My sportsman 500 ho will start and re in neutral fine but will cough and sputter when you try to drive it away. I had it in a garage and they figured it was the rev limiter which is built into the ecm control modual. I replaced that and it still won't run.
I had to fix the rewind, starter and the bendix this past spring. Just wondering if it could be the stator, coil or override switch.

 

Have you checked into the carb? Could be just the jets need to be cleaned,possible needle and seat? OPT

 

My father in-laws quad does this on a regular basis. It will idle and as soon as we hit the gas it dies. this is a easy fix. Just pull the carb off and i bet the needle has came loose and is stuck in the main jet. So i took apart the carb and sure enough the little c clip that holds the needle in place had came loose and the needle just fell into the main jet . all i did was put it back together and pinch the clip with a pair of plyers and it is working great now. hope this helps