I have a 2002 yz250fp runs great. on deceleration and down shifts bike will pop, but not if I keep revs up. I have looked at the valves not to long ago pretty sure I can rule them out. Any help will be appriciated.

Are you in gear while "deaccelerating"? and are you NOT using the breaks while on your decent?

Using a vehicles gear to slow down saves gas because the momentum is turning the engine and the engine isnt really doing much at all
((Example: the weird sound a car makes while in reverse))

To me it sounds like its fine unless it pops while in idle, next time you ride it and your descending try keeping albit of gas going while downshifting if it still pops while your keeping the small gas going then idk whats wrong hopefully someone with more exp will come in

overall i think its perfectly fine even my car makes poping sounds with its gutted exhuast while im down shifting

Using a vehicles gear to slow down saves gas because the momentum is turning the engine and the engine isnt really doing much at all
((Example: the weird sound a car makes while in reverse))


I think:
that weird sound a car makes in reverse is because reverse has straight cut gears. forward gears in a car are slash cut which hurts performance but makes cagers happy. also the reason sportbikes whine thru the gears because they are geared for performance straight cut .


and as far as the 250 pop noise tapil is probably right because mine does it too. I heard another 250 popping in the same corner under the same conditions.
I gotta admit If my bike keeps popping I may have to try the air screw kit :http://jdjetting.com/ that several people have recommended.
with my bike it popps under hard deceleration in the heavy chop. but If I keep the revv's up a bit it doesnt pop.
my guess would be its unburned fuel being ignited .
Like I said I heard at least one new bike other than mine popping under the same conditions in the same corner so It at least made me feel better knowing another was doing it.
my 450 never did it but I probably never entered a corner at such a high rpm on a 450. lol

Thanks guys. Feel a little better now. Thought it might need a carb. adjustment of some kind.

correct on all counts Mac. I wouldn't stress the popping too much. it's natural for a carbed engine being pulled by the wheel. It's wastefull of fuel, and the primary reason that fuel injected engines are way easier on fuel, when they sense a closed throttle, the injectors slam closed, period.