It was of course frequent and prolonged bad surging from the 3.01" stroke length Husqvarna 610 motors throughout 2016 and 2017 that prompted the development of a shorter stroke length version. Keeping the tuning just right on the 3.01" stroke length 610 motors helps alleviate surging, and often it was simply excess crispness and too much spark advance that was causing surging. In the end though it was the fact that only a more appropriate stroke length is capable of truely eliminating surging that clearly pointed to the necessity of going down to less than two and a half inches of stroke length.
The Junk 350
Why go Big?
Big Bore Details
Shorty Big Bore Cylinder
37mm Intake Valves
The 38mm DellOrto Carburetor
Grinding the Cases
Mild Dome Woessner Piston
Installing the 475 Big Bore Motor
Riding the 475 Big Bore Motor
If the stroke length has to be shorter, then the displacement per cylinder is also going to go down, and that tends to mean substantially less power output. If however the bore diameter is kept just as large and good flow capability is provided then reducing the stroke length results in only modest reductions in peak power output. A better way to reduce stroke length is of course to go up to more than one cylinder with smaller bores. Two, three or four small cylinders could work well on a dirt bike, and the increased cylinder count is a huge boon for transmission efficiency. Two, three and four cylinder dirt bikes don't however exist. Dirt bikes just have one cylinder. The obvious compromise for what is already defined as a big powerful single cylinder dirt bike is just to go radically over square. Look at the 100mm bore Beta 480/500. That's the most radically over square dirt bike ever built. The 88mm bore KTM 350 is also extremely radically over square. Those are both finger follower engines, if that coincidence means anything. The cam and bucket 97mm bore Yamaha YZF450F is also extremely radically over square. Pretty much all modern dirt bikes are radically over square, but some slightly more than others.
My idea for a 98mm bore on a 2.48" (63mm) stroke length Husqvarna 350 bottom end goes all the way back to March of 2017 when I saw a Colorado 1991 Husqvarna WXE 350 being parted out on eBay. At first I was interested in the motor just to get the 1991 six speed transmission, as I figured that would be a great upgrade for one of my five speed 610 motors at some time in the future.
I didn't buy the 1991 WXE 350 motor just for the transmission though, instead I bought the plastic, the swing arm, linkage and shock off that Colorado bike. I bought the plastic just to have since it was in fairly nice condition, and the linkage I also bought just for the spare bearings. The swing arm I bought so that I could put a kick stand on my main 1991 Husqvarna WMX 610 (my original bike that I put the 1997 six speed TE 610 motor in back in 2015). See Resurrecting the 1991 Husqvarna WMX 610.
The shock off the Colorado 1991 Husqvarna WXE 350 I bought just so see if it might be any good. It wasn't. As with the rest of the bike the shock looked to be in perfect like new condition, and it wasn't leaking oil. I installed it on my period correct 1991 Husqvarna WMX 610 just to see if it might be better than the suspension shop modified shock (See A Period Correct Big Cam 1991 WMX 610). The bone stock 1991 Husqvarna WXE 350 White Power shock wasn't better than the crappy race shop modified 1991 Husqvarna WMX 610 White Power shock. The stock 1991 Husqvarna WXE 350 White Power shock was actually a noticeably harsher ride under most conditions, and it was also much slower with that same less competent rebound damping as the 1991 Husqvarna WXE 350 shock that I suffered with throughout 2014. Back on went the crappy race shop modified 1991 Husqvarna WMX 610 White Power shock, and the Colorado 1991 WXE 350 White Power shock simply got shelved as another stupid waste of a hundred bucks. But then I came up with another idea about something I could do with the also seemingly junk 1991 Husqvarna WXE 350 motor.
I always had liked the shorter 2.48" stroke length, as it is just so much more appropriate for normal types of gasoline than the long 3.01" stroke length of the 610 motors. The big problem with those early 1990's Husqvarna 350 motors is just that the pistons and rods are ridiculously over weight with the same 313g 127mm connecting rod as the 610 motor and a very portly 270g Mahle piston. It is possible to go somewhat lighter on the 84mm piston, but that huge 610 connecting rod really holds the 350 motor back. Going up to an 85mm XR400 piston and a somewhat longer 2.68" stroke length with the significantly lighter 122.5mm XR400 connecting rod certainly had helped free up the power potential of the 350 motor (See Husqvarnas). The 386 stroker motor was a clear improvement in terms of freeing up power production potential from reduced reciprocating losses, but the longer 2.68" stroke length didn't always get along with cheap types of gasoline. The longer stroke length caused surging, and that wasn't good at all.
The obvious solution is to keep the stroke down at the stock 2.48" length of the 350 motor. That's also the stroke length of the modern 450F dirt bikes, and it generally seems just barely short enough to be able to run well on normal cheap types of gasoline. The 1984 through 2004 Husqvarna four stroke motor is however a big engine with parts sized for quite a lot of displacement. Everything about the Husqvarna 510/610 motor platform is designed for very large amounts of displacement. It's not only the sizing of the main bearings, rod bearing, transmission bearings and transmission gears, it's also the actual layout of the engine. The valves are spaced for large bore diameters of around 96 to 98mm, so running small bore diameters on that engine platform is inherently problematic. Yes, it can go down to an 84mm bore, but then the valves are small and pushed way off to the sides of the combustion chamber where flow is rather severely blocked. It's just not a good idea to use the wrong valve spacing.
So there are two separate problems with the early 1990's Husqvarna 350 motors; the reciprocating mass is way too heavy, and the layout of the 510/610 motor platform is for much larger bore diameters. Two problems, and one solution. The idea I came up with was just to slap a giant bore on the 350 motor. When I ran the numbers for compression ratios it seemed to work out to simply stick a 610 piston in the 350 motor with a 610 cylinder head for a 472.5cc displacement.
I also ran the numbers for a 95mm piston, and that required a compression height that simply wasn't available. There are lots of 95mm pistons, as that has been a popular bore diameter for 450F dirt bikes. The Japanese 450F bikes are all up at 96mm and even 97mm (Yamaha) these days, but back a few years there were 95mm bore Honda 450's. In the KTM model lineup the 95mm bore has been ubiquitous, with all of the KTM 450 motors as well as all of the KTM 510cc motors (520, 525, 535, 500) using the 95mm bore diameter. Then there are also the 94.5mm bore Kawasaki KLX/KFX 400 and Suzuki DRZ/LTZ 400 motors from the first decade of the 21st century. All of these 95mm bore motors even use 20mm piston pins (except the 19mm Honda piston pin). The problem is just the compression height. All of these 95mm pistons have rather short compression heights in the 21mm to 24.5mm range, and that doesn't work out for a 95mm bore Husqvarna motor. What would be required to get a workable compression ratio in a Husqvarna motor would actually be a compression height similar to the compression heights of the 1980's and 1990's Husqvarna pistons. Something in the 27mm to 30mm range. Way down at less than 25mm of compression height there just isn't any easy way to get the compression ratio to come out in the 10:1 to 12:1 range with a 95mm bore on the 2.48" stroke length. An obvious solution is to stroke the 350 bottom end out to about a 2.83" (72mm) stroke length, but then the stroke length is too long again. That's also a much more expensive project when the crankpin needs to be moved. Running KTM bore and stroke dimensions on a Husqvarna 510/610 platform does actually seem rather appealing, and the 40mm Husqvarna intake valve spacing is reasonable for the 95mm bore diameter. Another project for another time (or perhaps for another builder with more of a propensity for spending $800 to have a crank pin moved).
The specific build plan that I came up with before I even got the 1991 Husqvarna WXE 350 motor was to use a stock 1990 through 1998 Husqvarna 610 98mm piston with the shorter 350/410 cylinder height and the larger combustion chamber 510/610 cylinder head on the stock 2.48" stroke length bottom end for about a 10.5:1 or 11:1 compression ratio.
As far as actually installing the 22mm piston pin on the 350 rod, that turned out to be no problem at all. When I knocked the small end bushing out of my junk 1992 Husqvarna TE 350 connecting rod the bore was exactly correct for a 22mm piston pin. Slipped right in with just the smallest little bit of wiggle. Steel on steel might not be ideal for a piston pin bearing, but it certainly can work.
I knew that the compression ratio of the smaller displacement 98mm bore engine was going to need to be at least slightly higher than the minimum compression ratio that will work with the larger 577cc displacement 98mm bore 610 motor, but as usual the exact compression ratio required would depend on what type of gasoline actually happened to be available. The 98mm bore is huge really, and with those giant bore diameters getting the compression ratio correct is very important. Too low of a compression ratio and the spark timing gets pushed up past 25 degrees BTDC and torque in the 3,000 to 6,000RPM range suffers drastically. Too high of a compression ratio and tuning is very tricky, especially without load dependent spark advance. Smaller 40 to 70mm bore diameters are much easier to tune, and can actually well tolerate a much wider range of compression ratios. Anything above about a 70mm bore diameter is just harder and harder to tune, and a giant 98mm bore diameter is so huge that problems are almost guaranteed. It is possible to make power with large bore diameters, but the compression ratio and spark timing values do have to be fairly precisely set for the gasoline that is actually used.
Generally the 10.2:1 compression ratio of the stock Husqvarna 610 motors has been somewhat too low, with 11:1 or higher required to reliably keep the spark timing below 27 degrees BTDC. Down at low elevations though on cheap fast flame front travel speed types of premium pump gas the stock 10.2:1 compression ratio sometimes has not been excessively low for the 98mm bore diameter. It really does depend on what sort of gasoline products actually come out of the pumps. The ideal compression ratio for the 98mm bore Husqvarna 610 motor running pump gas generally seems to be in the 10.5:1 to 11.5:1 range, which agrees fairly well with traditional recommendations for four inch bore pushrod automotive engines as well. The four inch bore automotive engines have generally had longer 3.25" to 3.75" stroke lengths and rather small camshafts for 3,000 to 6,000RPM operation, and compression ratios in the 9.5:1 to 11:1 range were generally recommended for street use. Keeping the camshaft very small does help with running very low compression ratios, as long as only the lower engine speeds are considered. The combination of a small camshaft and a low compression ratio on a pushrod engine does mean that top end power is basically an impossibility.
