Ok, I've got a '00 400EX, completely stock, and most of my riding is done between 1000-4000 feet elevation. Here's the problem:

As long as I keep the RPMs up, it runs great, but when I'm powering in the low-to-mid RPMs the engine starts to bog. If I don't rev it a little to 'clean it out', it bogs so bad that I can stop in neutral, and it will idle ok, but as soon as I give it a little throttle it chugs, and the only way to bring it back to normal operating is to feather the throttle rapidly until the RPMs will finally come up and it cleans out.

So, it seems to me it's running too rich, but that contradicts what I've read on this msg board (that stock jetting on the 400EX is lean). Also, whether rich or lean, what do I need to adjust? Main jet or needle clip position? I suppose I could just go out and start messing with it, but I'd rather first hear an educated oppinion on what to do. Thanks in advance for the help.

 

Try higher octane fuel.

Tree Farmer

 

Clean your air filter (if very dirty or wet). Check for loose intake connections and debris in the intake tract before air is filtered. Next, Check the needle jet (from the intake side of the carb)and make sure that you can not move it up or down. I had that exact same problem one time and noticed that I had not put the needle in below the plate that operates the slide and the needle (that's what you get when working on your ride at 3am). Next, check the idle circuit adjustment (under the carb, on the cylinder side, behind the idle adjustment bracket). It should be about 2 turns out from lightly seated. Replace the spark plug. Check the valve clearance. You are not running old gas are you? Make sure the head gasket is not leaking. Good luck!

 

If the problem is indeed a jetting one, it is unlikely to be with the main jet as you state it occurs much lower in the band than the main would operate. I would check the position of the jet needle to be sure it is in the 3rd position, or center slot, which is the nominal position. This jet controls the fuel flow in the 1/4 to 3/4 part of the band where you're having trouble. Since it is idling okay the pilot jet should not be at fault.You want to make sure you have the boots clamped securely also to prevent air leaks.