My brother's 850 has less than 5 hrs on it and every once in a while it will backfire out the exhaust when he lets off the throttle. The quad runs greats, never misses or stalls out. He called the dealer and they said that he probably has a fouled plug due to the fuel system running after the machine is shut-off, WTF?! What is running after machine is shut off, the fuel pump, that doesn't matter unless the injector is open and allowing fuel to enter the cylinder. Anyways, I called BS on the the dealers statement but I still don't know why it is backfiring. I figured that it was probably a timing issue but not sure. Any of you ever experienced this problem?