In any case the stock 10:1 compression ratio has generally seemed like the absolute rock bottom lowest compression ratio for the Husqvarna 610 motors. That means that the smaller 472.5cc displacement is sure to require at least slightly more than a 10:1 compression ratio to run well on the same gasoline. I was guessing that somewhere around 11:1 would be ideal for the 475 big bore motor. With a stock Husqvarna 610 98mm piston the compression ratio was seeming to come out a bit on the low side.
To keep the compression ratio up sufficiently I came up with the idea of using a slightly shorter timing set. By taking two links out of the 3/8" early Husqvarna 610 timing set the cylinder head would be able to be placed 0.040" lower than using a stock 5/16" Husqvarna 350/410 timing set. That extra 0.040" seemed like a great idea for getting the compression ratio to easily come out in the desired 10.5:1 to 11:1 range without huge amounts of slack in the timing set.
When I had my 1999 Husqvarna TE 410 motor apart back in February of 2017 I had noticed that the 410 cylinder is in the same casting as the 610 cylinder. I wasn't really thinking along these lines back then, but I did notice that the 410 cylinder could be bored right out to the 98mm diameter as the coolant passages were in exactly the same locations as on the 610 cylinders of the same years. For the 475 big bore motor I figured I could just slap a bored out 410 cylinder on the 350 bottom end. Simple as that.
For a cylinder head I decided to use the old junk 1994 Husqvarna WXC 610 cylinder head that I had to buy back in March of 2015 just to get the camshaft to replace the broken 1997 camshaft that came with the 1997 TE 610 motor. The stock 35mm valves in the 1994 cylinder head were totally toasted, and the seats were damaged also. In looking at the 1994 cylinder head I realized that the seats had probably actually been cut for 36mm intake valves, and the stock 35mm intake valves had been sitting excessively low on the seats. Was the low sitting valves what caused them to pound in and fail? Perhaps partially, but there were also signs that this 1994 Husqvarna 610 motor had been overheating. There was some greenish white residue of baked on coolant in the cooling passages, which seemed to indicate that the engine had been running very hot. Probably just due to running the stock 33 degree BTDC spark timing with a mixture of 80% race gas and 30% premium pump gas. The valve lash might have been left excessively lose also, as that does cause the valves to smash into the seats more violently. In any case the stock 35mm valves were badly smashed and would have dropped had that engine been run much longer.
The idea I came up with was that the seats in the 1994 cylinder head actually could be repaired simply by going up to larger valves. The 36mm valves of later Husqvarna 610 cylinder heads might have worked, but none seemed to be available. Kibblewhite has been making 35mm Husqvarna 510/610 intake valves as well as both 30mm and 32mm exhaust valves, but no 36mm intake valves.
An idea that I had been toying with for many months seemed to be the ideal solution. In searching through valve specifications for various engines I found that the 2003 to 2007 Polaris Predator 500 intake valves are close enough to the same size as the Husqvarna 510/610 intake valves that they can be made to work. The Polaris Predator 500 intake valves are 37mm on the head, and the same 6mm on the stem. The length is not exactly the same, but close enough to get me interested. It seemed like I could probably just stick 37mm Polaris Predator 500 intake valves in the 1994 Husqvarna WXC 610 cylinder head.
Initially the Colorado 1991 Husqvarna WXE 350 motor had been advertised as the complete motor, for a very reasonable price. There were several separate reasons that I didn't just buy the whole motor though. One was that I really wanted to give someone else the chance to buy the complete motor if they needed it as a replacement for a 1991, 1992 or 1993 Husqvarna WXE/TE 350. Another was that I really didn't want more of those 84mm bore parts. I did buy an extra 84mm 1991 Husqvarna 350 cylinder and piston the year before just because they had been cheap and I figured that sometime I might want to put one of my 350 motors back to fully stock. I didn't want more of those small valve cylinder heads though, I already have two of them and that's about two too many. I just didn't want anything to do with incorrect valve spacing. So I really didn't want the complete 350 motor. I knew I was just going to tear it down for parts anyway, so it seemed appropriate to give someone else a chance at it as a complete motor.
By the time I came up with the 475 big bore motor build idea the guy in Colorado selling the 1991 Husqvarna WXE 350 parts had pulled the motor apart and was selling them separately. I just bought the bottom end, without the clutch or clutch cover.
What I was thinking was that I could make use of some of my other junk Husqvarna parts, and I was also hopping to use the 2.63:1 primary reduction set from the 250 two stroke. I happened to have several of those old junk 250 two stroke motors lying around, and the smaller amount of primary reduction seemed like a good match for the reduced displacement 475 big bore motor. Since the 610 motor uses 2.30:1 primary reduction going up to just 2.63:1 primary reduction would probably work very well on the 475 big bore motor.
I was also thinking that I would use a 1986 clutch cover and a crankshaft driven water pump on the 475 big bore motor build. It's not that I really wanted to convert from the more reliable camshaft driven 610 water pump to the older less reliable crankshaft driven water pump. It's just that using an old 1986 clutch cover and crankshaft driven water pump would mean that I could make use of that broken 1997 camshaft to get a centrifical de-compressor. The centrifical de-compressor camshafts are highly desirable for running a points ignition system as they pretty much totally prevent kicking back when kick starting even if the spark timing has to be set in the 23 to 30 degree BTDC range. It's not desirable at all to go up to 26 or 28 degrees BTDC spark timing, but it can happen sometimes. And actually really good engine performance can sometimes be obtained at 23 to 25 degree BTDC spark timing settings where kicking back when kick starting is a potential problem. Basically the centrifical de-compressor camshaft is just highly desirable on a points ignition Husqvarna as it essentially totally removes the problem of kicking back when kick starting.
When the 1991 Husqvarna WXE 350 bottom end showed up I was surprised to find that it didn't have the stock Husqvarna connecting rod in it. Someone had replaced the connecting rod with one of the new Royal Rods brand Taiwanese Husqvarna 610 rod kits using a custom small end bushing for the 20mm piston pin. The Taiwanese rod looked very much like the stock Husqvarna rods, but without the Husqvarna logo embossed on the side. The radial clearance of the big end bearing felt a bit on the tight side, but there was some substantial side to side wiggle on the rod. It seemed like it might break in and work.
The 1986 Husqvarna 250 uses the same spline count and the same diameter on the clutch basket as the 1991 Husqvarna, but the spacing is different. Since the 1986 Husqvarna 250 clutch basket does go onto the 1991 transmission input shaft it probably would be possible to come up with some way of doing the conversion with custom spacers. Instead though I decided to just order the 1991 Husqvarna WXE 350 clutch, which I probably should have bought in the first place just to have as a spare anyway.
The 1986 Husqvarna 250 clutch cover did go right onto the 1991 Husqvarna WXE 350 cases with essentially no difficulty as they are in fact fully interchangeable. The 1980's two stroke and 1991 four stroke clutch covers might be fully interchangeable, but it turns out they are not actually identical. The two strokes have mounting holes for a kick starter return bumper, where the four strokes instead have mounting holes for a kick starter guide. I have had to make custom kick start return bumpers out of pieces of old tires for all of my 1991 clutch covers, but the 1980's two strokes already have a functional rubber return bumper. So from that perspective the 1986 Husqvarna 250 clutch cover is a nice upgrade for the 1991 motor.
All that development work was done in the spring of 2017, most of it before I even got the 350 bottom end from Colorado. Then the project got stalled for a number of reasons. First of all I didn't have a bike to put the new motor in, so there didn't seem to be much imperative. Then some of the parts I needed were seeming very hard to come by. The 3/8" pitch crankshaft sprocket was all of a sudden hard to come by, and there weren't any 410 cylinders listed for sale used.
Then after having not done much with the 475 big bore motor build for quite a few months I was reminded that the 350 cases actually have a smaller opening in them than the 410 and 610 cases. I had the 350 bottom end sitting right there all along, but I just hadn't though about that difference. I guess I had been fixated on the fact that the 410 cases had the same opening as the 610 cases, and I just hadn't thought about the 350 cases even though I had both the complete 350 bottom end as well as my left over disassembled WXE 350 cases to look at.
No big deal, I figured I could just open up the hole in the 350 cases with a hand grinder without even bothering to split the cases. They are the same castings, there is just a half inch deep lip on the 350 cases where the smaller cylinder goes in.
Since I was going to have to take material off the 350 cases anyway it started to seem like using a 610 cylinder would be the easy way to put the 475 big bore motor together. With the shorter stroke length the piston skirts don't go as far down into the cases, so the lower cylinder extension doesn't need to be as long. With the shorter cylinder extension the wall thickness of the extension wouldn't need to be as thick, and that would mean that I wouldn't have to take as much material off of the 350 cases to get the bigger cylinder to go in. Using a 610 cylinder would just mean cutting it down to the shorter length, and that seemed like it would be easy enough to do in the lathe.
Again I had a complete build plan, but with no bike to put the motor in there was little imperative to move forward with the project.
Then in early 2018 I decided to go ahead and build the 475 big bore motor and figure out what bike to put it in later. I ordered a used 3/8" pitch crankshaft timing sprocket off of a 1993 Husqvarna 610 motor for what seemed like a very steep $37 delivered and I also ordered a used 610 cylinder and piston. I was hoping to get a usable cylinder and piston, but I was avoiding the higher priced used cylinders because I figured I might damage the plating by chucking the cylinder in the lathe anyway. I was hoping that I could turn the length of the 610 cylinder down without damaging the plating, but I really wasn't sure how it would work out.
The first used cylinder and piston I ordered was advertised as being in good usable condition. When I got it I found that the cylinder was in rough but perfectly usable condition. The problem was that the rings were missing. When I tried to order new rings I ran into tons of trouble. The stock Husqvarna 610 rings have not seemed to be available. There have been 98mm ring sets available from a variety of manufacturers, but they have the thinner rings that fit the aftermarket 98mm pistons such as those from Woessner and Athena.
