adding up to 10KOhms of total resistance wouldn't be a problem. Automotive engines have long used 8,000 to 12,000KOhm resistor wires with little difficulty. If there is enough voltage from the ignition system then 10KOhms of resistance isn't any kind of a problem. On the weak spark Swedish SEM Husqvarna CDI ignition system though that little bit of extra resistance just wouldn't work at all.
The spark plug was a dead end anyway. This particular stock 1991 Husqvarna WMX 610 Swedish SEM ignition system was firing and the engine was running. The problem was that the gasoline was so severely watered down that the engine would hardly run. The weak spark stock SEM ignition was very annoying to be sure, but it wasn't what was preventing the engine from running this time.
Wrong spark plug, wrong gasoline; dead Husky.
That was the end of the daylight for the day, so I didn't get a chance to tow start the 10.2:1 big cam SEM 610 motor on the fresh gasoline. Instead I took some of it out of the tank and put it in the nearly empty tank on my 72cc Jin Ching scooter. The 72cc Jin Ching scooter was then hard to start and ran very poorly with tons of lean stumbling at all throttle openings. It was lean stumbling very bad at small throttle openings, and it was even lean stumbling wide open on the oversized main jet at first when it was still cold. Once fully warmed up from riding around a bit the lean stumbling wide open on the big (stock) main jet was gone. It was able to make some power wide open, but at all smaller throttle openings there was tons of lean stumbling and a near total lack of torque. I went up from the first needle clip position to the second needle clip position, and this got the torque to come back. There was still some little bits of lean stumbling at very small throttle openings on the stock pilot jet, but for the most part it was running and pulling. The torque wasn't quite as strong as it usually is from the 72cc Jin Ching scooter, but it certainly was running and pulling without difficulty.
The next day the 10.2:1 big cam SEM 610 motor still wouldn't start. I got it to fire a little bit both with the kick starter and when rolling it down a little hill, but it wouldn't fire up and run. My old spark plugs were seeming to be fairly severely fouled by this point, and nothing was working.
Then I happened to fire the 72cc Jin Ching scooter up, and it ran without lean stumbling. Right from the first seconds it ran consistently and strongly. When I had gone 400 feet or so I noticed it was drastically too rich around 3/4 throttle. I put the needle clip back to the first (leanest) groove, and the 72cc engine pulled stronger at all throttle openings from 1/4 up to 3/4 throttle opening. More torque, and still no lean stumbling at any throttle openings. The gasoline had come back just as mysteriously as it had left. This had all been at very similar ambient temperatures in the 50 to 60 degree Fahrenheit range, so it wasn't anything to do with the ambient temperature. The gasoline had simply disappeared and then reappeared, very mysteriously.
Wherever it had gone and however it had come back the more normal energy density gasoline did get the 10.2:1 big cam SEM 610 motor running again, although not xactly easily without a new spark plug to instal. Even with the more normal gasoline I still had some difficulty in getting the weak spark SEM ignition 610 motor going again. I drained the tank, drained the carburetor bowel and rinsed both through with a bit of the more normal energy density gasoline, but the 10.2:1 big cam SEM 610 motor would barely fire at all. It would fire for just a short half second and stall.
As I had on previous days I tried various idle mixture screw settings, but nothing got it to continue to run. Then I tried backing off on the idle stop, and this at first didn't get it to fire up and run either. Then finally it fired up and ran. Not only did it run, but it ran without any lean stumbling and it was able to restart very easily on the first kick.
No lean stumbling at any engine speeds and no lean stumbling at any throttle openings. Not all was perfect though, there was tons of surging and the 10.2:1 big cam SEM 610 motor was horrendously loud. Loud and lean sounding. A hollow, raspy lean sound, and very very loud. It did make some power, but with a narrow and peaky unpredictable feel. The surging was so severe that torque was lower than usual around 3,000 to 5,000RPM, and then it seemed unwilling to rev out even with the big camshaft. It wasn't cutting out, instead it just went flat and sounded very bad up at the highest engine speeds. Lots of horrendous loud screaming on the top end, but not much power. At least it was running and starting, that's something anyway.
