my cousins got a 98' sp500 and a 99' sp500, they should sound ideticle but its far from it. The 98 has a loud knocking/pinging noise that i think is the infamouse camshaft??? And this is where the problem starts, when you let up on the gas from at least 1/4 throttle is backfires the entire time the engine is decelerating, dealer thought it was the ETC at first but that was fine with 1/8' throttle freeplay so he concluding that it must be vapors getting through the carb and said its normal. I thought in my head WTF? Any suggestions as to what it might be. Now then we go to this 99' and its just as smooth as can be engine wise, its crazy just thinking about it.

 

anybody?

 

how many hours on the engine? mabye it jumped time. probably needs rebuilt.

 

4000 miles

 

well from car knowledge, my guess would be its outta time, or there is a problem in the valves, i think that wud be why it backfires, i think dont quote me, maybe someone else can help you out better, but that may be the problem.



Haz!

 

i would at least pull the head & check the timing. i haven't done this so i would have to find out how to check it, but it can't be that hard. thats where i would start. you could also compare what you find to the other 500. hope this helps some. let us know what you find.

 

Polaris's do not use a timing chain like cars do. The timing is controled by the position of the stator plate vs. the flywheel. There is virtually no way that this can be changed without actually going in and moving it by hand. I think it is more something with the fuel or something else. If it is timing then the CDI may be bad. It could be carboned valves which can be taken care of by running a product designed to rid the engine of carbon deposits through a tank of gas. The valves could be out of adjustment. The air filter, fuel filter, oil filter could be clogged. A carb vent hose could be stopped up or pinched, and the air screw could be out of adjustment. The first thing I would do is check to see if both carbs are set-up identical with the same # of turns out on the airscrew, then check vent hoses. Another thing could be a stopped up or partially clogged silencer/muffler. I think for some reason it is running lean and that would cause the backfire on decel. The easiest way to check for a jetting problem is in the throttle range it is giving problems, while riding flip the choke on and see if it gets better or worse, If it gets better you are lean, if it gets worse you are rich. The pinging noise you described is also the sound of too lean. He could also have an airleak somewhere. WD-40 on a cool idling engine will tell you if that is the case, there would be a noticeable change in idle if there is an airleak. The places to check would be around the intake boot, base of the cylinder, etc. Also check to make sure the vent hose on the gas tank is not pinched and the air filter is clean and seated correctly with no leaks around the airbox. Hope this helps, let us know what you find.

 

there is a small hole about the size of the starter button on the exhaust pipe that curves from the front of the engine.

I don't think its the timeing because it starts real easy.

 

That hole could do it, not sure what effects that has on a 4 stroke???

 

like i said i wasnt for sure, i didnt know how the polaris 4 stroke operated, ive only wrenched on the polaris 2 strokes, i was just taking a guess, tryin to help, sorry for misleading ya, NCS thanks for correcting me, lol im dumb, shudnt have said anything.


Sorry, Haz!