By calling around I did finally find a supplier who had the stock 610 ring set in stock, ready to ship out the next day. For $67 for both the compression and oil rings the price was very high, but not as bad as I have run into in the past. When the rings showed up the next week though they were 410 rings. Then when I complained I got some crazy song and dance about Husqvarna 610 motors using 91.5mm bore diameters. At first all I could get over the phone was a begrudging agreement to refund the purchase price of the 410 ring set when it was received back at the retailer. Then when I complained further that I had initially been told that the 610 ring set was in fact in stock the sales person very vaguely said that he would send out the 610 ring set. I sent back the 410 ring set, and nothing happened.
Five weeks after the initial order was placed a 98mm bore 610 ring set showed up, but the package said "KTM" all over it. That was pretty funny, but it was in fact a real 610 ring set for the stock Husqvarna 610 pistons. "KTM...Made in Italy". That's interesting to see that KTM was in fact supplying KTM branded Italian made parts for the old Husqvarna motors.
By the time the "KTM" 98mm ring set finally showed up I had already returned that 610 cylinder and piston as it seemed useless without rings.
There had been one 410 cylinder listed on eBay, but it was in Australia. At first I had been reluctant to order used parts from other countries, partially because of the high shipping cost. I did order the 410 cylinder from Australia though, and when it showed up I got another surprise. The cutout at the bottom of the cylinder extension is too big to allow the 98mm bore. Yeah, I had seen that cutout on my 1999 Husqvarna TE 410 motor the year before but somehow I had been convincing myself that it was not going to interfere with cylinder being bored out to 98mm. Wrong! That cutout on the 410 cylinders is in fact huge, bringing the outside diameter of the cylinder extension down to 3.93". So at first I thought I had just wasted another two hundred dollars and ended up with a spare cylinder for my 1999 Husqvarna TE 410 motor.
I was again considering 610 cylinders, but it was looking like I was going to end up buying a used cylinder for about $150, a new Woessner 8522 piston for almost $200 and then on top of it all I was probably going to have to send the cylinder out to have it re-plated also. What I decided to do was ask U.S. Chrome to weld up the cut-out in the 410 cylinder. I wrote up a proposal and shipped the 410 cylinder, along with a new 98mm Woessner 8522DA piston, out to U.S. Chrome in Wisconsin. Sure enough they were willing to weld up the cylinder extension cut-out and bore the cylinder for the 98mm piston, and the custom welding was only an extra $50. A beautiful job really, with the weld material turned down to the same 4.14" diameter of the top part of the cylinder extension and then lightly bead blasted for a uniform factory look.
As soon as I decided to go ahead with the 475 big bore motor build I ordered some Polaris Predator 500 intake valves. The first set I actually got from the local Polaris dealer, which were in stock on the shelf. They only had two in stock, and they were not the same. They looked exactly the same, and came in exactly the same sealed plastic bags with exactly the same Polaris part numbers on them but the valves were subtly different from each other. Dimensionally they were both Polaris Predator 500 intake valves, but the material was not the same. One was harder on the valve head and softer on the valve stem and the other was harder on the stem and softer on the head. They looked exactly the same, but on one the heat treatment was an annealing process that softened the end of the stem and on the other the heat treatment was a case hardening that hardened the end of the stem. They both seemed like they would work fine as Husqvarna valves, but it was annoying that they were not the same as each other. When I complained the guys at the Polaris dealership mostly played dumb about valve hardness, but they did agree to order two more Polaris Predator 500 intake valves. When the two replacements showed up they were exactly the same as each other; both harder on the valve head and slightly softer on the end of stem. That makes sense for the cam and bucket Polaris Predator 500 motor, and the hardness of the annealed stem end was still about what is found on traditional automotive engines. Not quite as hard as the original Husqvarna valve stems, but pretty good anyway and probably perfectly sufficient. And the valve heads were spectacularly hard, as would be expected to be required for a high speed cam and bucket type camshaft that smashes the valves into the seats.
I actually needed an extra 37mm valve to use to cut the seats in the 1994 cylinder head, so I ordered some more Polaris Predator 500 valves also. I decided to order two more of the OEM Polaris Predator 500 valves just to have for some future 35mm valve upgrade project, as they are pretty cheap. As little as about $16 each from some mail order suppliers. The mail order OEM Polaris Predator 500 intake valves ended up being exactly like the matched set of two OEM Polaris Predator 500 intake valves I got from the local Polaris dealer. Hard on the valve heads and annealed slightly softer but still sufficiently hard on the ends of the stems.
I also ordered some Kibblewhite brand Polaris Predator 500 intake valves, but that didn't work out. The Kibblewhite brand Polaris Predator 500 intake valves showed up soft and defective. Extremely soft like the defective auto parts store Mazda Miata intake valves that didn't work in the 386 stroker motor. I mean not just a tiny bit softer than stock Husqvarna valves. No, I mean way softer than any automotive valves. I went around checking the approximate surface hardness of automotive valves from Suzuki, Toyota, Volkswagen, Dodge and Chrysler engines and they were all about the same. Fairly hard, but noticeably slightly softer than the original Husqvarna valves. The defective Italian Mazda Miata valves and the defective Kibblewhite Polaris Predator 500 valves are drastically softer, like butter compared to automotive valves.
At least the OEM Polaris Predator 37mm intake valves are usable. It's kind of annoying that they are annealed down to be softer on the stems than on the heads, but they are still good enough. The slightly soft stems and spectacularly hard heads will just mean slightly more frequent valve adjustment intervals in the Husqvarna SOHC roller cam motor.
About a year earlier I had bought some of the Kibblewhite Husqvarna valves, and they were disappointingly a bit softer than the original Husqvarna valves also. At least the Kibblewhite Husqvarna valves were as hard as automotive valves, so they also are usable if a bit annoying. Mostly annoying because the Kibblewhite brand valves are much more expensive at around $37 to $45 each. At a price like that I would expect very high quality valves like the original Husqvarna valves. That $40 each for the Kibblewhite valves is a lot more than $16 to $25 each for the OEM Polaris Predator 500 intake valves.
What's also a bit annoying is that the valve lock groove is cut a bit closer to the end of the stem than on the Husqvarna valves. The combination of the 37mm Polaris Predator 500 valves being a bit longer overall than the original 35mm Husqvarna valves combined with the groove being cut 1/32" closer to the stem end meant that I had to make up some custom spacers to go under the valve springs. Just 1/16" was enough, and it probably would have worked without the spacers. To get the pre-load on the valve springs back to the original Husqvarna spec though I had to make those custom spacers. No big deal, just one more small annoyance.
The 37mm intake valves are absolutely the largest size that works with the stock Husqvarna 610 valve seats. The outside diameter of the intake seats is 37.0mm, so any larger than 37mm intake valves obviously just doesn't work at all. In fact even going out to 37mm valves on seats with a 37.0mm outside diameter is a bit ridiculous. Normally the seats are at least a small bit larger than the valves, and there is good reason for this. The seats need to be strong enough to withstand being pressed into the head and they also need to be strong enough to hold their shape as the valves push into them.
Of course 37mm valves will go on the 37.0mm Husqvarna intake valve seats, it just requires that the valves sit a bit higher. And that's fine for 37mm intake valves to sit a bit high on the Husqvarna 610 cylinder head because there is actually room for valves as big as 39mm or even slightly larger with the 40mm valve spacing. Since the 37mm intake valves are still a bit on the small side with lots of room around them it make sense that they sit a bit high. The reality is that the 37mm valves do need to sit a bit high on those small seats for 35mm or 36mm valves, and it is also true that on those small seats the 37mm valves aren't really going to flow any better than 36.5mm valves.
I could cut the 37mm valves down to have 36.5mm heads and they would work every bit as well on those small seats. So that brings up an important question. Are 37mm valves any advantage at all over the 36mm valves when used on those same seats? The answer is yes. There is some small advantage to going even bigger than 36mm on those seats. Extracting that advantage though requires re-cutting the insides of the seats.
Re-cutting the seats is easy with the correct valve seat cutters, which I don't happen to have. I do have an old set of valve seat cutters, but they don't go down to the 6mm stem diameter. A good set of cutters does make valve seat work much easier.
Instead I opened up the intake seats on the 1994 Husqvarna WXC 610 cylinder head using a small grinder. That was more work, and it was somewhat tricky work. In just a few minutes (maybe 90 minutes) of grinding I had both intake seats opened up to 1.35" (34.3mm) at the valve contact end. At the upper end of the seats I left the diameter at the stock 1.23" (31.3mm) size, so the re-cut seats have a tapered shape that transitions into the stock Husqvarna 610 porting.
So what's the point of opening the seats up for larger valves if the ports stay the same size? The answer is very simple. It's all about flow as the intake valves are already substantially closing towards the end of the intake event. At the upper end of the range of engine speeds where a certain valve closing time still works well a significant portion of the intake charge is flowing in as the intake valves are substantially closing.
So going up to the larger 37mm intake valves on the stock seats for 35mm or 36mm valves doesn't do much to increase the instantaneous maximum flow rate of the ports or the valves, mostly the big valves on the same seats just extend significant flow so that the same camshaft can work over a wider range of engine speeds. Small camshafts for strong torque at 3,500 and 4,500RPM do well with big intake valves to extend flow capability up at 7,000 and 8,000RPM where the intake valve closing time is becoming too early. The biggest valves that will fit allows the use of a smaller camshaft without giving up top end power. And the converse is also true. Undersized intake valves require a larger camshaft, and that obviously cuts into torque production at lower engine speeds.
The 37mm intake valves are clearly a nice upgrade for the Husqvarna 610 cylinder heads. Once I actually got to see the 37mm valves in the 1994 Husqvarna cylinder head I was so impressed with the aesthetic improvement that I made sure to order another set of 37mm valves to have on hand for a future cylinder head upgrade project. It's actually a fairly easy upgrade once the parts are available.
