Originally Posted by MRDS650
I am betting you have sheared off the crankshaft keyway - it's a common problem in these bikes - I have had it happen a couple of times. When it does happen the timing goes out causing the engine to backfire. You need to get the engine to TDC and lock it- insert a special bolt under the gearshifter in the crank. Then remove the side cover by the brake pedal, Pay attention how everything comes apart, it needs to go back together a certain way. Then remove the flywheel, you will need a special tool to do this. They can be bought off ebay. Inspect the crank, I am betting that the timing has jumped when the key broke.
Yup. Agreed. Pretty common on a lot of different make ATV's. Timing is off, thus backfiring.