Finally a Husqvarna 510. I had seen about a half dozen of them advertised for sale in the past four years, but they were always either slightly out of my price range or I was busy with something else at the time. The going price seemed to be about $3000 for a reasonably clean looking running 510, although some had been advertised for as little as $1500. This one was advertised as a 1987 six speed, but it had the short rear fender and "CROSS COUNTRY" on the swing arm indicating that it was the racing version and not the Enduro version. The asking price was $1200, and it was said to run. Run it did, but barely.
Purchase
Midnight Jetting Thrash
Gasoline Disappears
Recommissioning
Suspension
The 510 Motor
Fit and Finish
Exhaust System
510 Clones
Fastener Facts
Spark Legal
Before I even got out to look at the bike I knew what was wrong with it. The Husqvarna spec sheets list a 52 size pilot jet for the 40mm DellOrto carburetors on all of the 1986 through 1990 Husqvarna 510 motors. That's too small, impossibly small. The stock pilot jet size for the 40mm DellOrto carburetors on the 1990 through 2004 Husqvarna 610 motors is a 62 size, which is a bit too large. Only a bit too large though. Going down to 60 or 61 is better, and it seems possible to go down to the 58 size also although this is tricky. Anyway it is looked at the stock 52 size pilot jet is at least a half dozen sizes too small. I was guessing that something related to this error in the literature was why it was being sold so cheap.
The guy selling the 1987 Husqvarna 510 XC in question was rather vague about how long he had had it and what he had ever done with it. He said he had had the bike for quite a few years, but that he only fired it up and rode it around his yard every once in a while. He did mention that the previous owner had put 1990 forks on it, and that the exhaust system was also off of that 1990 bike. What was obvious was that this bike had not been ridden much at all in many many years. He did have gasoline in the tank though, and it fired up when he kicked it a few times. When it didn't fire at all on the first few kicks I asked if he knew if it had the stock pilot jet in it or not. He said he thought it probably did. It gave one little pop on about the fifth small kick without the choke, and then when he put the choke on it fired and died. Then without the choke it fired up and ran. It low idled right away, and he let it warm up without blipping the throttle at all. After a minute or so I took it out for a test ride. It ran fine at very small throttle openings. I just rode it around slowly and checked to make sure all the gears were there. All six present and accounted for. It seemed like a usable bike just the way it was. The seat cover was in usable condition with just one small tear at the back, the 110/100-18 Kenda K760 rear tire was in nearly new condition, both brakes worked, the clutch disengaged fully and worked fairly smoothly idling around. Also included was a spare rear wheel with an old 10 paddle "The Hooker" brand sand tire on it. When I handed over the $1200 in cash he produced a spare swing arm which he said was the 1990 swing arm of the bike that had donated the forks and exhaust system. He also handed over an original 1990 Husqvarna 510 Owner's Service and Tuning manual, which I immediately identified as quite a find.
I had checked the cranking compression when the engine was cold before firing up, and it was substantial for the 503cc engine. Then after the motor was run the cranking compression came up even a bit more. Not a brand new top end by any means, but fairly good. Perhaps just another one of these Husqvarna motors with a 0.040" end gap set on the compression ring.
When I got home it was already dark, but I started messing with my new 510 anyway. First off I fired it up. It started rather easily for me at an ambient temperature of 45 degrees Fahrenheit, although it did require prodigious use of the choke. Once running it was idling fine, but there was a bit of instability at first at very small throttle openings. It seemed like the pilot jet was extremely small, but once warmed up it did run consistently. The bike had an aftermarket Acerbis head light that was very bright and once adjusted illuminated my long dirt driveway very nicely. The bike seemed to work quite well. The suspension seemed fairly plush over small bumps at all medium speeds, although there wasn't much rebound damping at the front end. The rebound clickers were all the way in at the #7 position, and turning them all the way out didn't seem to make any difference. It seemed like the forks were low on oil or something, although they did soak up the bumps rather well and didn't seem totally devoid of damping. The rear end was noticeably a bit harsh, but the suspension did work.
At small throttle openings the engine ran fine, and it was able to restart with the kick starter. I noticed though that it wouldn't start with the headlight turned on. The headlight switch had to be turned off to start the bike, then once running the headlight could be turned on and the engine didn't stall or do anything horrible.
When I opened the throttle there was torque and power, but then the engine loaded up and almost stalled. It seemed like the spark plug had fouled, and the ignition was cutting out at all wider throttle openings. I limped back and tore the carburetor apart to see what was going on. I noticed that I could get the top of the carburetor off just by loosening the clamps and tilting the carburetor towards the left hand side of the bike, but it hit on the gas tank and wouldn't tilt far enough to get the slide to come out. I was able to pull the throttle return spring out and grab the needle and clip with a piece of bent wire. The needle clip, unsurprisingly, was set at the stock #3 groove on the stock listed K32 needle. I figured that was a big part of the problem, and I reset the needle clip to the first groove (leanest position).
Next I turned to the other end of the carburetor. The main jet was marked 170, but the hole was obviously way oversized. A 0.0695" diameter #50 drill stock dropped right through with no resistance and there was quite a large amount of side to side wiggle but a 0.0713" diameter #49 drill stock wouldn't start in at all. Probably about a 182 to 184 size main jet. Way too big. I set another main jet at the 174 size by running the same #50 drill just through by hand. The 0.0695" shank end would just barely start in, but wouldn't go through the finished hole. Smaller than a 176 size, and perhaps all the way down at around 174 size. Substantially the same size that I have in all my other 40mm DellOrto carburetors at this point.
Next up was the pilot jet. Sure enough it was marked 52, but a 52 size it was not. A 0.0216" diameter #74 drill stock dropped through with no resistance and some considerable side to side wiggle. A 0.0227" #73 drill stock shank end just barely started in both ends of the hole. That's about a 56 or perhaps 57 size pilot jet. Certainly a whole lot bigger than the 52 size, which didn't surprise me in the slightest.
I was at first undecided about what to do with the pilot jet. It had been running with that 56 size pilot jet, but only on gasoline with such a high energy density that the engine flooded and wouldn't run at all with the stock #3 needle clip position. I wasn't going to run the #3 needle clip position, but this still seemed to indicate an excessively small pilot jet. Then there was also the fact that the engine was somewhat hard to start from cold and was obviously too lean at the first crack of the throttle. I decided to go a bit bigger on the pilot jet. At first I ran my 0.0230" #73 drill through by hand, but I had forgotten that I had broken the tip off of that particular drill when I was thrashing around with trying to get a hole punched through plumbers solder when I had been making a 61 size pilot jet for my period correct 1991 Husqvarna WMX 610. The broken tip pushed the hole out way too big, and I ended up with a 63 size pilot jet. Too much of a change from 56 up to 63, that's for sure.
Again I had to solder up and re-drill a pilot jet, which is a difficult and annoying process with cheap little drills that won't cut through even plumbers solder. I did get another pilot jet soldered up and I finally got a hole punched through with a #74 drill. Then I repaired the broken off #73 drill by twisting it in a high speed drill motor and running it along a piece of 400 grit silicon carbide wet/dry paper at about 30 degrees from vertical. This did the trick, and when I ran the repaired #73 drill through by hand it cut a reasonable size hole. The 0.0230" diameter #73 drill stock was just able to go into the hole with nearly zero resistance and no noticeable side to side wiggle. That's about a 59 size pilot jet, which seemed just right. Small, but certainly larger than the 56 or 57 size pilot jet that the bike was able to start and run on.
With the carburetor re-jetted it fired up easily with the kick starter, but did require the choke. The engine ran much stronger with the jetting sorted out. All of the jetting work had been done in the garage where the bike was sitting, so I was quite sure it was in fact still the same gasoline in the tank. The engine idled much the same as it had an hour earlier, and it also ran much the same at the first small throttle openings. It was however perfectly stable with no lean stumbling right from the first few seconds of operation after firing up cold, so the 59 size pilot jet obviously was a bit bigger than what had been in the carburetor when I got the bike.
The really big difference though was when I opened the throttle farther, and at first it wasn't all good. The engine made lots of power, but only at higher engine speeds. It wouldn't take any substantial throttle opening across the midrange around 4,000 and 5,000RPM. It ran consistently, smoothly and reasonably powerfully at small throttle openings at all engine speeds, but when the throttle was opened farther across the midrange engine speeds the ignition cut out and the engine wouldn't make power.