The next day the 10.2:1 big cam SEM 610 motor fired up very easily on the second kick. One small kick with the choke on, and then on the next kick without the choke it fired up and ran. And, wonder of wonders, the surging was nearly totally gone. Essentially no surging, and there was more torque also. Particularly around 4,500 to 5,500RPM the torque was much stronger. Top end power was also stronger, and the big cam engine revved out willingly. More torque, broader power and the 10.2:1 big cam SEM 610 motor was quieter also. Considerably more powerful gasoline. Higher temperature of combustion potential gasoline to be specific. The energy density was similar, but the temperature of combustion potential was substantially higher which eliminated the surging and delivered stronger and broader torque. It still wasn't exactly normal gasoline though. There still wasn't much in the way of 3,000 to 5,000RPM harshness from the huge 33 degree BTDC spark timing, and it was still giving a substantial loud scream with very early times of late compression ignition up at the top of the power band. The 10.2:1 big cam SEM 610 motor still restarted very easily on the first kick every time and ran without any lean stumbling, or at least with hardly any lean stumbling. There were a few times I noticed a slight bit of instability for a split second around 2,000RPM. It was hardly anything, and it seemed like just a bit of instability caused by the large amount of overlap on the big camshaft.
Then the next day it was back to surging and narrower power, but still a reasonably normal energy density and easy starting. This time without any hint of any lean stumbling anywhere at any engine speeds, and again the 10.2:1 big cam SEM 610 motor fired right up very easily with the kick starter. Again it was on the second kick when cold at a 53 degree Fahrenheit ambient temperature, and then it restarted very easily mostly on the first kick. One time when I stalled the motor it took two kicks, and another time I gave it a weak small kick and it didn't fire but did fire right up on the second kick. For the most part it was firing very easily on the first kick. Often even on a rather small first kick it just fired right up very easily.
Lots of surging seemed to cause the 10.2:1 big cam SEM 610 motor to be louder. It was loudest when the power was narrow with lots of surging, and it wasn't quite as loud when the power was stronger and broader with little or no surging. What was always true though was that the Uptite Racing brand single muffler was always very loud. Too loud really. So loud that it couldn't reasonably pass any sort of sound test. Illegal sort of loud, and annoying loud also. I decided to install a dual inline muffler system. The bike had come with the stock exhaust system in a box, so I put it back on with a Supertrapp secondary muffler.
I made the whole swap in one day after a test ride in the morning so that I could compare the two exhaust systems on the same gasoline. What was most striking when I first fired up the 10.2:1 big cam SEM 610 motor on the dual inline muffler system was how drastically quieter it was than on the Uptite Racing brand exhaust system. Wow, what a difference a muffler makes. Power and performance seemed very much the same. The Uptite Racing muffler had 12 diffuser disks in it, and when I first installed a Supertrapp secondary muffler on the stock exhaust system on the period correct 1991 Husqvarna WMX 610 I only had 11 diffuser disks. Adding the second muffler and running one fewer diffuser disks did seem to noticeably choke off the top end flow. I then took apart the Uptite Racing muffler and put three of the diffuser disks in the Supertrapp muffler on the bike for a total of 14 disks. This got the flow all the way back. In fact it seemed like it might have even been flowing a bit better than it had with the Uptite Racing brand exhaust system. The big camshaft on the small stock 1991 30mm exhaust valves seemed to favor more diffuser disks for a very free flowing exhaust system. The big camshaft allows for substantial power up above 7,500RPM, but then the 30mm 1991 exhaust valves are on the small side. Keeping the exhaust system very free flowing helps overcome the limitations of small exhaust valves.
I have been calling this bike my period correct 1991 Husqvarna WMX 610, but I have now stripped off quite a few of the unique features that led me to call it "period correct". The air box cover with the giant holes in it is gone, the stock intake screen is back in the air boot and the Uptite Racing exhaust system has now been replaced with my dual inline muffler system based on the stock 1991 Husqvarna WMX 610 pipes and primary muffler.
What is left to differentiate this bike from my other 1991 Husqvarna WMX 610 bikes? Pretty much just the big camshaft at this point. The big camshaft and the stock 33 degree BTDC spark timing. They go together, but I am still frequently tempted to pull that flywheel off and set the spark timing to something more reasonable. I might still do that one of these days, but I suspect that the next modification I would be making would be to advance the cam timing by two or three degrees of crankshaft rotation. Perhaps backing off to just 29 or 30 degree BTDC spark timing would allow the big camshaft to stay installed straight up with split overlap right at top dead center. It is in fact rather impressive the way that the big camshaft can overcome the small 35mm/30mm stock 1991 Husqvarna 610 valves and heavy stock 406g Mahle piston to deliver rather strong top end yank up to high engine speeds.
