since my blaster was broke last night i drove my dads 4 wheeler(350x motor in 250x frame)when i would give it full throttle then the engine Rpms went down there was like a backfire noise.

Could be a leaky headpipe. Be sure it is tight. If I interpreted your post correctly, I think you were saying you would be going full throttle then let off??? As a result, the RPMs would drop...right? If the pipe is loose, the deceleration will suck air in and cause a backfire.

i don't know if it was backfire, almost like a crackly noise.

It means it is a normal 350X. They all have a slight backfire on decel, nothing to worry about. Has to do with a slightly rich mixture..

My 350X never backfires unless it is about to run out of gas. Anyhow, if you haven't run out of fuel yet, obviously, that is not it........

nonono the quad is jetted to rich... look in the dirtwheels issue about the science of carburators

Hmmm, every 350X I've ever been around (probably 20 or more (including about 5 we owned), they were very popular around here at one time) does it on deceleration, kinda a soft popping sound, not a loud backfire. Most of them were all stock too, some with the spark arrestor insert, some without...

Skunky is right. An internal combustion engine will pop, crackle and snap, if the mixture is a little too rich. You could always back off fuel to air mixture some, but then the bike might run too lean. However, set it to lean and it will run too hot, and that is bad. Ride it, ride it hard and it ain't broke so don't fix it.

I heard it could be fuel fumes compressed in the cylinder instead of gas.
But I don't really know.

Ideally that's what you want, liquid gasoline will not explode. "Fumes" would be the perfect situation....

Backfiring on deceleration is typically attributed to a rich mixture, backfiring on acceleration is a severely lean condition that requires immediate correction or will destroy the engine. The lean backfire will usually occur thru the carburetor instead of the exhaust.