I suspected that the very rich jetting had been blocking flow through the intake valves and reducing cylinder filling, and this turned out to be exactly what was going on. I pulled the spark plug out, and the gap was at 0.030". Wider than the stock 0.024 to 0.027" gap listed for the 1987 through 1989 Husqvarna 510 motors. I set the spark plug gap down to 0.024", and this solved the cutting out problem completely. Now there was lots more midrange torque, really very impressive torque for the 503cc 510 motor. The motor was running well with no surging at all at any engine speed, no cutting out anywhere at any engine speed and the torque was building nicely from about 3,000RPM up to the big power above 6,000RPM. Throttle response at all small throttle openings was good, and the engine was pulling well over a very wide range of engine speeds. A very good running engine to be sure. And smooth and quiet down low also. It felt and sounded like about 20 to 24 degree BTDC spark timing. There wasn't any noticeable excess harshness due to excessive spark advance and there also was quite strong torque without any surging. That's probably later than 24 degree BTDC spark timing, although with the weak spark SEM ignition it could be 25 or 26 degrees BTDC I suppose.
Then when I really twisted it out to see what sort of overrev it had the headlight burned out. Just as the engine speed got all the way up to where the power was going flat at about 9,000RPM or so the headlight went dead. That was scary to be sure, but it was on a wide section of dirt road and I was able to jump on the brakes and come to a stop. Luckily I had a little LED flashlight in my shirt pocket that I had been using to work on the carburetor in the dark, and I was able to use that to ride back. That was pretty much the end of the midnight riding, although I did ride the bike around a bit more with my little LED flashlight held in my mouth. Not anywhere near as good as a real headlight, but I did get to mess with the bike a bit longer to make sure that everything was working.
I could feel the spark timing backing off substantially at about 1,500RPM or so, and the advance seemed to come all at once like on the later 1990 through 1994 Swedish SEM ignition systems on the Husqvarna 610 motors. This is a very similar Swedish SEM ignition on the 1980's Husqvarna 510 motors, but there is one significant difference. The flywheel is behind the stator on the 510 motors, where the stator is behind the flywheel on the 610 motors. That means that there is no easy way to watch the advance mechanism with a timing light on the 510 motors. The 1987 Husqvarna "WORKSHOP MANUAL - LIQUID COOLED FOUR STROKE ENGINE" says that the timing can be checked by shining a stroboscopic timing light through the little 6mm bolt hole. I didn't try this yet myself. The up side of the 510 ignition systems is that the spark timing can be adjusted simply by loosening the three screws on the outside of the timing cover and turning the stator. I didn't try this either as the spark timing seemed perfect just as it was. It looks to be right in the middle of the range of adjustment judging by the location of the screws relative to the flat spots on the timing cover casting. It looks like the spark timing has not been moved on this particular motor in a long time. It actually looks like the spark timing has never been moved. It looks like it is probably the original spark timing that the motor came set at way back in 1987. This SEM ignition does appear to work. Even with the spark plug gap set way out at 0.030" the engine was still able to start fairly easily with the kick starter.
The next day the 1987 Husqvarna 510 again fired up fairly easily with the kick starter at an ambient temperature of 60 degrees Fahrenheit, but there were signs of trouble right from the first seconds of operation. There was lean stumbling at small throttle openings, and it was difficult to keep the engine running. In fact it did stall after about 10 seconds, but it restarted easily on the first kick. It was still lean stumbling at small throttle openings for a few more seconds, but once warmed up it was running and idling. The low idle was however much more unstable than it had been the night before, and there was a slight bit of noticeable ethanol exhaust smell. Not the acrid alcohol exhaust smell that I associate with methanol, but the milder ethanol exhaust smell.
The 510 motor still was able to make some torque and power without any surging, but both torque and power were down compared to the night before. Instead of the instant strong torque of the night before there was now some lag and reluctance to get going and the engine was requiring much larger throttle openings to make torque. There was popping out the exhaust on deceleration. Not horrendous amounts of popping out the exhaust, but it was popping out the exhaust on deceleration. Obviously the gasoline had disappeared, and this was very disappointing.
Then four hours later when I rode the 1987 Husqvarna 510 again at an evening ambient temperature of 56 degrees Fahrenheit it wasn't so bad. It still seemed a bit lean and weak compared to the night before, but the popping out the exhaust was mostly gone and throttle response at small throttle openings was again quite good and overall the torque and power seemed quite impressive for the 91.5mm bore 510 motor. There still wasn't any surging, but there was some of that early sound to the power way up at the top of the power band. That sound of a big engine running a bit too high of a mean piston speed on pump gas.
On subsequent days the gasoline seemed to change back and forth between two different types. One had both a higher energy density and a higher temperature of combustion potential and the other had both a lower energy density and a lower temperatuer of combustion potential. There was still no surging from the 3.01" stroke length 510 motor at any time, and it was always able to make some substantial power. Some days more power than other days though, and the lower power output was always accompanied by that early screaming sound up at the higher engine speeds at the top of the power band. The lower power output and screaming sound was also always accompainied by more popping out the exhaust, less spectacular throttle response at small throttle openings and usually also harder starting. It was never such low energy density gasoline that the 59 size pilot jet in the 40mm DellOrto wouldn't work, but there were times when I noticed little bit of stumbling weakness at the first crack of the throttle even once the engine was fully warmed up.
One day when the power was seeming particularly impressive with great throttle response at small throttle openings I was getting the impression that the 174 size main jet was actually probably bigger than required on that particularly high energy density gasoline. It was still pulling great at all throttle openings with strong instant torque everywhere, but even down at 1,000 feet of elevation I got the impression that it was a bit more gasoline than strictly required to make power. That was however the gasoline that worked well with the 59 size pilot jet, as there wasn't the slightest bit of an overly lean feely at the first crack of the throttle and there was no popping out the exhaust on deceleration.
One day when the energy density was seeming severely low with poor throttle response at small throttle openings and lots of popping out the exhaust I also noticed a few other signs of low energy density gasoline. One was that the engine was requiring the choke to restart after sitting for a while even though the cooling jacket was still substantially warm to the touch. I also noticed that if I turned the idle mixture screw in at all the low idle became very unstable even with the engine fully warmed up.
As I looked over my new 1987 Husqvarna 510 XC I noticed a few strange things. The compression release was made up of strange mismatched parts. The arm on the engine isn't the stock type, but rather a clutch arm repurposed for the compression release, it does sort of work although the angle on the cable is wrong so the cable will tend to wear out more quickly. The manual compression release lever is a Magura unit mounted down under the left grip. This setup does sort of work, but it's not as good as the stock 1990/1991 manual compression release.
The Magura lever is a bit in the way, and several times I accidentally pulled in the compression release instead of the clutch after first firing up the engine. There was a piece of black electrical tape holding the cable end into the Magura lever. When I took this tape wrapping off the cable end fell out. My thoughts were that a German made product from that era would probably work without some funky added on electrical tape. I figured there must be some way to get the cable end to stay in. What I eventually figured out was that the cable end was just twisted up the wrong way. The cable end had been twisted in the direction to unwind the strands of the cable, which tilted visible side of the barrel away from the handle bar grip. I turned the cable end a few turns in the other direction, the direction that tends to wind the strands of the cable together tighter, and this caused the visible end of the barrel to tilt towards the handle bar. This did the trick. With the barrel tilted in the opposite direction the action of the lever tended to push the barrel end farther into the lever instead of popping it out as had happened when I first took the electrical tape off. Sure enough the traditional German product worked perfectly. The only problem was that the correct operating procedure hadn't been provided, or had been ignored.
