STOP!! PLEASE REPLY IF YOU HAVE ANY IDEAS. DON'T LEAVE A FELLOW RIDER HANGING. THANKS

I have a 2000 400ex and have done nothing to it at all as far as mods. It has serious problems with backfiring and sputtering. It will run great until the engine gets somewhat warm, then it backfires and sputters and stalls. From then on it won't stay running at all. I'm not to knowledgeable about fixing a problem like this, so if you do know how, please take the time to explain. THANKS ALOT.

Put a new plug in first than spray starting fluid around the intake if it revs up you have an intake leak hope that helps

sound like it is sucking a valve. A friend of mine had the same problem on a Wolverine. We removed the valve access covers and noticed that the intake valve was not closing. It tunred out that the oil pump had failed and the cam was not getting any oil. this caused the rocker arm on the intake side to seeze. Check the timing first.

Have you had the valves adjusted lately?
If the plug and filter check out...I would look into the valves and or timing

400EX'ers (HELP!!!) I NEED YOU
 

get a new plug!!!!!!!!