It is also interesting that the removed intake screen and giant holes in the air box cover weren't necessary. It's the big camshaft that delivers more top end power. With the air box back to stock and a second muffler installed the 10.2:1 big cam SEM 610 motor seems to have very similar flow capability up to high engine speeds. If anything the stock 1991 Husqvarna exhaust system based dual muffler setup seems to work a bit better than the Uptite Racing brand exhaust system. Perhaps that large megaphone type muffler just wasn't a good idea. Mega noise, but perhaps not any sort of real gains in flow capability. Since both muffler systems used the diffuser disk type spark arrestor a good direct comparison was possible. Adding as park arrestor does cut into flow a little bit, although with enough disks the Supertrapp spark arrestor is a minimal hindrance to flow capability and performance. Still though adding any type of spark arrestor does tend to choke off flow at least a small amount. Since both of the exhaust systems used the same type of spark arrestor the comparison was of the exhaust systems themselves, and not a spark arrestor comparison.
First of all it has to be said that Cagiva wasn't wrong in stipulating that the range of possible main jet sizes for the 40mm DellOrto carburetors is 170 to 180. That is the actual range of workable 40mm DellOrto main jet sizes for all types of alcohol free gasoline. What Cagiva was wrong about was in specifying a 170 size main jet for the 36mm DellOrto carburetors on Husqvarna 410 motors. Clearly that is way too big, much richer than a 180 main jet in a 40mm DellOrto. What Cagiva was also wrong about was specifiying a 138 size main jet for the later years of 34mm DellOrto equipped Husqvarna 350 motors. That's cearly way too small. The stock 145 main jet in the 34mm DellOrtos on the 1991 Husqvarna WXE 350 bikes was already on the small side, so going down to a 138 size main jet in the same 34mm DellOrto just isn't reasonable at all.
The range of workable main jet sizes for the 40mm DellOrto carburetors really is 170 to 180. The 180 size that Cagiva specified for the 1990/1991 Husqvarna WMX 610 is quite large. That is probably the maximum main jet size for maximum power output at sea level when the engine is running very crisply. The 180 main jet size is probably what makes peak top end power at sea level when running cheap fast flame front travel speed premium gasoline. It really is a very large main jet, and it usually blackens the exhaust even at low elevations. Why so rich? Simply because a three inch stroke length engine spinning at 6,000 to 8,000RPM is a very high mean piston speed for normal cheap types of gasoline to support. With a long stroke length and high mean piston speeds cheap gasoline needs to run rather rich to make power. Race gas can run leaner and still support higher mean piston speeds, but race gas is expensive and probably isn't what is supposed to be comming out of the pumps. The 1991 Husqvarna WMX 610 came stock with a main jet sized for absolute maximum power output on cheap premium gasoline.
The reality is that even for extremely cheap fast flame front travel speed gasoline the 175 size main jet works better for most purposes. The 175 main jet in the 40mm DellOrto carburetors is still on the rich side, but not quite so ridiculously rich as the 180 size main jet. The 175 size main jet is a better choice for efficiency and cleanliness at low elevations, and it is a much better choice for peak performance at higher elevations than the 180 size main jet. That's why the Husqvarna 610 motors came with 175 size main jets in later years. Overall the 175 size main jet is simply better, and that is true on all types of gasoline that don't contain large quantities of alcohol.
What about the 170 size main jet that was stock in the 40mm DellOrto carburetors on certain later years of Husqvarna 610 motors? That's way on the small side, but certainly reasonable from certain perspectives. Particularly for clean operation at higher elevations the 170 size main jet is probably a good choice, that's why Cagiva went down that extremely small. The 180 was too rich for most practical purposes, so they sort of overcorrected way down to a 170 size for certain later models. The difference between a 170 size main jet and a 180 size main jet is actually very substantial. It's more than a 12% higher fuel flow rate at the same intake air flow rate at wide open throttle. That's a big difference. As far as amounts of fuel versus available intake air that is something like the difference between 100% gasoline or a 70%/30% mixture of the same gasoline with ethanol. Yes, a very big difference.