Kibblewhite also makes oversize 38mm intake valves for the Polaris Predator 500 motor, but using 38mm intake valves on the Husqvarna 610 motor would require larger valve seats. And going up to larger valve seats with 38mm valves it starts to make a lot of sense to recess the valve seats back into the cylinder head casting a bit more. That would require slightly shorter 38mm valves. The Polaris Predator 500 valves are a bit longer than the stock Husqvarna intake valves, so that's going in the wrong direction. For an ultimate valve upgrade on a Husqvarna 610 cylinder head more appropriately sized 38mm intake valves would be required. Perhaps such valves are available for some other application, although I couldn't find any. Another possibility would be to simply cut 0.20" off the stem of the Polaris Predator 500 valve and re-cut the groove for the lock. That's really too much reduction in length, but it might be workable. There's actually quite a bit of extra material on the Husqvarna 510/610 cylinder heads that would allow for large adjustments like that. Too bad the 38mm Kibblewhite Polaris Predator 500 valves are soft and defective, but that problem is likely to simply go away. Either Kibblewhite will stop making Polaris Predator 500 valves, or future production runs will be at least as hard as the OEM Polaris Predator intake valves.
Clearly it's somewhat problematic to use valves for cam and bucket engines in roller valve train engines, but the problems would normally be rather minor. Obviously cam and bucket engines have no need for spectacularly hard valve stem ends, but the valves can't be all that soft either. High performance, high speed cam and bucket engines are actually very hard on valves and seats. Because the rate of opening and closing of the valves is severely limited by the cam and bucket architecture the only way to get any performance increases is to increase the acceleration of the valves when they are close to the seats. That means much larger forces on the valve stem and also means that the valves are going to crash into the seats when they close. Overhead cam engines with ratio rockers or finger followers can open and close the valves with constant acceleration to maximize valve opening "area under the curve" while keeping valve stem force fairly even as the valves open and close. There are of course reasons why cams work even better with some slight modifications from constant acceleration, but the point is that ratio rocker or finger follower OHC valve trains have the capability of providing whatever high level of constant acceleration of the valves that is required, where a cam and bucket valve train (or any valve train with flat tappet lifters) is constrained to a certain maximum rate of valve opening and closing set by the diameter of the lifters. Cam and bucket valve trains don't have good performance potential, and squeezing what performance potential is available out of a cam and bucket valve train results in smashing the valves and seats rather severely.
The 475 big bore motor would work great with a stock 40mm DellOrto carburetor from a Husqvarna 610 motor, there is no doubt about that. The carburetor that I happened to have though is an old 38mm DellOrto that I picked up cheap a few years ago. The 38mm size is just about big enough for the 475 big bore motor, especially if going after huge power at immense engine speeds isn't the goal. The high flow capability, radically over square 475 big bore motor certainly could be configured to have some very large power production potential. Even without all that huge of camshafts the 450F bikes with similar stroke lengths make maximum power at about 9,000 or 9,500RPM with substantial overrev to as much as 12,000RPM, and the 475 big bore Husqvarna motor actually has considerably more top end flow capability than the 450F motors. The extremely radically over square 97mm bore Yamaha YZF450F is held back by it's cam and bucket valve train. The KTM 450SX-F/Husqvarna FC450 on the other hand has lots of performance potential with it's SOHC valve train, but it's less radically over square with a 95mm bore. The 98mm bore of the 475 big bore motor makes room for bigger valves and bigger ports, and the roller follower SOHC valve train is able to through those big valves open and closed very rapidly.
But getting more power than the top race teams get out of 450F engines isn't really my goal with the 475 big bore motor build. To be perfectly honest my rather boring performing 386 stroker motor already makes more power than is strictly required for the type of single track riding I prefer, and if I just want to make tons of ridiculously excessive power then the very easy way to do that is with the substantially larger displacement of the 610 motor.
For me the 475 big bore motor build is about practicality, usability and getting the most of everything from the parts that are actually available. And practicality and efficiency certainly means keeping the camshaft reasonably small. By small I don't mean miniscule like traditional automotive engines. I just mean small enough that torque at 3,500 to 4,500RPM doesn't suffer. That pretty much means a stock Husqvarna camshaft like came in all of the Husqvarna 610 motors. Big enough to pull hard on the top end, but not so ridiculous that torque drops off at 3,500 and 4,500RPM.
So the 38mm DellOrto is a bit on the small side for the 475 big bore motor, but it's right in line with the carburetor sizing of the factory stock 1990's Husqvarna motors. The 36mm carburetor on the 410 motor and the 40mm carburetor on the 610 motor provide reasonable levels of flow capability without being oversized, and the 38mm carburetor on the 475 big bore motor falls right in along those lines.
The problem with the 38mm DellOrto I happened to have was that the jetting in it was totally screwed up. I had long since borrowed the main and pilot jets just to modify them to the sizes I happened to need at the time for repairing the jetting on my various Husqvarna 610 bikes, but those are easy to replace. The real problem with the 38mm DellOrto I happened to have was that the needle and needle jet were totally wrong. I don't know if those parts came stock on any factory 38mm DellOrto application or not, but I sure could tell that they were totally wrong. The mystery PHM38 DellOrto carburetor I happened have came with a K4 needle and an AB270 "atomizer". The K4 needle has a shank diameter more than 0.02mm smaller than the K32 needles used on all the Husqvarnas with PHM series DellOrto carburetors, and then that AB270 needle jet is just monstrously huge.
There was however plenty of evidence that the jetting in the 38mm DellOrto was simply wrong. That K4 needle has both a shorter tapper and a smaller tip diameter compared to the K32 needle used in all of the 34mm, 36mm and 40mm PHM series DellOrto carburetors on Husqvarna 350, 410 and 610 motors. The shorter tapper and smaller tip diameter means that the mixture is going to richen up drastically as the throttle is opened up from 1/2 to wide open.
To fix the jetting I ordered a new K32 needle and an AB262 needle jet. Since the 34mm, 36mm and 40mm PHM series DellOrto carburetors on the Husqvarnas all use K32 needles it seemed reasonable to go with the same K32 needle for the in-between sized 38mm PHM series DellOrto carburetor. For the needle jet size it was just sort of a guess, although the guess was also constrained by the lack of availability of AB263 size needle jets. For the 36mm DellOrto on the Husqvarna 410 motors it is an AB260 on the K32 needle and for the 34mm DellOrto on the Husqvarna 350 motors it's an AB258 on the K32 needle, so it's probably an AB262 that is going to work on the 38mm DellOrto. That's just a guess, but it seems like a reasonable starting point. I might have instead started with an AB263 if such a size were available, but the Ab262 seems likely to be the correct size. For a choke jet I decided to start out in the middle with a 55 size. That's smaller than the 60 size that was stock on the 40mm DellOrto carburetors on the Husqvarna 610 motors, but I know from experience just how drastically oversize that particular jet selection turned out to be.
Pilot jet selection is easy, as all of the 34mm, 36mm and 40mm PHM series DellOrto carburetors tend to work well with pilot jets sized at about 58 to 61. At the small end of that range for the smaller engines and more towards the large end for the larger engines, but they all tend to run well with 59 or 60 size pilot jets. Going bigger than 61 on the pilot jet really doesn't seem to make any sense at all, and in fact the only reason to go up from a 60 size pilot jet to a 61 is simply because the 62 size was stock on all the Husqvarna 610 motors for more than a dozen years. Getting very far from what has proven to be functional stock jetting does tend to be problematic, even when that stock jetting is somewhat too fat for best possible operation. The Husqvarna dirt bikes might have been rather low production, but 14 years of production for the iconic 610 motor does add up to a substantial presence on the scene.
The main jet is trickier as sizing is of course roughly proportional to the size of the carburetor. If the 34mm DellOrto runs with about a 145 to 150 size main jet and the 40mm DellOrto runs with about a 170 to 178 size main jet then the 38mm DellOrto is going to be somewhere in the neighborhood of 162 or 165 for the main jet size. Probably the only reason that a main jet larger than 165 would be required in a 38mm DellOrto would be to run more than 10% alcohol. Going down to a 160 size might sort of work on regular gasoline with no alcohol in it, but even at that 160 is probably going to be way on the small side for the 38mm DellOrto.
Another consideration with carburetor size is the engine speed at which the intake stack boost hits. The 38mm carburetor is going to result in the intake stack boost coming at a slightly higher engine speed than would be the case with a 40mm carburetor on the 475 big bore motor. That's actually a good thing though, as the intake stack boost is going to come down at lower engine speeds on the radically over square 475 big bore motor. With a 40mm carburetor the intake stack boost on the 475 big bore motor might end up hitting way down around 6200RPM. That's pretty darn low, even for a smallish stock Husqvarna 610 camshaft. With the 38mm carburetor there is a bit less total volume in the intake stack, so the intake stack boost is going to hit a bit higher. How much higher? Uh... Perhaps around 6,800RPM or so. Up around 6,500 or 6,800RPM actually seems like a better location for the intake stack boost, as 6,200RPM is hardly above the point of peak cylinder filling with a stock Husqvarna 610 camshaft. If the intake stack boost hits at the engine speed where the camshaft tends to deliver peak cylinder filling then the engine is too "peaky". Getting the intake stack boost to hit a bit above where the camshaft tends to deliver peak cylinder filling allows the engine to be less peaky with a broader torque band. Pulling very strong torque around 6,000 to 6,700RPM is great, but pulling very strong torque everywhere from 6,000 to 8,000RPM is clearly even better.
If a points ignition system is used then the rather interesting additional peculiarity of crankshaft wiggle advance also comes into play. On the 475 big bore motor the crankshaft wiggle advance is going to come way up high at around 8,500RPM or so. On the less radically over square Husqvarna motors the crankshaft wiggle advance comes at a lower engine speed than the intake stack boost. On the radically over square 475 big bore motor though the crankshaft wiggle advance is going to be quite a bit above the intake stack boost. That's interesting because it means that a lighter piston will actually stretch out the separation between the intake stack boost and the crankshaft wiggle advance where on the less radically over square engines a lighter piston tightens up the spacing between the crankshaft wiggle advance and the intake stack boost.
As soon as the bored out shorty big bore cylinder came back from U.S. Chrome (the next morning actually since the package came late in the afternoon) I tore into the 1991 Husqvarna WXE 350 cases with a 4" angle grinder. After removing the cylinder studs I laid out a scribed circle on the top of the 350 cases, two circles actually. One at the 4.15" required diameter and another out at 4.19" as a guide. With the cylinder studs out of the way I had fairly good access to the tops of the cases, but the connecting rod sticking up was of course rather in the way. The first problem I had was that I couldn't stay away from the rod with the other side of the grinding wheel. After just one small knick in the rod I realized I had to add something to protect the rod. The first attempt was just some sturdy reinforced tape, but the grinder went right through that. Next I added some small steel washers between layers of tape in the most vulnerable areas, and that did the trick. I could hit the steel washers pretty hard with the grinder without getting anywhere close to damaging the connecting rod.
