04 kx250f
 

hi, im new here, i'll try to explain this as short as i can. my 04 250f starts sometimes and other times it doesnt. checked out coil, stator, flywheel, cleaned carb, timing is dead on, has spark, everything checks out good. when i go to start the bike (and if it cranks) it backfires(which i know can be a timing thing, but its dead on). once it runs, its starts popping out the exhaust and sometimes flames come from it. i have a twin air system on it and when i place a rag in the air box into the carb and it cranks, there is no backfire or popping or flames. our local kawi dealer has a bike in there shop right now doing the same thing and they have tried everything and it still does the same thing. i talked to another mechanic and he told me to replace the timing chain and it should work. timing chain has been in for approx 1 year, but when you put the chain tightner in, the chain is tight. when i take the tightner out, it does not have the appearence of the chain streching. i have tried different ignitions and does the same thing. right now i have a racing vortex ignition on it. please help, very frustrated. any suggestions will be VERY appreciated. also i have an o5 head on the bike. bike runs good on top end, just bottom end bites. believe prob is in carb, could there be an air leak possibly. thanks