Why such a wide range of jetting sizes for the same 577cc motor with the same 40mm DellOrto carburetor? Simply because absolute peak power output at sea level on cheap fast flame front travel speed premium gasoline comes with rather rich mixture ratios. So rich that the exhaust gets somewhat blackened, and so rich that the engine actually doesn't run as well as it does with a somewhat smaller main jet size. Peak power output on big engines requires dumping lots of gasoline in, but for most purposes slightly leaner jetting works a whole lot better.
So the stock 180 size main jet really was too big for most purposes. To go along with that overly large main jet was the factory stock specification of the third needle clip position on the K32 needle. That also is simply wrong. Just as wrong as the 170 size main jet in the 36mm DellOrtos on the Husqvarna 410 motors. The reality is that with the stock 180 size main jet the second needle clip position on the stock K32 needle is a lot better for all purposes. Why then did Cagiva specify the third groove as stock? Simply because it is a rather large change in jetting from the second to the third groove. The third groove is way too fat, but going all the way down to the second groove just seems like a very large change. Why exactly they picked the 3rd groove as stock is somewhat of a mystery, but what is clear is that the 2nd groove is much more appropriate for the 180 size main jet. With the needle clip in the 2nd position the mixture still richens up slightly as the throttle is opened from 1/2 to wide open on the stock 180 size main jet, but it is a very slight richening of the mixture. That is just right for most purposes, but some tuners prefer the mixture to be every bit as rich at 1/2 throttle as at wide open throttle.
A more reasonable stock setup would have been the second groove on the K32 needle and a 175 or 178 size main jet. That is probably what Cagiva would have picked back in 1990 if they had more time for development. It seems that the new model was severely rushed for some reason though, and some things didn't get sorted out. Instead of fixing the jetting for the 1991 model year full release of the WMX 610 though they just stuck with what they had hastily picked earlier for the ill-fated 1990 Husqvarna WMX 610 model. That was a mistake, but there were other worse problems that should have been fixed also. Like the weak spark CDI ignition and the woefully incorrect specification of 10W oil in the 40mm USD White Power forks that were actually valved for 5W oil.
That second needle clip position on the stock K32 needle and a 178 size main jet is still very rich jetting. Ridiculously rich jetting for most purposes, especially when either running at high altitude or when running higher energy density slower flame front travel speed types of midgrade or regular gasoline.
The stock K32 needle in the stock 264 size needle jet gives another perspective on lean versus rich jetting. The 2.48" diameter shank of the stock K32 needle in the stock 264 size needle jet results in a mixture ratio at 1/8 and 1/5 throttle that is considerably leaner that what is delivered at 1/4 and 1/3 throttle with the needle clip in the second groove. The 40mm DellOrto carburetors on the 1991 Husqvarna WMX 610 model were specified as having a stock 265 size needle jet (atomizer in DellOrto parlance). The thing is though that the holes in these 265 marked needle jets actually measure more like 2.64mm in diameter. DellOrto seems to have been tweaking the jetting leaner back in 1991, and then in later years Cagiva went ahead and specified the 264 size needle jet as stock. Both the stock 265 marked needle jets from 1991 and the later true 264 size needle jets work very well, and it isn't exactly an extremely lean mixture at 1/8 and 1/5 throttle openings. It is in fact still a very slight bit richer than what is delivered at 1/4 and 1/3 throttle openings with the needle clip in the first groove on the stock K32 needle. Those stock needle jets work with either the first of second needle clip positions on the stock K32 needle, but they are closer to matched to the first groove needle clip position. With the needle clip in the second position the mixture does richen up noticeably from 1/5 to 1/4 throttle, and that is probably why someone drilled out the 264 marked needle net to about the 266 size on my 1997 Husqvarna TE 610 motor. They were running the 4th (richest) needle clip position with a 190 size main jet, so clearly the jetting was for a very low energy density alcohol mixture.
The stock 264 size needle jet and the 2.48mm diameter K32 needle delivers a mixture ratio that usually works very well, although it is certainly still somewhat rich compared to the jetting that is sometimes found in street bikes and small Japanese dirt bikes. Clearly this stock 264 size needle jet is matched to the first or second needle clip position on the stock K32 needle, not the supposedly stock but ridiculously overly rich 3rd position.