The Brembo front brake system had been working when I got the 1987 Husqvarna 510 XC, but there were signs of trouble. The action of the lever was a bit chunky. It felt like old thickened fluid. The screws holding the master cylinder reservoir cover on had been painted over, and under the paint they were rusty and stuck. I did get them to come out without stripping the heads, but it took a #2 Philips bit in a 1/4 socket and lots of down force. Sure enough the brake fluid was brown and a bit gloppy. The bleed screw was also quite stuck, but did break loose. Once I had flushed the old brown and thickened brake fluid through with fresh DOT 4 fluid the front brake system worked much better. The lever moved smoothly, and the braking action was consistent and powerful. The cross-drilled rotor appears to be in like new condition, although a bit rusty. (230mm vs 240mm front brake rotor UPDATE May 7th 2018 - At first I thought it was a 230mm rotor as it looks just like the 1987 230mm cross-drilled rotors, but it actually has the same 135mm bolt centers and 240mm outside diameter as the slotted 1991 240mm rotors)
In fact much of this bike is a bit rusty. Although it was obviously meticulously cared for by a dedicated owner it did get left out in the rain for a number of years at some point. Nearly all the bolt heads are rusty, and everything looks a bit weathered. Interestingly though many of the bolts I loosened had been installed with a bit of waterproof grease and came out very easily. Obviously this bike was well maintained for much of it's long 30 year history. Someone really went the extra mile, including installing quite a few fancy titanium aircraft bolts and nuts. The whole exhaust system was held together by fancy titanium aircraft bolts, and there were a few other titanium bolts here and there. Actually I ended up removing most of the other titanium bolts in the course of the initial overhaul. The lower chain roller was on the wrong side of the chain and had to be removed and reinstalled. I did this that first night before I rode the bike, and at first I was confused by the titanium bolt holding the roller on. I didn't know what it was, so I just took it off and replaced it with a regular grade 8.8 carbon steel 8mm bolt. When I got the titanium bolt out it was very light and I realized that it was in fact an aircraft fastener. At first though I was just confused by the strange 8mm nut with a small hex for a 10mm wrench. I thought someone had drilled a 6mm nut and tapped it for 8mm threads, so I just took it off. When I tried to put an 8mm nut on the bolt it went on only a short distance and jammed. Not an 8mm bolt at all but rather a 5/16 fine thread bolt.
It turned out that there were quite a few of these fancy titanium aircraft fasteners on this bike, but not really enough of them to make a significant difference in the overall weight. If every fastener on the engine cases had also been replaced with titanium that would have added up to a substantial weight savings. As it was though just a half dozen or so titanium bolts here and there was only enough to add a bit of exotic pizzazz.
One NAS675V-20 1-3/4" long 5/16-24 bolt with it's 3/8" hex flange nut weighed in at about 14.5g, and an equivalent 45mm long 8mm bolt weighs 27g. These titanium fasteners are about half the weight of normal bolts, which is a lot lighter. It does however take a lot of them to add up to a significant weight savings.
The other main upgrade that this 1987 Husqvarna 510 XC needed right away was a kick starter bump pad. The 1980's two stroke Husqvarnas come stock with a nice little rubber bumper to cushion the impact of the kick starter coming back up. Although the clutch covers on the four strokes are nearly identical to those on the two strokes they lack the mounting pad for the screw that holds that little bit of rubber bump stop in place. What I have figured out is that a piece of an old tire can be used as a kick starter bump stop if it is long enough to reach up to the 4mm mounting holes for the stock plastic kick starter holder. This isn't a stock piece, but I have one on all of my 1991 Husqvarna motors and it was a simple little project to make one up for the 1987 Husqvarna 510 motor also.
The old O-ring chain was a bit rusty, but in serviceable condition at 12.55" for a 20 link length. The stock size 14 tooth front sprocket and stock size 52 tooth rear sprocket were likewise in good condition, although a bit old and rusty. The aluminum rear sprocket is another one of those AFAM brand French built pieces, but not nearly as minimalistic and light as the spidery looking AFAM brand 50 tooth aluminum rear sprocket I have on my 1992 Husqvarna 610.
When I got the 1987 Husqvarna 510 XC the coolant was low, but there was still some coolant in the engine. I filled the cooling system with water, and the resulting mixture had a noticeable light green color. I rode the bike like that for several days, and then I drained cooling system and refilled it with fresh coolant. At the same time I took the funky homemade water pump guard off and replaced it with a stock 1980's Husqvarna water pump guard. Then I was having some trouble with coolant leaking out. At first I thought it was the water pump housing, but when I moved the radiator overflow tube to the right hand side of the bike I could clearly see that no water was leaking out anywhere around the water pump housing. It was the radiator overflow that was spurting, and I also noticed that there was a lot of pressure in the radiator even after the engine had cooled off. This looked like a blown head gasket, and when I put a wrench on the two exposed 10mm head bolts they were very loose. I decided to pull the rocker cover off to see if other two 10mm head bolts were also loose.
It's fairly easy to take the rocker cover off on the 510 motors as the crankshaft mounted water pump means that the cooling system doesn't even need to be disturbed. The rocker cover bolts on the 1987 Husqvarna 510 XC were rather stuck, but I did get them all out without breaking any or stripping any of the 6mm Allen heads out. The rocker cover was glued on with what looked like the factory original white RTV silicone. It looked like this rocker cover had never been off in over 30 years. All the head bolts turned out to be very loose, with the torque wrench reading less than 30 foot pounds as I started tightening them down. I torqued the 10mm bolts to the 36 foot pound spec for the 510 motors, and I tightened the lower 8mm bolts which also were very loose. The bolts went down nearly a half a turn, which seemed like a lot. The head and base gaskets didn't appear to be squished down much at all. It appeared that the bolts just hand't been torqued down tight when this engine was originally put together. It had been just barely enough torque to seal, but eventually the gaskets did squish down that small amount that allowed a small amount of high pressure combustion gasses to get past the sealing ring.
While I had the rocker cover off I had a chance to get a look at the camshaft, which appeared to be a stock type Husqvarna camshaft very similar to what came in the 1991 Husqvarna WMX 610 motors.
When I checked the valve lash I found the intake valves at 0.004" and the exhaust valves at 0.008", which is a fairly good setup and I retained these settings.
With the head bolts torqued down to the spec there was no more spurting coolant, and the excessive pressure under the radiator cap was also gone. Luckily all it took was re-torquing the head bolts, which was a lot easier than taking the cylinder head off.
The linkage and swing arm bearings on the 1987 Husqvarna 510 XC seemed to be in good condition, but they were in sore need of new grease. The easiest way to lube the swing arm bearings seemed to be just to pop in an extra 1990's swing arm pivot bolt that I happened to have. I took the zirk fitting out of the end and used the new bolt to drive the old bolt out. This went very easy, but then I realized that I needed to install the new pivot bolt from the right hand side of the bike since the 1980's brake lever didn't clear the nut on the 1990's pivot bolt. Then I remembered that I needed to take an inner race out of each side to cut grease slots, so I had to pull the pivot bolt out and tilt the swing arm up to gain access to the inner races. With the bolt in from the right side the upgrade was complete, although I did have to add two 9/16" washers under the nut since the threads on the 1990's bolt didn't extend quite far enough.
Next up was the linkage. One side of the 1985/1986 style linkage took grease without difficulty, but the other side was plugged and the grease wouldn't go in. I had to take that side of the linkage apart to get the grease flowing. It turned out that the linkage needed more substantial service anyway, as all the bolts were somewhat loose. Those bolts do need to be tightened down sufficiently so that the inner races are held securely and don't wiggle around and wear the bolts out.
The bearings looked to be in fairly good condition, although they were getting dry and a bit rusty. This linkage really was in sore need of new grease. While I had the linkage apart I noticed that someone had replaced the original seals with big thick O-rings. These big thick O-rings seemed to work fine, and once greased the linkage moved freely without any noticeable excess friction.
The 1986 style kick stand mounts on one of the linkage bolts with a long bolt and a spacer, so that bolt having been loose had caused the kick stand mount to be a bit floppy. I also reinstalled the lower kick stand mounting bolt with loctite, and this got the mount to stay solidly in place on the swing arm. While I was overhauling the kick stand I replaced the worn out old pivot bolt. As is common on these 1980's and early 1990's Husqvarnas someone had put a bolt with threads all the way up to the head in as a kick stand pivot bolt, and of course the threads had eventually worn out and introduced more wiggle into the kick stand. A new bolt with an unthreaded shank up through the kick stand reduced the wiggle, and the bike then sat more upright on it's stand.
That first night the forks on the 1987 Husqvarna 510 XC had felt plush and comfortable, but drastically under damped even with the compression clickers at the #3 setting and the rebound clickers all the way in at the #7 position. Then the next day the suspension again seemed quite good. The forks seemed a bit strange feeling, but substantially functional and very much like the stock White Power 40mm USD forks on my 1991 Husqvarna WMX 610 bikes.