Cutting the cases back by hand was a bit of work, but it went easily enough. Working out towards the scribed lines was fairly straight forward, but keeping the half inch deep cut perpendicular to the deck surface was tricky. I knew that I wasn't going to be able to keep the cut perfectly perpendicular, so I just accepted that the hole was going to at first be tapered out towards the top.
Once I had the top of the hole all the way out to where the 4.15" diameter line was removed and the cut was right up against the 4.19" diameter line I was able to start checking the taper and out of round with the 4.14" diameter cylinder extension. By turning the cylinder in the opened up cases I was able to make scuff marks on the high spots which served as a guide to where more material needed to be removed. Surprisingly it only took a half dozen or so trial installations of the cylinder before I had the hole in the cases opened up to where the cylinder fit loosely all the way down on the deck surface. Overall a rather easy modification considering that it was done by hand with a 4" hand grinder, and it only took me about two hours to accomplish. Not the sort of operation that one would want to attempt in bulk, but for a one off custom build it was actually very easy.
Next up was modification of the 98mm Woessner 8522DA piston. I knew theoretically what to expect in terms of piston location relative to the cylinder head, but once I got the bored out 410 cylinder actually sitting on the 1991 Husqvarna WXE 350 cases I was able to take a physical measurement. First I had to install a stock type 22mm small end bushing, but knocking the custom 20mm bushing out and knocking the new bushing in was a simple task. I only had to take one small pass by hand with an adjustable hand reamer to clean up the burr on the end of the new bushing from beating it into the rod. Once cleaned up just that small bit the 22mm Woessner pin dropped right in and fit perfectly.
Before I really knew exactly what was going to be required I also had to try the 3/8" pitch timing set. I did this first without a base gasket, and there was plenty of slack in the chain. Next I installed both a 0.057" used Husqvarna 610 head gasket and a used 0.050" thick used Husqvarna 610 base gasket. Amazingly the 3/8" pitch timing set fit on this configuration also, although with only a rather small bit of extra play in the chain. That was good news, that I was able to use the full thickness 410/610 base gasket, as it gave a bit of extra room to stuff the tall 30mm compression height Woessner 8522 piston in.
I started out with the as-yet unmodified Woessner 8522 piston bolted to the turn table on the milling matching. My little milling machine isn't really big enough for such a tall turn table, and that means that there was no room for a vise to be bolted to the top of the turn table. Instead I had to bolt the piston directly to the top of the turn table, and that was a bit tricky. What I came up with was a piece of 1/2" Schedule 40 pipe through the pin bosses with holes drilled in it's ends for 3/8" bolts. That worked, but the piston was not all that secure on the turn table and at first it was moving around a bit on me when I started cutting. Turning up the spindle speed of the milling machine helped, and I was eventually able to get a fairly uniform 0.057" deep by 0.125" wide cut around the outside of the crown of the piston.
For the next part of the crown milling I removed the turn table and clamped the pin bosses in the vise on the stock bed of the milling machine. This provided a much more stable work surface, and milling the required square edged flats went very easily. When I had the one large, one medium and two small flat areas milled down to the same 0.057" depth I realized that I could easily have dispensed with the entire turn table operation. It's only the four little corners that need to be round cut with the turn table, and for all the work it was to setup the turn table on the undersized milling machine I could have more easily cut those four corner radiuses by hand. Such is life. At least the finished product did come out fairly uniform and professional looking. Then when I cut the required angles on the sides of the new mini dome with a hand grinder I made the piston modification look rather homemade anyway. The next step of piston modification makes them look even funkier, so aesthetics on piston modification is sort of a relative thing in the end.
That next step was my standard pin boss modification regimen to hack off a large portion of the extraneous weight. This particular 98mm Woessner 8522DA piston weighed in at only 364g when new and unmodified, which seems to be about three grams lighter than the first two 98mm Woessner 8522 pistons that got back in 2015.
After cutting the mild dome in the top of the crown this Woessner 8522 piston was down another six grams, a substantial start on substantial lightening. When I was done with all the modifications I had it all the way down to 323g, and again I had stayed substantially far away from any potential weak spots
The end result is very nearly 1mm of squish band between the piston and the cylinder head surface, and the mild dome then sticks up into the combustion chamber by just under 0.020". I really don't know exactly what compression ratio that works out to as I don't know exactly what the volume of the stock 610 cylinder head combustion chamber is. If the stock Husqvarna 610 motor is at 10.0:1 though, then my 475 big bore motor running stock gaskets is going to be at about 12.0:1. That's a high compression ratio to be sure, but not all that high compared to a 12.3:1 KTM 350 dual sport motor or a 12.8:1 450F closed course competition only race bike. A better direct comparison might be with the 11.8:1 KTM/Husqvarna 450 and 500 street legal dual sport bikes. The 95mm bore would be expected to be able to get away with a slightly lower compression ratio than the 98mm bore, but on the flip side the load dependant spark timing on the new bikes also makes it much easier to run high compression ratios. I would call 12:1 a very high compression ratio. Higher than I had initially planned for the 475 big bore motor, but still substantially within what is considered normal for street legal motorcycles.
As some indication of just how high of a compression ratio the Woessner 8522 equipped 475 big bore motor ended up with is that the giant Woessner valve reliefs turned out to not even be quite big enough. I had to cut both the intake and exhaust reliefs another 1/32" or so deeper to get sufficient valve clearance at split overlap. That of course being with the big 1997 TE 610 camshaft that does have more overlap than a 1991 Husqvarna camshaft. And of course that's with the 37mm Polaris Predator 500 intake valves sitting fairly high on the seats. In the end I decided to cut the 37mm valves down to 36.5mm anyway, as the extra material wasn't doing anything other than getting in the way with the valves sitting so high on the seats. I also ended up taking about 0.015" of the combustion chamber side of the valve heads, again because the extra material wasn't doing anything but getting in the way. The cut down OEM Polaris Predator 500 intake valves came in at 51g after taking a little bit off of the intake port side of the valve heads also.
The question of what bike to put my new 475 big bore motor in has been a festering issue ever since I first came up with the 475 big bore motor build idea back in March of 2017. The main candidates were my 1991 Husqvarna WMX 386 and my bone stock Czech Republic CDI 1991 Husqvarna WMX 610. It was plainly obvious that it had to be a 1991 Husqvarna WMX 610 with stock suspension, or something with equally good or better suspension. That's what I am used to riding, so anything less just isn't a possibility at all. As much as I didn't like the idea of taking any of my good running Husqvarna motors out of their bikes there was the simple fact that I knew the 475 big bore motor was going to be better than any of the other Husqvarna four stroke motors. The 1991 WMX 386 and the bone stock Czech Republic CDI WMX 610 were the likely candidates because it is those lowest compression ratio engines that had proven to be the most un-reliable and the least able to be ridden seriously on the 3,000 to 5,000 foot elevation trails. The other problem with those two bikes was the flat shocks. It was alright messing around with those bikes a little bit without functional compression adjusters on the 1991 WMX 610 White Power shocks, but that was a situation that couldn't last. The recent times I rode those bikes I came to the conclusion that I just couldn't tolerate riding without the ability to dial in a bit more hold up by going up to the #2 compression setting. The flat shocks were just too much of a distraction, and something had to be done about it. Pumping up the shock on the 1991 WMX 386 had worked once, but then the next time I tried the same trick I didn't get the pressure up high enough and it didn't work. It didn't leak, but the compression adjuster didn't work. Then my "Husky" brand bicycle floor pump mysteriously failed and I didn't have any good way to try to pump the shock up anyway.
Instead I sent those two flat shocks out to suspension shops to have the seals replaced and the shocks serviced. I got some crazy stories about the shaft seals not being available, and both shocks came back cleaned up but leaking even worse out the shaft seals and still apparently flat with compression clickers still not doing anything. That's still somewhat up in the air so to speak. I sent both shocks back by refusing delivery on the packages. Then one of them came back again still leaking large amounts of oil out the shaft seal, so I refused delivery a second time. Then both shocks came back hardly leaking at all and holding pressure, but without the compression clickers working. The one off of the 1991 Husqvarna WMX 386 I poked into a bit to see what was wrong with it. First of all I let the pressure out of the reservoir, and there was substantial nitrogen gas pressure bellow the reservoir piston. But then even with the pressure released from the bottom of the reservoir the shock was still pressurized. When I unscrewed the reservoir from the shock body a huge amount of high pressure bubbles came squirting out from the oil side of the piston. So the shaft seal had finally been replaced and the shock re-charged with nitrogen gas pressure, but there was gas pressure on the wrong side of the reservoir piston also. When I put one of my Schraeder valve equipped reservoirs on and bled all the bubbles out the shock sort of worked again. It was able to hold 90psi without the shaft seal leaking at all, and there was a noticeable increase in hold up on jump landings. It appears that the slight oil leak past the new shaft seal was due to a huge amount of pressure having been applied, probably somewhere around 200psi. At 90psi the new shaft seal didn't leak at all. What I am worried about on that shock though is that they changed the valving while they had it apart to replace the shaft seal. It feels harsh, with excessive rebound damping, like the oil is too thick even with the shock filled with 3W light shock oil.
The shock off of my 1991 Husqvarna WMX 610 with the bone stock 10.2:1 Czech Republic CDI 610 motor worked better as delivered back from the suspension shop, but it also was not without problems. There was still a slight oil leak at the shaft seal, and hardly any response from the compression clicker. Going up from the #3 compression clicker position to the #5 compression clicker position did add some small amount of hold up on jump landings, but the shock felt strange, bouncy and a bit harsh regardless of where the compression clicker was placed. That suspension shop swore up and down that they had not changed the valving at all, but the valving didn't feel like a 1991 Husqvarna WMX 610 anymore. The bike seemed to be bouncing up a lot and riding high in the stroke, and then on jump landings there was hardly any hold up to absorb the impact. It did sort of work, but the bike felt strange and slow.
