So I made a very bad decision and purchased a 1987 Yamaha SRV 540 from Craigslist sight unseen. It has given me nothing but trouble and never run right since I got it. (having said that I've still had a blast riding around the local NH trails) When I got it it would barley run at all, and backfired a lot, but after a lot of work I got it to where I am now, and I'm stumped.
Here are the current symptoms;
When first started it runs perfectly, and acceleration seems very good, (to the extent where if you weren't paying attention you could almost get thrown off the back), then once it's warmed up it has a severe bog right around 4000 rpm. it spools up from a somewhat rough idle fine, but then has trouble getting past 4000 rpm. If you're on a flat surface once you get past that speed it runs fine until you let it idle down again. (on an uphill it will never get past that) If you punch the throttle from Idle the engine will momentarily cut out.
Opening the choke when the engine is warm stalls it, slightly opening the choke when it's bogging does nothing or makes it worse, adjusting the pilot (fuel) screw doesn't seem to have much effect, except for stalling it if it's closed all the way. The track turns freely by hand when up in the air, and the symptom still exist, although slightly less severe.
Here's what I've done;
When I first got it I drained the fuel tank and replaced all the fuel line, I cleaned a mouse nest out of the air box, and plugged some holes that were drilled in it, I've replaced the spark plugs and the plug boots. I've cleaned the carb and blown out all the jets with an air compressor, I found that the headlight wire was crossing over a plug wire and shorting it out so I rerouted it, I checked the reeds and found and air leak where where a gasket wasn't correctly installed (fixing this fixed some of the backfiring) I replaced the belt, and I also checked the exhaust to make sure it wasn't plugged.
A few things to note; the main jet is a
#145 , the exhaust is aftermarket, the tether kill switch has been removed, and the carb boot has been replaced with a pice of rubber hose. (not sure if that would be affecting airflow over the jets?)
Honestly the sled is a mess, and people are telling me I should sell it, but It's become a sort of personal vendetta that I don't want to let a silly snowmobile beat me, so if anyone has had any experience with these sleds, or has any advice for what I could try, I would much appreciate it.