i bought my 400ex a couple weeks ago and the owner said that just last summer he had the motor rebuilt into a 440. nothing else done to it no cam or anything. i kno of the weak timing chains on 400ex's and im afraid thats what my problem is. today since it was like 70 degrees i thought i would get it out for awhile after school and i rode it until about 6:30 and right before i put the thing up i started hearing this rattling on the left side of my engine and when i would give it gas somewhat aggressively it would jus spit and sputter some. not anything terribly bad but enough to give the bike some hesitation and get my attention. any help ASAP would be nice im supposed to go riding tomorrow and i need to nko if i should ride it or not.

 

if there was no noise i would say it would be a fuel ajustment. But since there is a noise, im gonna say its the timing chain. Does it get louder when you give it gas? Have a friend ride it and listen. If its very loud chances are its the chain.

 

umm, i never worked on a 400 ex.... but isnt there a tensioner adjustment on those? maybe it's just loose.

The hesitation could just be the 440 mod that's doing that though. If the cam is still stock it probably doesnt flow quite good enough. That, and you would likely have to re-jet the carburetor with a 440 kit, who know's if that was done either.