Then the compression clicker on one of my other 1991 Husqvarna WMX 610 bikes mysteriously stopped working at the same time that the forks got very harsh and uncomfortable. The bike was riding high and harsh in the front, and the rear end was bottoming out and flopping around as if the compression clicker was all the way out, but turning the compression clicker in did nothing. The forks I was able to fix by replacing the years old 5W BelRay fork oil with fresh 5W BelRay fork oil, and then mysteriously the rear end felt better also with more hold up and less bouncing around.
Next up for consideration as a bike for the new 475 big bore motor was my period correct big cam 1991 Husqvarna WMX 610. Taking that big cam motor out was actually an appealing idea, as the big camshaft had proven to be rather problematic. Even with the carburetor sorted out and running well without lean stumbling or popping out the intake there were still problems. The big cam but otherwise stock 10.2:1 SEM ignition 610 motor was kick starting easily and reliably both hot and cold, but the idle quality was very poor. In fact it wasn't idling at all, and that was not working out very well.
The really big problem with that period correct 1991 Husqvarna WMX 610 was that the race shop modified suspension was very bad. Just very harsh and slow in all conditions, much worse than any of the other 1991 Husqvarna WMX 610 suspension and also much worse than any of the KTM White Power "MX" suspension. That race shop modified 1991 Husqvarna WMX 610 shock had also started leaking out the shaft seal, so I sent it and the race shop modified damper cartridges out to another suspension shop for a revalve. After hemming and hawing about it being an old bike for several weeks and not doing anything they just sent the shock and damper cartridges back without having taken them apart. No charge, but no revalve either. Still no suspension for that bike.
Amazingly I found a White Power shock for sale that fit on a 1991 Husqvarna chassis. It was clearly stamped "TC", so that was encouraging, but there were some strange things about this mystery shock. It was advertised as being a new unused shock, but when the package arrived I saw that it was clearly made up of old used parts that had looked new enough to pass for new in a small eBay photograph. It wasn't a new shock at all, but it was very mysterious. It had an old 1980's one-piece red anodized reservoir on it, but on that 1980's reservoir was a 1991 "Husqvarna Super Adjuster" White Power sticker. Very strange. The shock did indeed seem to have the good "TC" damping, but there was no response at all from the compression clicker and the rear end bottomed out extremely easily.
For the forks I used the damper cartridges out of a set of 1992 KTM 250 MX 40mm USD White Power forks. I got those forks very cheap as the lower stanchion on one side was broken and the plastic rebound adjuster knobs were missing. The fork caps had been modified to take air bleed valves, but then the stock red plastic rebound adjuster knobs didn't fit. Instead they had used small black plastic rebound knobs, but one of them was missing when I got the forks. I could have just used a set of Husqvarna fork caps, but I like to keep the stamped fork caps with the damper cartridges that they go with. I took the aftermarket bleed valves out and plugged the threaded holes with plastic plugs so that a set of stock red plastic rebound adjusters could be used.
These 1992 KTM 250 MX damper cartridges were not however stock. The damping had been completely removed from the left side cartridge, leaving damping only in the right hand side cartridge. Once installed in the Husqvarna forks I saw that these 1992 KTM 250 MX cartridges are the same KTM length as the other 1991 through 1994 KTM 40mm USD White Power damper cartridges. That is, about 1/4 inch longer than the 1991 Husqvarna WXE 350 and 1991 Cagiva 250 40mmm USD White Power damper cartridges, but also about 1/4 inch shorter than the 1991 Husqvarna WMX 610 and 1989 Husqvarna 510 XC damper cartridges.
With approximately 15W ATF in the forks the modified one-sided 1992 KTM 250 MX damper cartridges did sort of work. The ride was fairly smooth at all bike speeds, and the front end felt planted and well in control. The ride was however a bit harsh and stiff feeling, and then the forks blew through the stroke very easily on jump landings. When I turned the rebound clicker nothing happened. No response at all from the rebound clicker. There was substantial rebound damping, but it didn't change at all turning the clicker from the #1 position to the #7 position. Just no response at all from the rebound clicker. The compression clicker did work, but there was hardly any response. Even with the thick ATF, going in on the compression clicker only very slightly increased the compression damping. Going in on the compression clicker didn't increase harshness, but it did very little to increase hold up either. The big problem though is just the harsh ride. The modified 1992 KTM 250 MX damper cartridges don't ride quite as smoothly as the stock 1991 Husqvarna WMX 610 forks, and that was disappointing. All in all though the 1992 KTM 250 MX damper cartridges were fairly good. Certainly a substantial improvement over the very harsh riding and slow, badly modified damper cartridges that I took out of those forks.
Installing the 475 Big Bore motor in the 1991 Husqvarna WMX 610 chassis was fairly strait forward, with the only custom work required being the coolant hoses to connect the low 1980's water pump to the 1991 radiators. For the coolant inlet fitting on the cylinder head I used a piece of 3/4" brass pipe threaded into a 1/4" aluminum plate. I drilled and tapped the plate for the 3/4" NPT thread on the pipe, and I drilled the plate for the 6mm mounting bolts to bolt it to the cylinder head. This worked well, and provided a hose barb sticking straight out the side of the cylinder head. A piece of 3/4" hose bent around nicely from this custom fitting down to the outlet of the 1980's water pump.
Picking gearing for the 475 big bore motor was a bit of a challenge. I thought initially that I would run a 13 tooth front sprocket, but the numbers just didn't work out for the 13 tooth. With 13/48 gearing on a 14 link chain the rear axle is too far forward. I have 13/48 gearing on the 1999 Husqvarna 410 and it seems to work fairly well, but that's a smaller and less powerful engine. On the 475 big bore I wanted to keep the rear axle back farther. I thought about 13/47 gearing, but the rear axle is still very far forward with that gearing. Going down to 13/46 gearing might work, but I was thinking I wanted to keep it geared down lower than that. I thought about 13/50 gearing with a 116 link chain, but that's lower than I wanted to go. In the end it seemed like the only reasonable option was 14/52 gearing with a 118 link chain. That also happened to be very convenient as I already had a good 118 link O-ring chain and 52 tooth rear sprocket on the bike. All I had to do was add a new 14 tooth sprocket in place of the 13 tooth I had been running with the five speed big cam 610 motor. Taller gearing for the smaller motor in the same bike? Well it's a six speed, so it can tolerate much taller gearing.
As I already had a 40mm DellOrto carburetor on that bike I decided to use that instead of the 38mm DellOrto I had been planning to use. For an ignition system I ordered a Czech Republic CDI ignition system from PowerDynamo in Germany. This is a new ignition system based on the old Czech Republic CDI ignition system, but it is listed specifically for 1980's and 1990's Husqvarna four stroke engines.
The first PowerDynamo CDI system I got actually didn't fit. The flywheel stuck out so that the ignition cover wouldn't go on. I put this first PowerDynamo CDI ignition system on my 12.2:1 hot rod 610 motor, but then I ran into trouble with the advance curve. The advance curve had a shoulder up at about 3,700RPM, and this was causing a severe loss of torque at 2,500 to 3,500RPM. Not only was the advance curve shoulder up way too high at about 3,700RPM, but then it continued to advance gradually from 3,7000RPM up to higher engine speeds. This just couldn't work at all, on any engine. If it was such a small engine that a 3,700RPM advance curve shoulder would work then it wouldn't be able to use additional advance up to 5,000RPM. An advance curve that just doesn't work at all. And on top of that it was advancing much too far, like more than 20 degrees from low idle up to 5,000RPM. These Czech Republic CDI control modules do have four little switches on them though to select different advance curves. With no information about what the switches do I never moved them from the 0010 possition that they came set at on the old Czech Republic CDI ignition system that came on my 1991 Husqvarna WMX 610 with the bone stock 10.2:1 Czech Republic CDI 610 motor. The switches are labeled one through four, and are also labeled "on" in the up position. The 0010 setting is switch number one off, switch number two off, switch number three on and switch number four off. That 0010 setting was what delivered the 3,000RPM advance curve shoulder and 15 degrees of advance from 1,500 to 3,000RPM.
That first PowerDynamo brand CDI module looked exactly like the old Czech Republic CDI module, but the advance curves were not the same. At the PowerDynamo recommended 0110 "Husqvarna" switch position it was advancing more than 20 degrees with an advance curve shoulder way up at about 3,700RPM that caused a severe loss of torque over a wide range of engine speeds from 2,500 to 3,500RPM. That didn't work at all. But then there was also the as-delivered switch position of 1111. At the 1111 setting there was no advance curve at all! Just one fixed spark timing value. That wouldn't work for any stock Husqvarna engine, but it was able to sort of work on my 12.2:1 hot rod 610 motor with the centrifical de-compressor camshaft. The PowerDynamo CDI ignition system with the CDI module set to the 1111 setting for no advance curve at all worked pretty much exactly like a points ignition system on the 12.2:1 hot rod 610 motor. The engine ran fine with the spark timing set at 23 degrees BTDC, but that much spark timing did actually make it noticeably harder to start. By switching to the 0110 setting for 7 degree BTDC cranking and low idling spark timing the engine fired up much more reliably on the first kick when hot. At the 1111 setting for 23 degree BTDC cranking and low idle spark timing it was often taking a second or even third kick to start hot. Obviously 23 degree BTDC cranking spark timing is way too much. And then there was the problem of the ignition cover not fitting.
It wasn't much of an ignition system, but not having to deal with adjusting points all the time is very appealing. I also tried the 0010 setting on that first PowerDynamo CDI unit, but that didn't deliver the 3,000RPM advance curve shoulder that the 0010 setting delivered on the older Czech Republic CDI module. Even at the 0010 setting that first PowerDynamo CDI unit was still delivering an advance curve shoulder way up at about 3,700RPM that caused a severe loss of torque from 2,500 to 3,500RPM. The 1111 setting was the only one that worked, but with no advance curve idle quality was poor with a high and unstable low idle and more difficult hot starting.
