I rode my 2003 P650 a few weeks ago and it started to pop back through the carbs and back-fire.It would idle and run fairly smooth at lower rpm's but when I would throttle it up it would hesitate and pop.The atv has 103 hours on the it,until this happened ,nothing has ever happened to this motor to give me any concern and it started to run rough all of a sudden.I know it's not the belt re-set switch,I installed new plugs(gapped correctly),air filter is fine,bad gas maybe?
After I installed the new plugs,I ran the engine for a minute or so,re-checked the plugs,one of the plugs was slightly fouled and the other plug was clean.
Where do I go from here?
any suggestions would be appreciated,
thanks
Cleve

 

I shimmed .04 with the ratshack shims, solved the airbox popping at low speeds also made a bit more responsive. I also installed new NGKs. In winter I run 3.25 and 3.5 for the air screws, now with it warm I run 3 and 3.25 ish.

03 prairie