My friends 400ex has a hard time idling but when you get it into gear and start accelerating it goes but when you give it over half power it boggs out and dies or almost dies. He just replaced the carb thinking the float was stuck and it was getting too much fuel. Any help would be appreciated!


It ran great then sat for a month, I believe he has drained the fuel

 

He replaced the carb? Like bought a new one? That's a bit expensive and extreme if he thought it was a stuck float. If it sat for awhile, won't idle, bogs when it gets gas..that's bang on for a dirty carb.

 

Yes he put in a new one, still had the same problem. I sent him a diagram i found online on how to adjust the carb. he adjusted it and still seemed to be getting too much fuel. he then bought a smaller main jet and says its running a little better but not to all the way fixed.

 

Any mods?

 

i believe he has a stage 3 exhaust

 

I"m not sure I can give much advise without knowing specifics. I don't know why anyone would spend the cash for a new carb for a stuck float but, if it's new and clean than you can rule that out. If it has an exhaust (not familiar with stage 3) I can get jetting close if you can tell me the what kind of exhaust, the elevation your at, if the spark arrestor was left in (Some remove it) and if airbox lid is on or off.

In my experience 99% of the time when it runs then sits for a period and then runs poorly, it's fuel related.

 

Sounds like a dirty carb. Like koop said we need more info all motor, intake and exhaust mods, what jets are in the carb, elevation. What does the plug look like black, white or brown.