These forks are very much like the forks on the 1991 Husqvarna WMX 610 bikes. They are the longer version of the 40mm USD White Power 4054 Multi-Adjuster forks like on the 1990 and 1991 Husqvarna WMX 610 as opposed to the shorter version of the 40mm USD White Power 4054 Multi-Adjuster forks used on the 1991 Husqvarna WXE and the 1990/1991 Cagiva Husqvarna two strokes. It is the longer version that has the competent valving, and this seems to have been true in the 1980's as well as in the early 1990's.
These forks on my 1987 Husqvarna 510 XC do appear to be off of a 1990 model Husqvarna 510 as the caps are stamped "11-89" and "TC/TX 510". There was however obviously something strange going on with these forks. There were signs that large amounts of fork oil had been leaking at some time in the past, but then there wasn't much oil leaking out when I rode the bike. I thought the oil level was low.
Then the next day though the forks felt a little bit different. It seemed like there was a bit more damping, although the rebound damping was still extremely minimalistic even with the rebound clickers all the way in at the #7 position. Then there was no oil leaking out of the fork seals at all that next day either. A small bit of oil had been leaking the night before, but then all of a sudden there wasn't the slightest bit of a leak. When I cracked open the bolts on the bottoms of the fork legs I got a big surprise. The oil smelled strongly of some kind of light mineral oil, and it was very thin. Much thinner than 5W oil, and it seemed thinner than 2.5W BelRay fork oil also.
I decided to drain this strange oil out, which I did with the forks installed on the bike as the fork seals didn't appear to need replacing. I pulled the damper cartridges out the tops of the forks with the handle bar removed, and this allowed me to look over the springs as well. What I found was one spring like came stock on the 1991 Husqvarna WMX 610 with 0.187" diameter wire and 0.315" between the coils and one of the lighter weight springs with 0.182" diameter wire and 0.365" between the coils. The lighter spring was damaged, with a free length of just 20-11/16". All of the different springs for the 40mm USD White Power forks are about 20-15/16" long when new and in good condition, so the substantially shorter length was not a good sign. It had probably been run with a very large amount of pre-load at some point in the past. I replaced this damaged spring so that both legs have the slightly softer (but slightly heavier) WMX 610 springs. Two of the softer springs seemed appropriate for the 1987 Husqvarna 510 as it is in fact a slight bit lighter than a 1991 Husqvarna WMX 610. For oil I used the 4W Motorex oil that I had briefly tried in my 1992 Showa forks. After sitting for months there was some gunk on the bottom of the container, but the oil that came off the top looked substantially clean and clear.
I used 15mm pre-load spacers when I put the forks back together, but I noticed that this resulted in substantially less total pre-load versus the same spacers in the 1991 Husqvarna WMX 610 White Power forks. They look very much like the same damper cartridges, but the there is a slight difference. The 1990/1991 Husqvarna 610 White Power forks have welded on spring mounting points, where the 1990 Husqvarna 510 forks have separate lower spring mounts. The significant difference is that the 510 White Power 4054 forks mount the bottoms of the springs lower by about 5mm or so compared to the 610 White Power 4054 forks. This means that the 510 ends up with less pre-load on the springs with the same pre-load spacers. The 1987 Husqvarna 510 does seem a bit lighter. It feels more like the 1986 and 1987 Husqvarna two strokes. Even though the frames are essentially identical the 1986 and 1987 models do feel a bit smaller than the 1990/1991 Husqvarna 610. I had previously thought that was just the weight difference and shorter 40mm Husqvarna conventional forks. Then the 1987 Husqvarna 510 XC with the exact same long 40mm USD White Power forks as the 1991 Husqvarna WMX 610 does in fact feel a bit smaller than the 1991 Husqvarna WMX 610. There might be a steering head angle difference. The 1980's Husqvarnas are listed as having 27 degree steering head angles, where the 1991 Husqvarna WMX 610 is listed as having a 30 degree steering head angle. The 1987 Husqvarna 510 XC does feel lighter also. The 1987 Husqvarna 510 XC sits a bit lower in the back despite the long 17.25" XC/Motocross version of the Ohlins shock, so that is part of it also. A lower center of gravity tends to make a heavy bike feel a bit lighter. The 1987 Husqvarna 510 XC does really feel a bit lighter though. Surprisingly the scale doesn't really bear this out.
On a cheap spring scale mechanical dial type bathroom scale a bone stock 1991 Husqvarna WMX 610 appears to weigh very close to 257 pounds ready to ride without gasoline. Assuming 9 pounds for the lube oil, fork oil, shock oil, coolant and brake fluid this is very close to the 247 pound dry weight listed for a 1991 Husqvarna WMX 610.
On the same cheap bathroom scale my 1987 Husqvarna 510 XC seems to weight perhaps one pound less ready to ride but without gasoline. That's with a kick stand, headlight and six speed gear box on the 1987 Husqvarna 510 XC and no headlight or kick stand on the five speed 1991 Husqvarna WMX 610. Yes, the 1987 Husqvarna 510 XC is slightly lighter, but it is a very small difference. In riding the bikes though it feels like a bigger difference. The 510 feels much smaller and considerably lighter.
With 15mm pre-load spacers and the springs that are stock for the 1991 Husqvarna WMX 610 the 1987 Husqvarna 510 XC was then sitting with 1-3/8" of bike sag at the front, which is very similar to what I usually run on the 1991 Husqvarna WMX 610 bikes. With the 4W oil the forks on the 1987 Husqvarna 510 XC worked better, but there still wasn't much damping. I started out with the rebound clickers at the #3 setting and the compression clickers at the #2 setting. The ride was just as smooth as with the even thinner mineral oil, but the front end still felt drastically under damped. I then gave the rebound adjusters two more clicks in for the #5 setting. This helped, but there wasn't much difference. Noticeably a bit more rebound damping, but still not much. Still perhaps not quite enough rebound damping. Then I went in on the compression clickers to the #4 setting. This also made a noticeable improvement with increased hold up and no noticeable extra harshness over small sharp bumps, but again it wasn't much of a change. I think these 1990 Husqvarna 510 40mm USD White Power forks may actually be valved for 10W oil. The 4W certainly does work, but it was seeming like they were going to be better with even thicker oil.
The Ohlins shock on this 1987 Husqvarna 510 XC seems to work. It hasn't been leaking, and there are no visible signs of it having leaked oil in the past. There certainly is a bit of that strange Ohlins harshness, but not nearly as bad as on the other 1986 and 1987 Ohlins shocks on the two stroke enduro bikes with conventional Husqvarna forks. This Ohlins shock on the 1987 Husqvarna 510 XC does really seem to work rather well, and it remains plush and comfortable over all types of bumps up to high speeds. Going out from 15 clicks in on the compression adjuster to just 3 clicks in on the compression adjuster did seem to smooth out the ride. Then later it was still seeming like too much compression damping and the rear end was sort of kicking up over medium sized bumps at higher speeds. I then went all the way out on the compression clicker, and again this smoothed out the ride and eliminated the kicking up. Clearly this more competent Ohlins shock still isn't as fast and competent as the 1991 Husqvarna WMX 610 White Power shocks, but it's fairly good. Sort of a second best I would say. Is the Ohlins shock on the 1987 Husqvarna 510 XC better than the Sachs shock on the 1999 Husqvarna TE 410? Perhaps. The Ohlins shock on the 1987 Husqvarna 510 XC does seem plusher and more comfortable than the 1999 Sachs shock, but perhaps the 1999 Sachs shock does do a bit better with very aggressive riding over rough terrain at high speeds. It's hard to say which is faster, but the Ohlins shock on the 1987 Husqvarna 510 XC certainly is a lot more comfortable across all lower bike speeds compared to the 1999 Sachs shock.
