1997 Polaris Scrambler 500 - Engine Trouble

[matbogbry]

I am troubleshooting a problem with an 1997 Polaris Scrambler 500 and I could use some help. It will start and idle with no problems. When I give it throttle, it sputters, cuts out, backfires, etc. (very similar to having water in the gas). I have done the following: cleaned the carb & checked settings/jets (it is perfect), adjusted fuel screw (no change), checked intake for leaks (all good), replaced & gapped spark plug, fresh fuel, new fuel filter, fuel pump holds pressure and appears to be pumping fuel, exhaust is not plugged, air filter is in good shape, cam chain tensioner & cam timing (good shape), pulled the black wire on the rev limited (no change), added a battery charger to see if voltage was causing problems (no change). I ran a leak down test and found that I am leaking approx. 10%. I could not detect air coming out of the intake or exhaust ports, so I am assuming that I am leaking past the rings into the bottom end. Another thing I found was that the valves were very loose. They should be at .006" and the clearance I measured ranged from .008" to .014". I believe the exhaust side was the higher measurement. Would the rings and potentially worn valvetrain cause this issue? Should I look elsewhere, like at the stator? Any help is appreciated.

[oxidized_black]

worn valve train would give you white smoke in exhaust, you have spark so the stator is doing its job. i would do the most economical fixes first, possibly a new coil or look at condition of plug wire and a new plug. i would reclean the carb again if a new plug or coil didnt fix it ... i know you said its perfect, but i did a carb clean too and did it a dozen times because i thought this time was perfect ... i think youre ok with 10% leak down, but youre getting close to a ring job ... jump in guys with some tips !!

[Mudflap64]

Sounds like it could be loosing spark consistency. Id start with the cheap stuff too. A weak coil would do this. Do what OX said , then re-clean the carb if nothing changes. It;s easy to miss a passage here or there. Be mind full of that if you pull the carb apart again.

Debris in or around the pickup/ stator could also be an issue. Ive seen a dirty fly wheel cause higher RPM misses. ( especially if the rings are getting weak & the crankshaft seal starts seeping a little bit of oil inside. )

Edited December 6, 2013 by Mudflap64

[matbogbry]

I pulled the rocker assembly, so I could get a good look at the exhaust lobe on the cam. The exhaust lobe was warn down to nothing. I also noticed that the rocker shaft had shifted over and was rubbing on the back of the cam sprocket.