Just rebuilt my XLT 1200, it had a blown piston, since I purchased it this way, I went ahead and replaced the Crank Seals, deleted the oiler, Cat Delete, and drilled out the top portion of the intake housing, also had to change out one set of reeds. I went ahead and rebuild the carbs not knowing what burned up the original piston. I took it out yesterday for the first time, and it seems to idles fine and runs good at High throttle, but when I go through a no wake zone under low throttle, It backfires or sounds like it backfires quite a bit. Reed valves are good, had to change one set during the rebuild. Also FYI it seems like it is cavitating quite a bit at this throttle range, the prop shows a lot of abuse on the leading edge and trailing edge and I will probably get a new one.

So what could be causing the backfiring. I don't want to burn it up trying to diagnose the problem. I am leaning toward a clogged idle jet since it runs good at full throttle, but this is just taking a wild guess.