After riding the 1987 Husqvarna 510 XC around for a few days it was clear that the 4W oil wasn't thick enough for these "TC/TX 510" White Power forks. Going all the way in on the compression clickers hardly made any noticeable difference, and although the forks worked fairly well they just felt very under damped most of the time. Even with the rebound clickers nearly all the way in the forks still came up so fast that they topped out with a noticeable clank. Amazingly the rebound damping was still feeling sufficient to keep the bike in control, but only barely. I had been planning to switch to 10W fork oil, but then one afternoon when I was messing around with going all the way in on the clickers I decided to just go ahead and switch to the Dextron II/ Dextron III automatic transmission fluid that I already had on hand. I drained the 4W Motorex fork oil out, and added 500ml of the ATF from a sealed container. As I poured the ATF in I was struck by how much thicker it looked than 4W or 5W fork oil. I stared out with both the compression and the rebound clickers at the #3 setting. Right away I noticed an improvement in the ride and performance, but I was very surprised that the change was not all that large. I went all the way in on the compression clickers, and again there wasn't all that much difference. Then I went all the way out on the compression clickers, and again it wasn't a huge change. When I went all the way out on the rebound clickers the forks were noticeably very fast to come up, and then when I went all the way in on the rebound clickers there was quite a bit of rebound damping. It still didn't seem like much of a change from all the way out to all the way in on the rebound clickers, but it was enough of a change that there was obviously too much rebound damping with the clickers all the way in and obviously not enough rebound damping with the clickers all the way out. In the middle at the #3 position the rebound damping felt pretty good. I rode the bike around for a while like this with the compression clickers all the way out and the rebound clickers at the #3 setting. The suspension seemed to work fairly well, although there wasn't much compression damping. When I went in on the compression clickers to the #3 setting there was a significant and very noticeable increase in hold up over rolling bumps and on jump landings without much of any increase in excess harshness. Then I went one more click in for the #4 compression setting, and again it seemed like an improvement without much of any increase in harshness. I also went in one more click on the rebound adjusters for good measure, which made only a slight difference.
The forks were then feeling substantially damped, but still fairly plush and comfortable. Clearly these "TC/TX 510" 4054 Multi-Adjuster White Power forks are valved quite a bit differently than the 4054 Multi-Adjuster White Power forks on the 1991 Husqvarna WMX 610 bikes. On the 1987 Husqvarna 510 XC with the much thicker ATF (approximately 15W) in the forks I still ended up at the #4 compression setting and the #4 rebound setting, where on the stock 1991 Husqvarna WMX 610 White Power forks with 5W fork oil I usually run the #2 compression setting and the #3 rebound setting. Sometimes I go up to the #3 compression setting on the stock 1991 Husqvarna WMX 610 White Power forks, but that is tons of hold up that usually is only required for very aggressive riding on a motocross track.
As different as the valving between the 510 White Power forks and the WMX 610 White Power forks the net result is similar. Once dialed in they both work well. They are not identical, but they both work well. The "TC/TX 510" White Power forks are easier to get to be extremely plush over all types of sharp bumps, where the 1991 Husqvarna WMX 610 White Power forks perhaps a bit faster for aggressive riding.
Once I got the suspension really dialed in and I was riding the 1987 Husqvarna 510 XC a bit more aggressively the old 3.00-21 Dunlop K130 front tire started to show it's age. Traction on hard dry terrain was not all that great, and the hard tread blocks were also chunking off pretty bad. Not the worst tire I have seen, but not great either.
With the same 40mm DellOrto Carburetor and the same approximately 7" total intake stack length as the 577cc 610 motor the 503cc 510 motor would be expected to have the intake stack boost come on at a 15% higher engine speed. The intake stack boost hits at about 7,500RPM on the 610 motor, so the intake stack boost on the 510 motor would be expected to first come on at about 8,600RPM. At that higher 8,600RPM the intake stack boost wouldn't tend to be as noticeable.
The 510 motor does in fact rev easily and reliably beyond 9,000RPM, but the power isn't all that spectacular up there. The power does usually stay strong up to at least 9,000RPM, but it's not increasing much past 7,000RPM either. The heavy piston and big heavy connecting rod obviously cut into power output up at those very high mean piston speeds, but the long 3" stroke length also does better down lower. The combination of a heavy reciprocating assembly, a long 3" stroke length and a rather small camshaft all conspire to keep the strongest torque down bellow about 7,000RPM. Power does usually increase a bit as the engine speed is increased past 8,000RPM on the 510 motor, but it obviously is a more bottom end and midrange oriented motor. The spectacular aspect of the 510 motor is the strong torque everywhere from about 3,000RPM up to 7,000RPM, and the strong overrev past 9,000RPM means that the strong power keeps pulling long and hard if shifting needs to be put off a bit for some reason.
The water cooled 1987 Husqvarna 510 four stroke is a very good running motor, as long as the gasoline has a high enough temperature of combustion potential to support the long 3" stroke length that is. On that rather powerful gasoline that doesn't cause any surging at the 3.01" stroke length the 510 motor pulls some very good torque across a wide range of engine speeds. It's not quite as monstrously powerful as the 610, but it does pull hard. The 503cc 510 motor is 87% the displacement of the 577cc 610 motor. With the same stroke length the power would be expected to be proportional to the displacement. The reality though is that the 510 is closer to the output of the 610 than 87%. They have the same 35mm intake valves, 30mm exhaust valves, the same 40mm DellOrto carburetor and similar camshafts. The 40mm DellOrto is big enough for the 610, but only barely. On the 510 it is an oversized carburetor and it doesn't quite need wide open throttle to pull it's hardest. The valves though are a different story. The 35mm intake valves choke off the 610 motor a bit, and that means that the 510 with the same valves does a bit better. It's probably something like 91% as much power from the 510 as from a bone stock 1990-1995 Husqvarna 610 motor with the same 35mm/30mm valves. The big valve 1996 and later Husqvarna 610 motor makes quite a bit more power, and a big camshaft on the 1991 Husqvarna 610 motor also makes quite a bit more power.
Direct comparisons are tricky though, as both the 1987 Husqvarna 510 and the 1990 through 2004 Husqvarna 610 motors share the same 10:1 compression ratio. With a smaller 91.5mm bore the 510 motor has an easier time running that low 10:1 compression ratio, where the big 98mm bore 610 motor tends to have a bit more difficulty dealing with that low of a compression ratio.
When I actually tried the same gasoline in both the 1987 Husqvarna 510 XC and a Husqvarna 610 motor though I got a few surprises. The big surprise was that on gasoline that the 510 motor had not been surging at all on the 610 motor was exhibiting some little bits of surging here and there. Not much surging at all, but noticeably some little bits of surging that had been absent when running the same gasoline in the 510 motor. They both have the exact same 76.5mm stroke length, so this was a bit of a surprise. The difference might have been a bit of a surprise, but it was easy to explain from the perspective of spark timing.
On a quart of gasoline out of the 1987 Husqvarna 510 XC my bone stock 10.2:1 Czech Republic CDI 610 motor ran well and pulled quite strong to 7,800RPM without any cutting out anywhere. The 510 had been running fairly strong on that gasoline also, with good throttle response at small throttle openings, smooth powerful torque over a wide range of engine speeds and fairly good top end power that revved out freely. Not quite as strong as that first night that I ran the 1987 Husqvarna 510 motor with the 174 main jet and the needle clip in the first groove position, but still reasonably good performance. Interestingly, and perhaps unexpectedly, the bone stock 10.2:1 Czech Republic CDI 610 motor running 26 degree BTDC spark timing was actually crisper than the 10:1 510 motor had been on the same gasoline. The 510 motor had been running sufficiently crisply with instant reliable torque, no hesitation and hardly any lag anywhere; but the 610 motor was even crisper. Overly crisp even, and it was this excessive crispness that seemed to be causing some slight surging around 5,000 and 5,500RPM. It's actually nearly 28 degree BTDC spark timing up there at 5,000RPM with the extra degree and a half or so of advance from 3,000 to 5,000RPM, and that is quite a lot of spark advance. It was seeming like the surging was due to excess crispness and also just the large amount of spark advance. This gasoline was making power and throttle response seemed reasonably good, but it did have a bit of ethanol in it. The 10.2:1 Czech Republic CDI 610 motor was popping out the exhaust a little bit on deceleration, and I did notice a very slight bit of ethanol exhaust smell. It obviously wasn't severely watered down gasoline, but there was that little bit of ethanol exhaust smell and the slight popping out the exhaust.
There was also a bit of a surging feeling right at 3,000RPM, but that was just that old problem of the 3,000RPM advance curve shoulder being a bit too high for anything but extremely high temperature of combustion potential gasoline on the three inch stroke length. On this moderate gasoline the 3,000RPM advancer curve shoulder certainly felt too high, and there was a bit of strange surging right there at 3,000RPM sometimes. Particularly when the throttle was opened aggressively right from 2,600RPM the response was a bit awkward until the engine speed came up above 3,100RPM.
