at about 1/4 throttal my 400ex starts to bog. i have 6 airhole in my airbox and and 38mm keihin carb running 155main.
what shoud i do?

 

you are running 38 carb is that stock??

 

nope aftermarket

 

that is right the stock carb on a 4000 is a 34 so... Your main just is too small. I would recommend a 190 to 210

 

no a stock carb is 35.5mm. and by putting a bigger carb on you give it more gas spo you have to put a smaller main jet. i check my spark plug and it was still running rich. i'm gonna try a smaller jet yet.

 

I'm not a jetting expert but at 1/4-3/4 throttle I believe it's the needle position, not the main jet, that affects the air/fuel mixture. The main only comes into play after 3/4 throttle (though there is some overlap).

Sounds like you may be running a bit lean and you might try raising the needle a position or two.

Regards,

Dennis

 

I could not remember the size so i guessed..hehe But usually i see bigger carbs sold with huge main jets. But dhines is right at 1/4 throttle the needle is what is in play..

 

I have always understood bigger carb= less vacume=lean running. If you are not getting black smoke then it is lean. Raise the needle clip if it is indeed lean

 

nope its not lean i have keep consint watch on the plug.

 

Do remember though that the plug only shows how the machine is running for a given throttle position. In order to check the mixture at 1/4 throttle you would have to do a run in that range and then kill the engine and check it. You may have already done that but the bog you're talking about sure sounds like a lean condition - though as I've said before I ain't no mechanic. The best thing I could recommend is to go ahead and try raising the needle a position or two. If it gets better, you may have solved it. If not, you can start looking elsewhere.

Regards,

Dennis