i recently picked up a used 01' 600F4i. i rode it for about a month problem free, no back firing or anything,and out of no where it started back firing when i decelerate right around 4000rpm. also with the backfiring it seems to be blowingmore smoke than normal out the exhuast.the bike has about 20k miles on it, an aftermarket exhaust and no power commander(ran great w/o one). i had it looked at by the dealer i purchased it from and they think it could be the exhaust core could be shot or i would need a power commander to possibly tune it better but like i said it ran great without one. i have no idea where to start, it is just turning into a bit of a headache.all i know is when i boughtthe bike it had nosuch problems,any ideas as to what could be the cause of the problem and possible suggestions would be appreciated.
thanks,
nick

 

Just a wild guess here, backfiring on the overun (backing off) is usually caused by air getting into the exhaust system from some where (leaks drag the cooler air "in" causing a seconday detonation). so check the fittings for leaks. A stuffed core would only make the pipe louder.