whats up guys im new to the forum. on my 02 400ex i cant get it to fire up and when it does it wont stay on. ive cleaned the carb. and changed the plug and it still doesnt want to work right. what else should i check or do? thanks

 

Have you done a compression check? Checked the valves? Hows your spark look?

 

well?

its an EX...can't be to hard to get it to run

 

i went out and messed with it a little more and i can get it to fire up but when it firsts turns over black smoke comes out the exhaust and back fires and cuts out if i dont hold down the throttle what could be causing this

 

does the choke lever on the carb work ?
is the gas/fuel fairly new and not stale ?
try to start it with the air filter box lid off .....

 

A little history would help too. Was it just sitting there for 2 years, untouched? Have you or anyone else done any wrenching on it, and then the problems started? Did it get swamped, or was it in an accident?
Sound carb related to me. Try starting it at 1/4 throttle, and no choke. Try new , fresh gas, if the gas in it is over 6 months old.

 

back fires? black smoke? i'm not to savy yet on 4 stroke atv motors, but i remember an lt1 firebird i worked on, backfired with black smoke...the rotor in the optispark was just freewheeling(not bolted onto the gear) messed up the timing, check your timing, may have jumped? Before that though, check your carb, maybe the choke is stuck?