There is more to the jetting intrigue on the 40mm DellOrto carburetors also. The stock K32 needle provides more even mixture control with the clip at the second groove. With the clip down at the first groove the mixture richens up perhaps a bit too aggressively as the throttle is opened from 1/4 to 1/2 throttle openings. It's too rich at the second groove, but the mixture control is more even at the second groove than at the first groove. Again though this has something to do with running cheap fast flame front travel speed premium that needs very rich mixtures to support high mean piston speeds. The cheap fast flame front travel speed premium needs somewhat overly rich mixtures as the engine speed increases on the three inch stroke length engine. A big engine does benefit from the mixture richening up slightly as the throttle is opened, so from that perspective the stock K32 needle in the 40mm DellOrto carburetors might be very appropriate.
It is beginning too look like all normal types of gasoline that don't contain large quantities of alcohol do work better with the needle clip in the first groove on the stock K32 needle in the 40mm DellOrto versus in the second groove. It is easy to call that too lean of a mixture at 1/4 and 1/3 throttle openings with the needle clip in the first position, but only by comparison to the drastically overly rich supposedly stock 3rd groove needle clip position. Ultimately it depends on the style of tuning and the goals of the tuner. Dumping in more gasoline with the needle clip in the second position can very easily seem better on the long three inch stroke length 577cc monster of a dirt bike engine. If the goal is avoiding even the slightest hint of a lean feeling under all conditions even down at sea level then dumping in extra gasoline with the needle clip in the second groove seems to make sense.
The reality though is that the 40mm DellOrto is in fact too rich at 1/4 and 1/3 throttle openings with the needle clip in the second groove on the stock K32 needle. So rich that the exhaust gets somewhat blackened even when running at small throttle openings, and perhaps even more importantly so rich that it causes extra lag and a more choppy response from the engine.
The harsh reality is that it probably is in fact large quantities of ethanol or other expensive specialty low energy density additives that makes the 40mm DellOrto seem too lean at 1/4 and 1/3 throttle openings with the needle clip in the first groove. On what would be considered normal types of gasoline there isn't any excess lean feeling with the needle clip in the first groove. My guess is that the first groove delivers the ideal clean and efficient mixture at 1/4 and 1/3 throttle openings on cheap fast flame front travel speed premium gasoline. On higher energy density slower flame front travel speed types of midgrade and regular gasoline the mixture is probably a bit too rich at 1/4 and 1/3 throttle openings even with the clip in the first groove on the stock K32 needle. That is just another one of those harsh realities. Carburetors have generally been jetted overly rich, and sometimes there is little that can be done about this problem. The recompense here is that the 40mm DellOrto never seems too rich to make torque at 1/4 and 1/3 throttle openings with the needle clip in the first groove. It might be a bit too rich for ideal clean and efficient operation, but it's not so extremely overly rich that it won't make good torque. On slow flame front travel speed regular with no alcohol the Husqvarna 610 might run a bit overly rich at 1/4 and 1/3 throttle openings with the needle in the first groove, but not so rich that torque increases if 10% ethanol is added to the gasoline.
It seems that there are only two reasons to go up to the second needle clip position on the 40mm DellOrto with the stock K32 needle. One is alcohol in the gasoline. With 20% or 30% ethanol in the gasoline it is necessary to run fatter jetting to avoid an overly lean feeling to the power delivery. The other reason to go richer on the needle clip position is just to even out the mixture control. Some people strongly believe that the mixture should not richen up noticeably as the throttle is opened, and that can require running the clip in the second groove on the stock K32 needle in the 40mm DellOrto. For me there is a third reason that I still have the needle clip in the same second groove on the Husqvarna 610 motor that I have had since the 1990's (the one that is now in my 1992 Husqvarna chassis). That reason is simply that this particular 1991 Husqvarna WMX 610 motor has never had the needle clip in the first groove. It has (nearly) always been overly rich, but it does work. It runs, it pulls and it doesn't usually give any trouble at low elevations other than a blackened exhaust system, extra lag and a more abrupt hit to the power at around 2,900 or 3,300RPM. At high elevations that overly rich mixture is another story. I just don't take that bike up to high elevations anymore. The 1992 suspension sucks anyway, so why would I chose to ride that bike instead of one of my much better 1991 Husqvarna WMX 610 bikes?
For running rich and making peak torque at sea level the second needle clip position makes sense. A 175 main jet can be run with the needle clip in the second position, but a 178 or 180 main jet may make a bit more peak power output at sea level. The mixture may actually lean out very slightly from 1/2 to wide open throttle with a 175 main jet and the needle clip in the second position, so a 178 or 180 main jet seems to make sense for delivering absolute maximum power output at sea level.
