HELP!! Not Starting

[400exrapes]

my 400ex just got rejetted and ive ridden it for about a week now with the hmf (slip on) on the quad. i was riding and then all of the sudden sounded weird and then eventually in my driveway just stopped and i start it. its turning over but other than that i dont know what else it can be please any info would be good i have a 152 main and 42 pilot if that helps

[GPracer2500]

There are three fundamentals that make engines start and run: compression, fuel, and a spark. Look into those three things and you'll find the problem.

Start with the simple and easy things first. A sudden loss of compression is unlikely so assume that is fine for now.

Is the carb getting and delivering fuel?
Is the spark plug fireing?

[BlackTrx440ex]

turn your gas on :) i went down the road b4 with the gas off it stalled and i was confused cuz it wouldnt fire back up :)

[400exrapes]

it started but very hard something burning to its like a two stroke starting up it looks like smoke is coming out and its backfiring once i started it if i have to rejet it agian thats fine i just want to make sure its running right

[NRTHBUSH]

sounds like your running lean adjst your air fuel mixture screw. turn it all the way in then out about 2 1/4 turns . that should make it a bit richer. and stop the back fireing... if not put in a 155 main... took me awhile to get my bike jetted right to. but now its starting and idleing like it should. now i hafta do it all over again when it warms up darn... i shoud buy an extra carb n just swich out carbs much easyer

[400exrapes]

thanks nrthbush thats what i was thinking it was i was getting worried the piston was worn cause i was reading in the manual and i did not wanna bore it too much work and money