MY bike has been backfiring and hesitating alot, so i took the carb off and i was lookin at it and i took the main jet out and its a 200. I was saying to my self that sounds really big maybe thats why its running like sh*t. Guys help me out here, I have a 99 400ex with white bros R4 pipe, k&n air filter, what size jet should i be running.

 

What kind of carb? ifs its a kehien then wow thats huge! But it shouldnt make your bike backfire. That would mean its to lean. If its really a 200 it should make it sputter and not run smooth, and also will make it hard to rev hi. Im not sure but I think the stock 400ex jet is in the 150s, or low 160s

 

my buddy has 04 400ex with white bros e2 slip on and k a n filter and a 170 main jet the stock jet is a 148 on the 04 s tri a 165 to a 170 should be good.

 

The main jet wont effect backfiring and hesitation. Sounds like the mixture screw is too lean. I think that its too big also, (200 main).But the backfiring and hesitation is a separate issue from the main jet.

 

Acually no if your main OR pilot circut is to lean it will back fire. I know what you mean though, if the main is to lean and you dont rev it up enough so the motor is running on the main then it might not backfire but if its lean it can definetly back fire.

 

with the R4, K&N, and the air box lid ON, i would put in a 152 or 155 Keihin mainjet. try turning the fuel screw on the bottom of the carb out 3 turns from lightly seated. the fuel screw should fix the backfiring problem.

 

i run a k&n filter with no air box lid,fmf power bomb header and power core 4 pipe with a rev box on my wifes bike and we run the fmf jet kit as told with the exception that i like to run the main about 10 points (175-185) higher and run a hotter plug.