For lower fuel consumption and better performance over a wide range of altitudes the first needle clip position on the stock K32 needle is clearly a better choice. The main jet size to go with the first needle clip position is 175 or smaller. Going up to a 178 size main jet with the clip in the first groove the mixture richens up excessively from 2/3 to wide open throttle. A 178 size main jet can be run with the needle clip in the first groove, but it is not really a good match. With a 175 or 176 size main jet the mixture only richens up very slightly as the throttle is opened from 2/3 to wide open, and this works well. Going down a bit smaller than 175 might also make sense if an even mixture across all wider throttle openings was desired. The problem is that the jet sizes jump all the way down to 172, and that's probably really a bit too small. An even mixture from 2/3 to wide open throttle would probably be with about a 173 or 174 size main jet, but a 172 might also sort of work. It seems that a 170 size main jet would be too small for the stock K32 needle, as the mixture would then lean out substantially from 2/3 throttle up to wide open throttle. In my opinion it's not a good idea for the mixture to lean out at all as the throttle is opened past 1/2 and 3/4 throttle openings.
Then there is the pilot jet. The idle mixture screw draws from the pilot jet circuit so the pilot jet does have some small effect on low idle mixture. Mostly what the pilot jet does though is feed the transition port that controls the mixture from the first little crack of the throttle up to about 1/8 throttle openings. It seems like it is mostly the very first little crack of the throttle that the pilot jet strongly controls, and as soon as the throttle is open more substantially the needle shank and needle jet become very important in controlling the mixture. There is of course overlap, and the pilot jet does have some small effect on mixture control up to and even beyond 1/8 throttle openings. As the throttle is opened more and more the pilot jet is less and less significant. All of the circuits overlap like that. The main jet affects mixture ratios down as small as 1/2 throttle, but it is mostly up above 3/4 throttle openings that the main jet controls the mixture. The only type of adjustment that really has a very hard cutoff is the needle clip position. Bellow about 1/5 throttle the needle clip position has no effect at all on mixture control. Just exactly how far down the needle clip position affects mixture depends on the needle clip position. The more the needle is lifted by the needle clip the earlier the carburetor transitions from the straight shank of the needle to the tapered portion of the needle. While the straight shank of the needle is fully covering the needle jet though it matters not in the slightest where the needle clip is located. This results in a hard cutoff down around 1/5 throttle, below which the mixture is not controlled even slightly by the needle clip position.
Down there at small throttle openings below about 1/5 throttle it is only the needle jet and the pilot jet that control the mixture. The needle jet is mostly up at 1/8 to 1/5 throttle, and the pilot jet is mostly down at the very first crack of the throttle. In between around 1/8 throttle and somewhat bellow both the pilot jet and the needle jet are significant. At some point in this overlap the pilot jet and the needle jet could be said to be equally significant, with neither having a stronger effect on mixture than the other. That point of equal significance is probably at or slightly below 1/8 throttle on the 40mm DellOrto carburetors.
The stock 62 size pilot jet in the 40mm DellOrto is certainly on the rich side. That stock 62 pilot jet is in fact so rich that it seems well matched to running the needle clip in the second groove on the stock K32 needle with a stock 180 main jet. The 62 pilot jet does deliver a bit leaner mixture at the first crack of the throttle than what is delivered at 1/4 and 1/3 throttle openings with the needle clip in the second groove, but not to an extreme. The stock 62 pilot jet usually seems to work just fine with the needle clip in the second position, and the reason for this is again that it is the big 3" stroke length that requires rich mixtures at wider throttle openings. Way down at 1,500 and 2,000RPM at the first crack of the throttle the mixture can be leaner and work just fine.
The stock 62 pilot jet is in fact on the rich side though as evidenced by the normal cold starting and cold running performance of the Husqvarna 610 motors. On that rather large 62 pilot jet there has (almost) never been any lean stumbling even when the engine first fires up cold. It's rich enough that the engine just fires up and runs strong right from the first two seconds.
