I just purchased a KFX 400 to play in the sand with and have put the 10 hours on it they recommend for breakin (ok maybe 91/2). So my partners and i are waiting patiently for this time to come and when it does I opt to take her on a flat open patch in the desert and look to see what she does. I go through the gears at an aggressive pace but not totally throttled out ... about 90%. I hit 5th and she is flying (she is still all stock) and I take her full throttle for a bit. Then when i let off the gas she lets out a very loud backfire. I let her cool down and ride around for a bit and do the same thing and she does it again...is this a really bad thing...is it not jetted properly....did i do this to soon in the break-in procedure and possibly done something to it...any info would be appreciated. Other then this I am totally impressed with my Kawasuki 400...she handles great...has spunk for a 400 and is extremely light compared to the prairie....prairie is still my beast though and will be my mountain and rough rider.

 

My buddy's 400 was doing that. We adjusted the idle mixture screw and it went away.