Ok, I'm still having some issues with my 2000 400ex. I have completely rebuilt the top end. New valve job, new heavy duty timing chain, new cylinder, new piston and rings etc.... But, I continue to have problems with it running at low revs. I have cleaned the carb numerous times and rebuilt it as well. I put a 40 jet in the slow jet. I just had it out and was struggling to keep it running at low RPM's and it definately wouldn't run at idle. As it was running though, I noticed that at about 1/4 or 1/3 throttle it ran great and had tons of power. I assume this is when the main jet is kicking in. Does this sound right?

If that's the case, I must still have something caught in the slow jet. Please let me know if you have any ideas.

 

Could also be in the mixture screw, which is the fine tune for the pilot jet ( slow jet). Is the inner o ring still there, and correctly in place? Is the steel washer just above it? Is it about 2 turns out?
If that all looks ok, then make sure the air bleed is clear. Its the small hole in the intake side, that runs parallel to the air flow. Its where the air for mixing with idle fuel comes from.