I knew back in 2015 that the stock 62 size pilot jet was on the large size, so I figured that the 60 size pilot jet would be able to at least sort of work. I was correct about that. The 60 size pilot jet is in fact big enough for the 40mm DellOrto on the Husqvarna 610 motor. Again there normally isn't any lean stumbling when the motor first fires up cold, although the 60 size pilot jet can also very easily seem too small. On weak watered down gasoline the 60 size pilot jet is sometimes noticeably too lean to the point where the engine will hardly run at the first crack of the throttle when first fired up. It seems to take more than 20% ethanol in the gasoline to cause really severe problems even with the 60 size pilot jet, which is sort of unexpected and ridiculous. That means that the 60 size pilot jet is in fact still on the rich side, which in turn means that the stock 62 size pilot jet is actually quite ridiculously overly rich. The difference between 60 and 62 might not seem like much, but it is in fact more than a 7% difference in fuel flow rates at the intake air flow rate at the same small crack of the throttle. That's at the same mass air flow rate, not a volumetric air flow rate by the way.
A 7% higher fuel flow rate is substantial. That's like the difference between 100% gasoline and the same gasoline watered down with about 15% ethanol. Could the pilot jet go smaller than the 60 size on the 40mm DellOrto? Probably; but not with the stock K32 needle. The 60 pilot jet is already substantially leaner than what the first clip position on the stock K32 needle delivers at 1/4 and 1/3 throttle openings. With the 60 size pilot jet the mixture certainly does tend to richen up substantially as the throttle is opened from 1/8 to 1/4 even with the stock 264 size needle jet and the clip in the first position on the stock K32 needle.
The 61 size pilot jet actually seems best matched to the stock K32 needle. The 62 size pilot jet is great for running in the second needle clip position, but the 61 size is noticeably better for running in the first needle clip position. Anything bigger than the stock 62 pilot jet is just extremely ridiculous. A 64 size pilot jet in the 40mm DellOrto is not for gasoline. A 64 size pilot jet in the 40mm DellOrto is only for alcohol mixtures. Like 30 to 50% ethanol, which is not something that would tend to be a normal type of motor vehicle fuel.
The general conclusions are that the 40mm DellOrto carburetors came jetted too rich with the 180 size main jet and 62 size pilot jet, but not so extremely overly rich that the Husqvarna 610 motor wouldn't still run and make big power on normal types of gasoline. Very rich, but not so rich as to preclude torque and power production.
Different types of gasoline have different response characteristics in terms of how they flow through various size orifices. Even just changing the temperature of the liquid gasoline results in some small changes in mechanical properties. That means that carburetors are never perfectly consistent, and very precise jetting only remains very precise on exactly the same gasoline run under approximately the same operating conditions. The general conclusion that can be drawn from this particular reality is that it is ultimately impossible to delineate exactly what jetting works best for a certain carburetor on a certain motor if there is any inconsistency at all in the gasoline supply.
Small inconsistencies in the gasoline supply and slight changes in mechanical properties over a range of ambient temperatures are however only small problems. The difference, on the other hand, between 100% gasoline versus 20% ethanol mixed with 80% gasoline is a whole different level of inconsistency. And 30% ethanol with 70% gasoline is obviously even farther from compatible with 100% gasoline.
The traditional thinking on carburetor jetting seems to have been that it has to be able to at least sort of run on 10% ethanol and 90% gasoline, and this is reflected in the current situation of permitting up to a 10% ethanol content in gasoline sold as motor vehicle fuel. The harsh reality though is that many stock carburetors have in fact been jetted to run on about 15 or 20% ethanol mixtures, and this then results in drastically overly rich mixtures when running on 100% gasoline. Running a bit rich is fine from a performance perspective, but it does come with compromises. An overly rich mixture first and foremost means higher fuel consumption and dirtier exhaust emissions. An overly rich mixture also results in worse performance over a range of altitudes. Jetting that results in somewhat overly rich operation at sea level then results in much more drastically overly rich operation up at 5,000 feet of elevation. Carbureted engines actually do run a bit richer at higher elevations on the same jetting, but overly rich operation also tends to cause more severe problems at higher elevations. With less air in the combustion chamber it is harder to get an overly rich running engine to clear out, so it may be much more prone to "loading up" and stalling. A carburetor that is jetted just right for sea level tends to do fine up to much higher elevations. A carburetor that is already jetted too rich at sea level though tends to cause much more severe problems at high altitudes.