Both 76.5mm stroke length engines were running amazingly smooth way down supper low. Torque down to 2,500 and even around 2,300RPM was surprisingly smooth and also quite strong. Torque at small throttle openings way down at 1,200 and 1,500RPM was also very smooth and quiet and very usable in both engines. Sort of amazingly smooth and quiet torque way down at very low engine speeds.
Both three inch stroke length engines ran very similarly on the same gasoline, but the difference in bore diameter and the difference in spark timing certainly was noticeable. It seems like the 1987 Husqvarna 510 XC is probably running less than 24 degree BTDC spark timing. I don't know what the exact spark timing value is, but it might be way down at 21 or 22 degrees BTDC. What ever it is it works, and the 26 degree BTDC spark timing that I have set on the 10.2:1 Czech Republic CDI 610 motor is obviously on the early side. Not so early that it can't work at all, but earlier than would be ideal for best possible torque delivery.
On the same gasoline the 510 motor did run better. The 577cc motor is simply too much displacement per cylinder. Even with the same stroke length the smaller bore engine was able to run a bit better, with more easily modulated power delivery and overall better performance. As far as power goes though the 610 motor did certainly make more. Was it 8% more or was it 13% more power than the 510 motor on the same gasoline? Actually it seemed like 20% more power, it's just the power was a bit more unpredictable and harder to use from the bigger motor. With the same bottom end and the same transmission the smaller displacement motor is at a bit of a disadvantage to be sure. The 610 motor I was using has the heavy 406g stock Mahle piston, but then the 1987 Husqvarna 510 motor probably also has more weight on the piston than would be ideal. And that brings up another interesting comparison.
The 1987 Husqvarna 510 motor runs smooth and feels well balanced. It seems to run as smooth as my modified 610 motors with the drastically lightened cut-down 332g Woessner pistons. I am guessing that the bottom end on the 510 really is exactly the same as the bottom end on the 610, even to the extent of having exactly the same balance job. The 610 needs a lightened 330g piston to run smooth, and I am guessing that the 91.5mm piston in the this old 1987 Husqvarna 510 motor probably weighs quite a bit. It's probably one of those heavy old Mahle pistons, and it might be as much as 350g.
The old 510 motor might be a bit on the heavy side, but it is certainly a free flowing high revving high performance motor and it does make some substantial top end power. Interestingly though what is most impressive about the 510 motor is that it manages to feel a lot smaller than the 610 motor. There is less power and the power is consistent and easy to use. On top of that the 510 runs down to just as low engine speeds as the 610 motor since they both have exactly the same stroke length. As long as the gasoline has a high enough temperature of combustion potential the 3.01 inch stroke length does work rather well. It might be a perceptual mistake, but there certainly is something more appealing about the sound of torque at 3,000 and 4,000RPM from a three inch stroke length engine versus torque at 4,500 to 5,500RPM from a two inch stroke length engine.
That's the magic of the 510 motor, it has that long 3" stroke length to keep the engine speed down somewhat but the displacement is also low enough that it is sort of reasonable for a dirt bike. Or at least compared to the wild tire shredding and front end lifting of a strong running 610 motor the 510 motor tends to seem much more appropriate.
Another difference that has to be mentioned are the rear tires. The 1987 Husqvarna 510 has that new Kenda K760 on it, where my bone stock 10.2:1 Czech Republic CDI 610 motor is still spinning the same tired old Dunlop D707 rear tire that was on the bike when I got it. More torque and less tire, that equates to lots of breaking traction and a wilder ride when the throttle is substantially opened.
And finally the 1987 Husqvarna 510 XC Ohlins shock was actually seeming to do a better job than the flat White Power shock on that particular 1991 Husqvarna WMX 610. Compared to some of my other 1991 Husqvarna WMX 610 White Power shocks that flat one on the bike with the 10.2:1 Czech Republic CDI 610 motor is certainly not as good. The compression clicker doesn't work, and it is a bit harsher with the thickened old oil. The 1987 Husqvarna 510 XC Ohlins shock felt smoother and more comfortable, and it also had noticeably better hold up over rolling bumps and landing form little jumps. This Ohlins shock actually seems to work, even at higher speeds where the 1986 and 1987 Ohlins Enduro shocks get harsh and unpleasant. No, this Ohlins shock actually seems to work. It might not be as amazingly fast as a stock 1991 Husqvarna WMX 610 shock, but pretty darn good and very smooth and comfortable over all types of bumps at all speeds.
The main disadvantage of the 1987 Husqvarna 510 XC is that drum rear brake. It works, but it's no disk either. The drum rear brake on this 1987 Husqvarna 510 XC actually works great as far as drum rear brakes go. Especially after I lubed up the pivot shaft and engagement lobes the action of the rear brake was especially smooth and precise for a drum brake. I can't say I have ever seen a better drum brake on a dirt bike, so from that perspective it's a high performance Husqvarna part. But then that awesome four stroke motor and plush but well controlled and fast suspension is something I associate with a disk rear brake. I very easily forget that I am on a 1987 Husqvarna. It feels so extremely similar to the 1991 Husqvarna WMX 610 that I totally forget about the drum brake; until I stomp on it that is. It works, but it's not as powerful as the disk brake and the modulation is different and not quite as good. Mostly it is just different. If I had never ridden a bike with a disk rear brake I wouldn't have anything to complain about. It seems to work just fine for most purposes, but a disk brake it most certainly is not.
The rest of the 1987 Husqvarna 510 XC is, well just like a 1987 Husqvarna really. Squint this way and it looks like the 1987 Husqvarna 430 two stroke that shares much of the same plastic. Squint that way though and it looks just like a 1991 Husqvarna WMX 610 that has the same forks, same front brake system and a largely similar looking motor.
The motor does look a bit different though. The ignition cover is huge compared to the Husqvarna 610 motors, and it looks a bit vulnerable the way it sticks out farther than the ignition covers on the Husqvarna 610 motors. Then there is the water pump. The 1987 Husqvarna 510 has the same crankshaft driven water pump as the 1985 through 1989 Husqvarna two strokes, so that makes it look quite a bit different than the Husqvarna 610 motors with their camshaft driven water pumps. The rest of the motor is essentially exactly the same, but there are a few more small visual differences. The 1987 Husqvarna 510 has an internal oil squirter so there is no external oil line like on all of the later kick start Husqvarna 610, 350 and 410 motors. It's the exact same oil reed valve lubrication system, and the oil reed valve is in the exact same higher location as on the 610 motors. It's the very early 1984 through 1986 air cooled Husqvarna 510 motors that have lower oil reed valve placement. As far as the oil reed valve goes the 510 motor is just like the 610 motor. But what about the oil squirter itself? The 510 motor seems to rev out very freely to over 9,000RPM, but no metal shavings come out with the drain oil. The rod bearing seems better lubricated on the 510 motor, so the internal oil squirter may actually be better placed to get the job done than the later bolted in oil squirter on the 610 motors.
And the oil squirter location does seem to matter. When I put my 1999 Husqvarna 410 motor together a year ago I was very worried about the shorter stroke length and the insufficient oil reed valve lubrication system. Since my 386 stroker motor has held up much better than the stock 1991 and 1992 Husqvarna 350 motors did I got to thinking that the extra distance from the oil squirter to the rod bearing really might be very significant. To test out this idea I brazed an extension on the oil squirter on my 1999 Husqvarna 410 motor before putting the new piston in. With the end of the oil squirter as close to the rod bearing as in the 610 motor I hopped for better oiling, and that is exactly what happened. Every time I have ridden the 1999 Husqvarna 410 I have revved it out very high. A few times when the gasoline was seeming particularly powerful and the 410 motor was pulling hard way up to very high engine speeds I just stayed on it and revved the motor out beyond where the power started to let up. That's way up at around 10,000 or perhaps even 11,000RPM, although I never did put a tach on the 410 motor. I know it's way up there though as these Husqvarna transmissions don't shift all that well above about 10,000RPM. They will shift with a solid kick, but not nearly as smoothly as down bellow 9,500RPM. So that noticeable difference in transmission performance does provide a crude sort of gauge of engine speed. If the short stroke length 350 and 410 motors are revving out so far that the transmission no longer shifts smoothly then the engine is likely revving way out past 10,000RPM. I also just have a pretty good idea of what 10,000RPM screaming sounds like by this point, so I haven't felt the need to put a tach on the 410 motor. Anyway the 1999 Husqvarna 410 motor has held up to this abuse. I haven't ridden it all that much, but I have intentionally pushed the engine very hard up through higher gears just to see what would happen. The oil squirter extension does seem to largely fix the failings of the oil reed valve lubrication system.