Something interesting that I noticed about installing the PowerDynamo CDI ignition system on the 12.2:1 hot rod 610 motor was that light load performance at 3,500 and 4,000RPM seemed worse than with the points ignition. I have often noticed that small amounts of spark advance isn't good for very light load performance at small throttle openings and elevated engine speeds, but with a rather substantial (for the 12.2:1 hot rod 610 motor) 23 degrees BTDC light load performance seemed worse than it had been with the points ignition. This is of course also something that varies with different types of gasoline. One day the PowerDynamo CDI equipped 12.2:1 hot rod 610 motor would hardly run at all at 3,500RPM at small throttle openings at 23 degrees BTDC and then on another day there was hardly any noticeable problem at the same 3,500RPM with the same 23 degree BTDC spark timing. All in all though it was seeming like the CDI ignition system was much more likely to cause problems at small throttle openings at 3,500 and 4,000RPM with 23 degree BTDC spark timing. It was actually seeming like the points ignition was getting crankshaft wiggle advance all the way down to about 3,500RPM at very small throttle openings. I had noticed something similar back in 2015 when I switched from the stock SEM ignition to a points ignition on the then 11:1 hot rod 610 motor. Installing the points ignition system seemed to deliver better throttle response at small throttle openings over an amazingly wide range of engine speeds. I really don't know how low the crankshaft wiggle advance might pull in at small throttle openings, but several times I have gotten the impression that there may be some small throttle opening crankshaft wiggle advance way down to less than 4,000RPM. Another part of the advantage of the points ignition system has clearly been the simple fact that with adjustable spark timing I have tended to set the spark timing to the exact value that happens to work best. With the 12.2:1 hot rod 610 motor running crisply and instantly on the PowerDynamo CDI ignition system with the single fixed 23 degree BTDC spark timing performance actually was very good. Really the only complaint was just that I expected a sophisticated CDI ignition to have an advance curve so that the engine would low idle smoother and lower. Having no advance curve at all seemed totally out of place with the CDI ignition.
The next PowerDynamo brand Czech Republic CDI ignition system I ordered came with a lighter 984g flywheel that did indeed fit under the stock ignition cover. Unfortunately it also came with a totally defective CDI control module. Not just lacking an advance curve, but lacking any setting that came even close to working. The module looked exactly the same as the first PowerDynamo brand CDI module, with all of the same numbers on it, but the advance curves were totally non-functional.
At the as delivered 1111 setting it was low idling at 20 degrees BTDC, and then backing off to 16 degrees BTDC as the engine was revved up just a little bit to about 3,000RPM. That obviously wouldn't' work at all. Worse yet though at the 1111 setting the defective PowerDynamo module was actually continuing to advance as the engine speed was reduced, so that the cranking spark timing was up at more than 25 degrees BTDC.
This PowerDynamo brand Czech Republic CDI ignition system came with the same instruction manual that also recommended a 0110 setting for use on a "Husqvarna". On this totally defective CDI module the 0110 setting yielded a low idle spark timing of 10 degrees BTDC that then advanced very rapidly to 35 degrees BTDC by about 1,700RPM. That's a very bad advance curve, but it was even worse than that. The spark timing at low idle looked good, but when the low idle speed was reduced the spark timing backed off rather steeply all the way down to about 5 degrees BTDC before the engine stalled at a very low idle. It was obviously backing off considerably as the low idle speed was reduced, and it seemed to continue to back off to later and later spark timing values down to cranking speed. Obviously this couldn't work.
When I contacted PowerDynamo in Berlin about the defective CDI unit I at first just got a bunch of song and dance about how they had sold a large number of these CDI ignition systems so there couldn't possibly be any problem. When I continued to insist that the unit was both advancing much too far and also backing off steeply as the low idle speed was reduced I was told to try using either a 1000 or 0010 setting. At the 1000 setting the defective CDI unit was again low idling nicely at 10 degrees BTDC, but then it advanced very steeply at lower engine speeds so that the cranking spark timing was way up at like 30 degrees BTDC. That totally didn't work at all. The 0010 setting on the defective CDI unit looked a lot like the 0010 setting on the old Czech Republic CDI unit in that it had only 15 degrees of advance with about a 3,000RPM advance curve shoulder. On the defective unit though this advance curve couldn't actually be used. Each time the engine was shut off it switched to low idling at 25 degrees BTDC and advancing to 40 degrees BTDC at 3,000RPM. Only by switching to the 0010 setting with the engine running was the advance curve able to be used, which was a horrible distraction and very confusing.
These problems were so severe that, after repeated and increasingly ominous complaints on my part, PowerDynamo agreed to provide me with a custom module free of charge with any 16 advance curves I wanted. I immediately drew up 16 advance curves and emailed the drawings off to Berlin. I wasn't at all sure if I would ever see this promised custom module, but I got the curves off right away on that same day that it was offered. It then took another week to sort out transcription errors, but amazingly PowerDynamo did eventually supply a CDI module with my custom curves.
I had made up very clear scale drawings of the curves, and I even included (X,Y) labels on the low idle points, advance curve shoulder points and at 8,000RPM to make reading fast and easy. There were still about a half dozen errors in the digitized curves that PowerDynamo emailed back to me, and then even on the second set of emailed curves a few errors persisted.
I got the impression that they were trying to get me to ask for changes in my original curves, but I stuck to the 16 curves exactly as I had specified on that first day. They weren't perfect, but they were usable. I wasn't even really thinking about the 475 big bore motor when I drew the 16 curves. It was such a large number of curves that instead I just tried to make up a generic set of curves that would work for all of the stock 510, 610, 350 and 410 motors that this PowerDynamo CDI ignition system had been advertised as working with.
The basic strategy I used was to specify different curves for regular, cheap premium and race gas. For the big 3.01" stroke length 510 and 610 motors I used 2400RPM advance curve shoulders for "cheap premium", 2800RPM advance curve shoulders for "regular" and 3200RPM advance curve shoulders for "race gas". For the big bore 610 I specified a slight little three degrees of additional advance from 2,400RPM to 8,000RPM for "cheap premium", no additional advance beyond 3,200RPM for "race gas" and a big 10 degrees of additional advance from 2,800RPM to 8,000RPM for "regular". For the smaller bore 510 motor I specified no additional advance for "cheap premium", but a modest six degrees of additional advance for the 510 on regular. I also specified "High Altitude" variants of some of these curves, which mostly just moved the entire curve up a few degrees at the flip of a switch. I tried to keep the high altitude variant just one switch throw from the base curve to make switching easy, so each drawing was labeled with 0100 or 0101 etc. to make sure that they ended up in the correct positions.
For the curves for "regular" that continue to advance substantially out to 8,000RPM the high altitude variants actually advance even more up at 8,000RPM. Three degrees more at 2,800RPM for the high altitude variant of the "610 Regular" curve, but a whopping 7 degrees more at 8,000RPM.
I also included curves that I called "Small Engine Special" that use a 3,500RPM advance curve shoulder and back off slightly above 3,500RPM. The high altitude variant of the "Small Engine Special" backs off slightly less above 3,500RPM in addition to being two degrees earlier at 3,500RPM. For good measure I also specified some "Giant Engine Special" curves with 2,000RPM advance curve shoulders, both with steep additional advance to 8,000RPM for big 4.5 inch bores and with gradual additional advance to 8,000RPM for 3.5 inch bores.
When the new custom CDI module arrived I was surprised to find that it is in a smaller plastic case and also has a USP port on it. So that's how they can provide custom curves; they also have a programmable module. PowerDynamo also now has a note on their website that they will provide a CDI module with up to 16 customer specified custom curves free of charge with any new order of a PowerDynamo CDI ignition system. That's quite a bargain for a $450 ignition system complete with flywheel, stator, coil, CDI control module, DC regulator for lighting and a wiring harness.
The new PowerDynamo CDI module did indeed work, but not without some initial frustration. When I first switched from one curve to another the module did indeed obviously then run at the new curve. But then as luck would have it the gas tank happened to be nearly empty, and the bike ran out of gas 1/4 mile later. I pushed it back, added more gasoline and got it going again without much difficulty. But then the switches didn't seem to do anything, it was just staying stuck on the first curve I had selected.
What I eventually figured out after a few frustrating hours of head scratching was that this new programmable PowerDynamo CDI unit could only be switched with the engine shut off. The other two PowerDynamo CDI units could be switched with the engine running, and they immediately switched advance curves. This new programmable PowerDynamo CDI unit though couldn't be switched with the engine running. If the switches were moved with the engine running nothing happened, it just stayed at the previous advance curve. Not really a problem, but frustrating at first none the less.
I hadn't really been thinking about the 475 big bore motor when I drew the 16 curves, but I knew I was providing enough options that I could make something work. What turned out to work was the "610 race gas" curves. The 3,200RPM advance curve shoulder was just right for the short 2.48" stroke length 475 big bore motor as was the 1400RPM low idle point. What really amazed me was just how smooth and quiet the short stroke length 475 big bore motor was able to idle with a good advance curve.
I actually ended up running less spark advance than I had expected. What worked well was 21 degrees BTDC at 3,200RPM and 7 degrees BTDC at a 1400RPM low idle on the 475 big bore motor. Wow, just a really extremely low idle speed and also smooth and stable with no stalling at all. In fact the 475 big bore motor never stalled at low idle, even when it was low idling so crazy low that I thought it just wasn't possible. And operation at 1,500 to 3,000RPM is very smooth and very quiet at small throttle openings. It really is amazing what an appropriate advance curve can do on a good running engine.
The 3,200RPM advance curve shoulder did feel like it was up rather high, but there doesn't seem to be any problem at all on the short 2.48" stroke length. Power delivery is very predictable without any surging. No surging feeling anywhere around or bellow the 3,200RPM advance curve shoulder, and also no surging anywhere up higher around 4,000 to 7,000RPM. Just no surging anywhere at any engine speeds. The short 2.48" stroke length really does the trick for totally eliminating surging. Just like with the 2.39" stroke length 410 motor there is just never any surging at all from the 2.48" stroke length 475 big bore motor.
