Yeah, the airbox takes some technique, especially to get back on! If you really want to lose your mind, put on a new pair of "donuts" (the rubber pieces that slip over the carb throat) on and try and get it on. Anyway, to get to the fuel pump, pull the airbox and carbs; then you should have no trouble getting to it. That can cause flooding in the crankcase and possibly beyond if it's parked downhill. If the diaphragm gets a hole in it, fuel can go past the pump and through the impulse line. Like Jim said, stuck needles in the carbs are another likely cause. I've had very few issues on my sled, but I only run ethanol-free, and I clean the carbs at least once a year. While you have the carbs out, it's also a good time to check what you've got for jetting and where the idle and air screws are set. I wouldn't ride a new-to-me sled without doing that; even if that's not the case, it's real easy for them to end up somewhere in left field if you don't keep carful track of what you've done.