1999 450 foreman wont start

Alright, this weekend i went riding any my foreman started back firing really bad and if i pressed the throttle it would bog down. We let it sit for a while and it was really hard to start back and i ran fine. We got it back to the shop and then it wouldnt start at all. We checked to see if it was sparking and its not. So far we have changed the plug and even took another coil off a junk fourwheeler. Still nothing. If you crank it over for about 30 secs it will hit once but thats it. We also know its getting gas. What could be the problem. Im stuck