Taking the 475 big bore motor for a longer ride on some medium sized dirt roads I was amazed by a number of different aspects of performance. Cruising along at moderate speeds on big roads in sixth gear the 14/52 gearing felt rather low. The engine seemed to have plenty of power to pull taller gearing, and it seemed to rev up unnecessarily cruising along at 50mph. Shifting down and riding more aggressively with heavy acceleration out of the turns though the gearing felt very tall. The 475 big bore motor revs out very freely and makes power up to very high engine speeds. The bike was able to go very fast in fourth gear, so from that perspective the gearing didn't seem too low at all.
The 475 big bore motor was revving out so freely in fact that I was quite worried about ruining the rod bearing. I had added a 1/4" oil squirter extension, and I have every confidence that is going to do wonders for rod bearing longevity on the short stroke length motor. Still though I didn't want to be splitting the cases to replace the connecting rod. New rod kits are now available for these Husqvarna motors, but that doesn't mean that I want to install one just yet. Splitting the cases is a big nasty job, and one that I would just assume not have to do at all.
My main concern was the rather tight Royal Rods rod bearing. I wanted to let it break in gently, so I was trying to avoid prolonged use of the highest engine speeds. There was plenty of power at around 6,000 to 8,000RPM anyway, so there wasn't much need to rev the engine to the stratosphere. In fact power from the 475 big bore motor was seeming quite strong at all engine speeds. Torque really took off as the engine speed was increased from 4,000 to 6,000RPM, but the amount of torque available way down to 3,000RPM was impressive also. In fact torque down to crazy low engine speeds was surprisingly usable.
With the smooth advance curve torque at small throttle openings in full flame front travel mode was very usable way down to 2,000RPM or so. What I kept being very impressed with was that cruising along in sixth gear I didn't need to shift down even for rather tight turns. I could just let the engine speed drop way down to 2,500 or even 2,000RPM and smoothly accelerate out of the turn at small throttle openings. Not huge amounts of torque, but very smooth and very usable. Then as the engine speed was increased up past 3,000RPM opening the throttle farther delivered much larger amounts of torque that kept building as the engine was revved farther.
Something else that struck me was that it didn't feel like a 98mm bore engine. The bore diameter felt smaller. There are a lot of different factors that add up to the 475 big bore motor running surprisingly well. One is the big valves, big 40mm carburetor and large exhaust system. And oh, about that exhaust system. It's the same stock 1991 exhaust system with an added Supertrapp secondary muffler that I run on all my 1991 Husqvarnas. The amazing thing is how much quieter it is on the 475 big bore motor. Not only super quiet at low idle at small throttle openings, but just very quiet everywhere. Even opening the 475 big bore motor up and blasting big power the exhaust note remained surprisingly quiet. Much quieter than the exact same exhaust system had ever been on the big cam 610 motor in the same bike.
Anyway, the big valves, big carburetor and big exhaust system add up to substantial flow capability from a moderate sized stock Husqvarna cam shaft. The broken 1997 Husqvarna TE610 camshaft that I used in the 475 big bore motor has exactly the same lobe profiles as the 1994 camshaft that I have in the rebuilt 1997 Husqvarna 610 motor, and it also came out about 4 degrees of crankshaft rotation advanced. Advanced like that the slightly bigger camshaft really isn't all that big, but in the 475 big bore motor it turned out to be plenty of camshaft. With the big valves and big ports on the smaller 472.5cc displacement it's lots of flow capability to be sure. What I was noticing was that it actually was sort of seeming like a bigger camshaft. The 475 big bore motor was taking off considerably as engine speeds were increased from 5,000 to 7,000RPM in a way that did feel much like a bigger camshaft. The explanation for this is multi-faceted. Of course the shorter stroke length allows the 475 big bore motor to run up to higher engine speeds, and of course the big valves and big exhaust system allow it to flow well up to higher engine speeds, but it seemed like more than that. It really felt like a slightly bigger camshaft in terms of cylinder filling continuing to increase up to higher engine speeds. Part of the explanation is that the bigger valves actually do result in the camshaft seeming slightly larger. The bigger valves allow slightly more of the intake charge to escape back into the intake ports at low engine speeds, which makes the camshaft seem like the intake valves are closing a bit later. Does that mean that bigger intake valves are worse for low end and midrange torque production? No, absolutely not. The bigger valves allow the camshaft to be smaller or installed farther advanced without giving up top end power, and that means stronger and broader power. No, the big valves are always an advantage; they do however slightly change the character of any particular camshaft.
For the 475 big bore motor the result is that the 1997 camshaft installed quite substantially advanced results in an amazingly free revving engine that feels like it has a lot more camshaft than it actually does without giving up low end and midrange torque. Then there is also the intake stack boost coming on way down at a rather low 6,300 or 6,500RPM engine speed. I can't say I really noticed the intake stack boost hitting at any particular engine speed, but that might be part of why the cylinder filling was seeming to continue to increase up past 6,000RPM.
So, the 475 big bore motor flows well and revs out freely in a way that tends to hide the fact that its' a huge 98mm bore engine. That's one aspect of the bore diameter seeming smaller than it actually is. But then there is also the fact that the 475 big bore motor runs surprisingly well at small throttle openings with only 21 degree BTDC spark timing. With the throttle closed this can't be explained by substantial flow capability. There are actually several different things that might explain why the 475 big bore motor seemed to run surprisingly well at small throttle openings with just 21 degree BTDC spark timing. One is that the much smaller squish band means that more of the intake charge is up inside the combustion chamber in the cylinder head. On the stock 10:1 Husqvarna 610 motors the squish band is a massive 0.14", but on the 12:1 475 big bore motor the squish band is a miniscule 0.04". The much smaller squish band means that the bulk of the intake charge is up inside the combustion chamber in the cylinder head, and that combustion chamber is only 3-3/8" across. During the time that the flame front is expanding when the piston is near stationary close to TDC (approximately 20 degrees BTDC to 20 degrees ATDC) the combustion is taking place as if it were a 3.4" bore diameter engine, not a 3.9" bore diameter engine.
Then there is also the concept of "squish band mixing". When the squish band is small it causes a jet of pressurized intake charge to squirt out into the combustion chamber as the piston approaches TDC. This jet of compressed intake charge acts to stir the combustion chamber, and some people believe that this can actually speed up the apparent flame front travel speed. There might be something to this, and if there is then a small 0.04" squish band is certainly going to be more likely to produce this effect than a huge 0.14" squish band. The small squish band may be part of why the 475 big bore motor seems to run so well at small throttle openings with just 21 degree BTDC spark timing.
And yes, the shorter stroke length itself does in fact allow the engine to run up to higher engine speeds at small throttle openings in full flame front travel mode with small amounts of spark advance. And this is true for two different reasons. One is that the shorter stroke length means that the piston will accelerate away from the expanding flame front more slowly, giving a bit more time for the flame front to travel across the combustion chamber. The other reason that the shorter stroke length itself allows good performance with small amounts of spark advance at small throttle openings in full flame front travel mode with the same bore diameter is that excessively high mean piston speeds show up as a problem much sooner in full flame front travel mode than in late compression ignition mode. In the 3.01" stroke length 610 motor a 3,500RPM engine speed is an excessively high mean piston speed for full flame front travel mode operation, where the same 3,500RPM engine speed with the shorter 2.48" stroke length of the 475 big bore motor is a much more reasonable mean piston speed for full flame front travel mode operation.
Then finally there is the fact that the smaller 472.5cc displacement is going to be inherently better at supporting small loads than the big 577cc displacement of the 610 motor. Clearly less displacement can support smaller loads, so part of the reason why the 475 big bore motor seems to do so well under light loads is simply that it isn't such a light load for the smaller motor.
Which one of these factors is most significant in explaining the amazing small throttle opening performance of the 475 big bore motor running only 21 degree BTDC spark timing? Who knows. It's probably a combination of all of those factors adding up to surprisingly good performance at small throttle openings.
When I hit some single track trails I was even more impressed with the low end torque of the 475 big bore motor. Wow, third gear up steep climbs! Not only does the 475 big bore motor have plenty of torque to pull steep climbs at 3,000 to 4,000RPM, but even dropping way down to 2,000RPM around tight turns there was a surprisingly large amount of smooth torque available. Of course to get any really substantial amount of torque the engine speed had to be above about 3,000RPM, but on flatter trails I was amazed how slow I could let the bike speed drop in second and third gear around tight turns and just power right out without shifting and without slipping the clutch. An advance curve really is a huge advantage!
The only trouble I had at all with riding the 475 big bore motor on tight technical trails was that it got so quiet at very low engine speeds that I tended to lose track of what gear to be in. There just wasn't much of any indicator of what the engine was doing, and over and over again I found myself trying to open the throttle at excessively low engine speeds. And it wasn't just ending up in such a high gear that acceleration was impossible. Other strange things were happening also. One was that I actually found myself downshifting too much going down steep hills and then the compression braking was excessive and I couldn't keep the rear end in control. Normally I like to keep the engine speed down fairly low when on difficult steep descents to use the rear brake instead of the engine braking. At first I had some trouble with this on the new 475 big bore motor. It was just so quiet that I kept thinking that it was up a gear (or two) higher than it needed to be, but then after I downshifted the engine braking was so strong that the rear wheel was locking up and dancing around out of control. Part of this of course was the nearly bald rear tire I was riding on, and a big part of it is just getting used to a new motor.
It did take some getting used to to accept that the 475 big bore motor was going to be very quiet. Part of it is also just habits I have developed in riding the big 610 motors. With the longer stroke length the engine speed is much more critical, so I have tended to be very diligent about staying in just the right gear for each individual situation. On the 475 big bore engine speed isn't so critical, so the proper technique is just to let the engine speed go up and down with the terrain without shifting so much.
On a 200 mile mixed ride with lots of dirt roads and several short sections of single track the 475 big bore motor turned in a bit better than 50mpg, which is reasonably impressive. The 610 motors can sometimes do 50mpg in similar mixed rides, but the 475 big bore motor turned in better than 50mpg seemingly effortlessly on two different tanks of gasoline with quite a bit of tight second gear single track thrown in. And that's with just 21 degree BTDC spark timing above 3,200RPM on the big 98mm bore diameter. It's not quite the 65mpg of the 91.5mm bore 410 motor, but it seemed like pretty impressive mileage none the less.