2004 raptor 660 allien pipe stock engine 150-155 jets . It backefires bad on decell need some help please...

 

your bike is running lean if it is backfiring when letting off the throttle try turning your air fuel mixture screws out a 1/2 turn at a time if it does not go away you probably need to go up on your jetting.

 

Thanks for the help I'll try that.

 

Also check the head pipe gaskets. If there is an exhaust leak it will pop when decelerating.

 

Just to clarify what tpsycho is saying, you should find where your idle mixture screws are set, turn them in until they seat gently, counting the turns, ideally, you should be at about 2 or 2.5 turns from seated, if you have to go 3.5 to 4 turns out to stop your backfiring, you need to go up one size on your pilot jet, the little long tube like jet in front of your main jet, it feeds the idle circuit when the throttle is closed. Just turn them out enough to stop the backfiring, no more,no less, that will give you the best throttle response off the bottom end. Air is also correct, exhaust leaks will lean out the motor a little, and can cause some problems, and they can be hard to track down.Two strokes are affected a lot more than four strokes, but an air leak is an air leak.