None of the oil reed valve lubricated Husqvarna motors are immune from rod bearing failure due to the underperforming lube system, but it's not like they can't work at all. The 350 and 410 motors may just need extensions on the oil squirter, a piece of information I sure could have used four years ago.
This 1987 Husqvarna 510 XC is a bit confusing for a couple of other reasons also. It has a 1986 rear wheel and a 1986 swing arm and linkage. Is it a 1986 frame? No, the 1986 Husqvarna 510 bikes were all air cooled. The radiator mounting tabs on this frame are obviously original. The paint and rust matches the rest of the frame perfectly. Those radiator mounting tabs really don't look added on. They look entirely factory. It is a 1987 Husqvarna 510, but somehow it has a 1986 "Cross Country" swing arm and a 1986 rear wheel. Those parts could have been added at any later time, but there are reasons to believe it is in fact just an early 1987 Husqvarna 510. It's got that riveted on aluminum gas tank mount. I have heard about them. It seems like these early 1987 Husqvarna 510 frames were welded up with radiator mounts before the new 1987 black Acerbis gas tank design was finalized. Once they had the new gas tanks mounting tabs were added to the frame design; but the frames that had already been welded and painted just got a riveted on aluminum mount.
In the end perhaps the most controversial part of my new 1987 Husqvarna 510 XC might be the exhaust system. It's the over and under dual parallel muffler 1990 Husqvarna 510 exhaust system. It works fairly well, and isn't even all that extremely loud. I thought it would be louder with those small 11" long by 3" mufflers, but it actually does work. It's not whisper quiet like my custom dual inline muffler systems I have on all my 1991 Husqvarna bikes, but it does keep the sound down fairly reasonably. The dual header pipes are 1-1/4" O.D., but the mufflers are bigger. The pipes step up to 1-3/8" O.D. tubing two inches before the mufflers, and the muffler cores are even slightly bigger at about 1-5/8". Unfortunately though they are just straight through mufflers with no USFS approval stamped on them. That's the problem. No USFS approved spark arrestors, and apparently no easy way to add spark arrestors. That's not much of a problem in February, but...
Come late summer and dry grass and dry leaves everywhere... OK, so what is the real fire danger with gasoline engines. To be honest it's alcohol and huge main jets that's the dangerous combination. You know, giant blue flames out the exhaust. With it's big 35mm intake valves and huge 40mm carburetor the Husqvarna 510 motor certainly could blast some flames with big jet sizes on alcohol. So what size main jet would be safe for the 510 motor? I don't know exactly, but the 174 size that seems to be the smallest that will work well with the stock K32 needle is in fact pretty big in the 40mm DellOrto. Not huge, but pretty big. Big enough to shoot flames? I doubt it, but...
From a legal perspective though it's much simpler. A USFS approved spark arrestor is required regardless of what sort of engine setup is used. Is a simple screen type spark arrestor enough to prevent flames from shooting out the exhaust outlet when huge main jet sizes are used? Perhaps not, but it helps. All of the traditional USFS approved spark arrestors break up the flow of exhaust and spread it out over a large outlet area. This certainly helps keep any flame generation tucked in close to the exhaust outlet. If a bike falls over in dry grass with the throttle pinned wide open though... It seems to me that the only sure way to keep a high performance dirt bike engine totally fire safe is to keep the main jet size small enough that no flames ever come out the exhaust. From a legal perspective though it's much simpler. A USFS approved spark arrestor is required for most public lands riding areas, end of story.
The 1987 Husqvarna 510 XC frame has a bracket on the right hand side that is obviously for the stock 1987 exhaust system. There are a couple of different exhaust systems on the 1980's Husqvarna 510 bikes, and most of them are dual parallel under and over muffler setups. What exhaust system came on the 1990 Husqvarna 510 bikes is a bit hard to figure out. The 1990 Husqvarna Owner's Service and Tuning Manual that I got with this 1987 Husqvarna 510 XC is very confusing as it shows both the 1987-1999 Husqvarna 510, and it also shows parts of the mythical rear disk brake 1990 Husqvarna WMX 510 that was never actually built.
There are those photographs that have been circulating of the prototype 1990 Husqvarna WMX 510 with the disk rear brake and the custom exhaust system built on a 1987-1989 Husqvarna 510 frame. That's only a prototype though. The 1990 Husqvarna 510 Owner's Service and Tuning Manual shows parts for this mythical rear disk brake equipped 1990 Husqvarna WMX 510, but it also shows lots of drawings and photographs of the 1989/1990 Husqvarna 510 with the drum rear brake and the 1987-1989 plastic. One page talks about wear limits on the brake shoes and the brake drum inside diameter service limit, but then another page shows a drawing of a White Power shock. The same 1990 Husqvarna Owner's Service and Tuning Manual shows drawings of bikes with the over and under dual parallel muffler system, and most of the pictures and drawings depict the 1987 through 1989 Husqvarna 510 bikes with the drum rear brake. The 1990 Husqvarna Owner's Service and Tuning Manual lists "Marzocchi 40mm forks" for a 1990 Husqvarna WXE 510, and also lists "White Power 4054 Multi-Adjuster 40mm forks" for a 1990 Husqvarna WMX 510. Everything under the sun appears to be depicted in this magical 1990 Husqvarna Owner's Service and Tuning Manual. Then the 1990 Husqvarna 510 parts manual is even more confusing. The parts catalog is part number 66113, and is titled "Parts Catalog Husqvarna TE-TC 510/90" Every drawing in this manual shows a bike with the later 1991 Husqvarna WMX 610 plastic. They are actually drawings of the prototype 1990 Husqvarna WMX 510 with the custom exhaust system, but then the drawing for the exhaust system parts shows a 1991 exhaust system with either a race tip or a secondary muffler. Yes, dual inline mufflers just like came on the 1991 Husqvarna WXE bikes that were street legal in Europe. The "TE-TC 510/90" parts catalog shows nothing but disk rear brakes, with no drawings or part numbers for anything drum brake of any sort. They built a prototype, took pictures of it, published those photographs and printed a parts catalog but the disk rear brake 1990 Husqvarna WMX 510 never actually went into production. Or at least that's what it seems like to me. I have never seen any direct evidence of production 1990 Husqvarna 510 bikes with disk rear brakes.
Could a disk rear brake be added to a 1987 through 1990 Husqvarna 510 bike? Sure, but it's not all that simple. Any 1990 through about 2003 Husqvarna 610, 350 or 410 swing arm will go on any 1985 through 1989 Husqvarna frame, but only the later 1987 through 1989 linkage and Ohlins shock can be used. The 1987 through 1989 linkage does go on the 1990 through 2004 swing arms without any difficulty. That's the easy part. The swing arm and the rear wheel go right on, but mounting the rear brake master cylinder is a different matter. It requires fabricating some sort of a mount like what the 1990 through 2004 Husqvarna frames have. That's arguably the most awkward part of the 1990 through 2004 Husqvarna frames, that funky added on rear brake master cylinder mount. So in order to put a rear disk brake on a 1987 through 1989 Husqvarna 510 you have to duplicate, or redesign that funky rear brake master cylinder mount.
What about just using the whole frame, swing arm and all. Yeah, just use the whole bike. Or more specifically, put a 510 motor in any 1990 through 2004 Husqvarna frame. What about converting a 610 motor to a 510 motor? It could be done, but it's not as easy as one might at first think. The 510 apparently uses a longer 133.5mm connecting rod. The 510 has the same cylinder height and cylinder head position as the 610 motor, so the longer connecting rod moves the piston farther up to get the same 10:1 compression ratio with the smaller 91.5mm bore diameter. The 91.5mm piston also has a shorter compression height, so it is a substantially longer rod that is required to get the 10:1 compression ratio.