It is also possible to jet a carburetor to be somewhat lean at sea level so that it richens up towards an ideal power production mixture ratio at higher elevations, which would work best for carbureted engines that actually are regularly operated over a wide range of elevations. There is a rather substantial range of mixture ratios between lean stumbling and flooding, so jetting carburetors to work over a wide range of elevations is in fact possible.
Generalities about exact jetting are impossible, but some approximations are possible. It seems to me that the range of jetting options for the 40mm DellOrto carburetors on the Husqvarna 610 motors runs from a 60 size pilot jet, a 175 main jet and the needle clip in the leanest groove up to a stock 62 size pilot jet and the needle clip in the second groove with a 178 or 180 main jet.
What are the third and fourth needle clip positions for then? Mostly they are useless of course. The fourth needle clip position on the stock K32 needle in the 40mm DellOrto is so rich that it will actually run on 100% ethanol. It's still lean on 100% ethanol, but it will run and make some power. The fourth needle clip position is probably more realistically for E85, which has apparently been a popular fuel in some areas. The third needle clip position on the stock K32 needle would then be for some sort of a half and half mixture of gasoline and alcohol. If the fourth groove is somewhat too lean on 100% ethanol and the first groove is very slightly rich on 100% gasoline then it might be expected that the third groove would be for about 50 or 60% ethanol mixed with 40 or 50% gasoline. The reality though is that moving the needle clip position has more of an effect on mixture ratios down at 1/4 to 1/3 throttle and less of an effect on 1/2 and 3/4 throttle openings. The other reality is that going from the first to second needle clip positions has a larger effect on overall mixture than going from the second to the third needle clip positions. What this means is that although the fourth needle clip position appears to be for 100% ethanol or E85 the third groove might be ideal for making power on anywhere from 40% ethanol with 60% gasoline to 60% ethanol with 40% gasoline. At 70% ethanol with 30% gasoline the fourth needle clip position is probably still going to work best, although this is entirely ridiculous because there is no good reason why anyone would want to run 70% alcohol and 30% gasoline. Gasoline and alcohol are best run separately, not mixed together. If gasoline and alcohol are mixed it makes much more sense to use one or the other as an additive in small quantities instead of half and half. Again 10% ethanol added to gasoline doesn't really accomplish much of anything beneficial, but it doesn't cause anywhere near as bad problems as 30% ethanol or 50% ethanol.
Of course the 40mm DellOrto only works at it's absolute best on the type of gasoline that it was designed to work with, and that appears to be 100% gasoline or up to about 20% ethanol mixed with 80% gasoline. It is possible to richen the 40mm DellOrto even more to run more alcohol, but the mixture control is not optimized for those large amounts of alcohol.
As far as the pilot jets go there is more wiggle room, so to speak. The stock 62 pilot jet will sort of work with up to about 20% ethanol, but it really isn't all that extremely overly rich. To run 40 or 50% ethanol would probably require more like a 64 or 65 size pilot jet. Running 100% ethanol or E85 without the choke on would require an even larger pilot jet. I don't know how big, but probably up around 67 or 69 size. The minimum size pilot jet for 100% gasoline might be way down at the 58 size or so. It does seem that the pilot jet circuits in the 34mm, 36mm and 40mm DellOrto carburetors are very similar in size and they can all run similar pilot jet sizes. A stock 58 size pilot jet does work in the 34mm and 36mm DellOrto carburetors, and even going down to a 56 pilot jet on the 34mm DellOrto might work on 100% gasoline. There is some slight difference in required pilot jet sizes between the 34mm DellOrto and the 40mm DellOrto, but it's not as large a difference as a 58 pilot jet in the 34mm DellOrto and a 62 pilot jet in the 40mm DellOrto. It's more like a 58 pilot jet in the 34mm and a 60 pilot jet in the 40mm DellOrto for similar mixture control at the first crack of the throttle. Overall the best pilot jet sizes I have found so far is a 59 size in the 34mm DellOrto and a 61 size in the 40mm DellOrto. The 60 size pilot jet in the 40mm DellOrto also usually works very well, but it is such a large change from the stock 62 size pilot jet that it tends to be a little bit annoying. The 61 size pilot jet seems to be better in every way compared to the stock 62 size pilot jet in the 40mm DellOrto, so that is the size I would pick if I were buying jets. And yes, the pilot jets are available in 59, 60 and 61 sizes. Just one size change does make a significant difference, although that small difference might not be all that noticeable under normal conditions.