I need help quick. what is the stock main jet for a 99 400ex and what is the stock pilot jet for the 99 400ex. Thanx in advance I appreciate the info.

 

Stock main is a 148, and the pilot is a 38. Hope this helps

 

i sure do appreciate it I was reading 4 wheel action june of 1999 and they say to go 1 step up on pilot jet and 3 steps up on the main jet. I just bought a fmf powercore 4 and a uni air filter and i am trying to decide what i should rejet to if that is enough or not. any other info would be appreciated. these are the only mods at all so far

 

when they say to go up 3 steps do they mean in intervals of 5 because every one seems to be running right around 160 and what would u do on the pilot jet then? gotta buy some jets tomorrow going riding on thursday thanks for the quick help

 

Please re-jet properly, don't go by what a magazine or "directions" tell you to do. You may get close, but close may still seize your motor. Take an extra 30-60 minutes and read how to correctly jet your motor. You'll find out how to on the atv connection's main page. Click on Tech Connection, then advanced search, then select "all models" and "carburation;" then, start reading. By the way, jet sizes for the 400ex go as follows: 148 stock, 150, 152, 155, 158, 160, 162, 165, 168, 170, etc. You may have to raise your needle one clip for your midrange, I did. Also, I didn't need to replace my pilot, I just turned my mixture (air/fuel, not idle) out 1/2 turn. I'm at a 165 main with a supertrapp pipe. Your results may differ.

ogre

 

I agree with you ogre completely. Take your time to jet it correctly. I would start by purchasing a range of jets from a 160 - 170. They are about $15 for the set. Everyones bike and riding conditions differ, but I understand you need a starting point. I am running a White Bros E-Series slip on with 12 discs, a Uni filter, and 6 air vents in the airbox. I started with a 158, way too lean, the bike stumbled hard on top. Keep going up in mains until the bike does not pop on the top end and test the plug from there. You should be looking for a medium brown color. I am currrently running a 165 main with a 42 pilot. I live in the cold northeast and cold starting with the stock pilot was very difficult. Going to the 42 the bike was much better starting in the cold. Hope this info helps you with jetting your machine.

 

yesterday I put a LRD pipe, uni filter, 4 1" airbox holes and a white bros. rev box on my 400EX, After a few tries I think I have mine right with a 165 main jet and a 40 pilot. This thing REALLY runs now.