The best candidate for turning a 610 motor into a 510 motor would be one of the later 1999 through 2004 610 motors that use the longer 131mm connecting rod. That gets the piston up close to where it needs to be, but then the compression ratio is still going to tend to come out a bit on the low side. A slightly lower compression ratio is less of a problem with the smaller 91.5mm bore diameter, but it's not possible to go much lower without the spark timing getting pushed way up to where torque is lost at 3,000 to 5,000RPM.
Turning a 610 motor into a 510 motor isn't all that easy, but there are other options also. A shorter cylinder for a 410 motor is one possibility. With the 350/410 timing set and the lower cylinder head position it's easy to get the piston up far enough with the stock 127mm 1990 through 1998 Husqvarna 610 connecting rod. With that drastically shorter cylinder and much lower cylinder head position though the opposite problem comes up, getting the piston to clear the cylinder head and keeping the compression ratio reasonably low. With one of the 24.5mm compression height 410 pistons this seems workable, but then the small end on the 610 rod has to be bushed down to take the 20mm piston pin also. This could probably be done, but it's going to require taking some material out of the combustion chamber to get the compression ratio down low enough. It's actually a lot of material that needs to come out of the cylinder head, like about one cubic inch of material to get the compression ratio down bellow 12:1. If it's a 10:1 compression ratio that is required then a thicker base gasket probably needs to be added also. A bit of material might be able to be taken off the top of the crown of the piston also. It is close to workable, but a tight squeeze to be sure. Much easier is just using the original 510 connecting rod. Splitting the cases to install a 510 connecting rod in a 610 motor is one possibility, but there are other options also if the crankshaft is going to be taken apart. The 127mm connecting rod and 91.5mm bore could also be used with a slightly shorter stroke length. Not all the way down to the 2.39" stroke length of the 410 motor. No, just enough of a stroke length reduction to get it to fit in the shorter 410 cylinder. What stroke length exactly would work best? This is a bit of a shocker, but it's about a 72mm stroke length that gets the compression ratio on a 127mm connecting rod 91.5mm Husqvarna motor to easily come out in the 10:1 to 11:1 range for normal types of gasoline. Sound familiar? That 72mm (2.83") stroke length is what all of the recent 250 two strokes use. It's also the stroke length of the 510cc KTM 520/525 RFS motors, as well as the stroke length of the modern 510cc KTM 500/ Husqvarna 501 motors. And what's the displacement of a 91.5mm bore engine with a 72mm stroke length. It's 475cc (473.4cc to be exact).
At first I was only removing the fancy titanium aircraft bolts from the 1987 Husqvarna as they got in the way. Mostly they were 5/16-24 and 1/4-28 titanium bolts of various lengths holding the exhaust system, air box and seat on. There were however also a few metric titanium bolts threaded into stock metric threads. The titanium bolts were annoying mostly just because they took different size wrenches than everything else on the bike. A few of them also had odd-ball heads like a giant slot for a screw driver larger than the largest 5/16" size commonly available.
Then when I took the seat off to check the valve lash I found a much larger titanium fastener problem. The sub-frame mounting bolts had also been replaced with NAS 675 titanium 5/16-24 bolts. I had no idea what the national aircraft standard #675 denoted, but it looked like the softer pure titanium as opposed to the nearly twice as strong 7% manganese titanium alloy.
What I was worried about mostly was that the titanium bolts wouldn't be strong enough to wrench out of the long steel tubes if they got stuck in the rusty Husqvarna frame. Stuck sub-frame mounting bolts are often a significant challenge on these old 1980's and 1990's Husqvarnas, and the only way to get them out is just to twist them around in and out to work some light oil down between the bolt and the rusty cross tube on the frame. The small heads on the titanium bolts looked severely problematic from the perspective of trying to get long stuck bolts to come out.
As it turned out the titanium sub-frame bolts con the 1987 Husqvarna 510 XC were only moderately stuck, and I got them out without all that much difficulty. To pull the long stuck bolt out I put a large pair of vise grips on the head and pulled hard while twisting. Sure enough they were the softer pure titanium, and the vise grip jaws very easily dented the heads in. Obviously the softer and substantially weaker pure titanium, not a good choice for the heavily loaded sub-frame mounting bolts.
If the sub-frame mounting bolts are installed dry they rust and stick. If they are put in with grease on them then they require lots of clamping force to prevent the sub-frame from moving on it's mounts. When I got my first Husqvarna back in the late 1990's I at first had a lot of trouble with those sub-frame mounting bolts, they kept loosening up. I made them tighter, but eventually they loosened up again even with loctite on the threads. I seriously considered replacing the long bolts with 5/16-24 fine thread bolts to get more clamping force, but first I figured I would just keep tightening the stock grade 8.8 metric 8mm bolt until it either broke or held the sub-frame securely. The stock 8mm grade 8.8 bolt held, although it did take torquing it all the way down to where it was obviously loaded right up to the limit of what a standard size 8mm nut can handle.
I always put lots of water proof grease on these sub-frame mounting bolts and torque them down tight, and I haven't had any further trouble with the sub frames coming loose. The old 1980's Husqvarnas have sub frames with the mounting points considerably closer together, which loads the mounting bolts even more severely. By torquing the stock grade 8.8 bolts down tight though I haven't had any trouble with the 1980's Husqvarna sub-frames either.
To replace the soft titanium nightmare bolts I used a stock 8mm grade 8.8 sub-frame mounting bolt off of my junk 1985 Husqvarna 500 XC two stroke chassis. Out of curiosity I torqued this stock 8mm grade 8.8 bolt down with a torque wrench just to see what sort of torque I had been using. I was not a little bit surprised to find that I went right down to 45 foot pounds before it felt tight. That's a lot of torque for an 8mm bolt, especially a grade 8.8 carbon steel 8mm bolt with just a standard size 8mm nut on it.
In the end I decided to just go ahead and remove all of the titanium bolts from the 1987 Husqvarna 510 XC. The little handful of assorted 5/16 and 1/4 titanium bolts, nuts and washers I took off of the 1987 Husqvarna 510 XC weighed in at 140g. That's only a 1/4 pound weight savings versus the stock type steel bolts. That's peanuts.
Having added a quarter pound of steel bolts the next project was to add several more pounds to make the 1987 Husqvarna 510 XC spark arrestor legal. I decided that the only reasonable modification would be to somehow stick two of my old Cobra Sparky USFS approved spark arrestors on the muffler outlets on the 1987 Husqvarna 510 XC. The 1-3/8" O.D. exhaust outlet pipes didn't seem compatible with these 2-1/4" spark arrestor inlets, but I was determined to get legal spark arrestors on somehow. My first idea was to make up aluminum spacers to go from the 1-3/8" pipes to the 2-1/4" spark arrestors. This was going to be quite a bit of work, and I figured I would need to drill and tap three 6mm holes into each of the 1-3/8" outlet pipes to hold it all together.
Then I just happened to try sticking a Cobra Sparky on backwards, and it went onto the muffler outlet nicely with hardly any side to side wiggle. I grabbed eight little springs, bolted them under the rear muffler bolts and pulled the ends out to the slots on what used to be the inlet side of the Cobra Sparky spark arrestors. In just a few minutes I had a set of USFS approved spark arrestors installed rather nicely. Are they still USFS approved when run backwards? There is no direction of flow marked on them, so I guess they are legal in both directions.
When I took the bike for a test ride I was pleasantly surprised to find that the backwards Cobra Sparky spark arrestors added a substantial amount of muffler capacity. The 510 motor got quite a lot quieter when screaming in the power band. At low engine speeds there wasn't much of any difference, but the screaming noise at higher engine speeds was considerably better muffled with the added on spark arrestors.
Those same spark arrestors noticeably choke off a 610 motor when just one is used, but two of them installed parallel on the 510 motor seemed to work great. Much quieter, substantially the same flow capability and spark arrestor legal all for only 1.8 pounds of added weight.
Old catalogs from a few years ago list Cobra Sparky spark arrestors with a three inch inlet that would likely slip right over the 3" O.D. mufflers on the 510. When I tried to track down some of these three inch inlet Cobra Sparky spark arrestors though I couldn't find any for sale new or used.
The Cobra Sparky spark arrestors installed backwards seem to quiet the 510 motor down quite a lot, but being installed backwards they might not flow as well as being installed correctly. It's the slight cone shape of the screen. Screen cones in dirt bike spark arrestors always point towards the engine with the large end facing the exhaust outlet. Turning the screen around the other way does seem backwards, and it may increase back pressure slightly.