I recently put an aftermarket exhaust system, and K&N air filter on my 400EX. The problem I am having is that when I get to about 3/4 to full throttle the bike starts to sputter and doesn't make any power. I changed my piolt jet to a 40, moved my needle clip to the middle grove, and tried 3 different main jets, a 155, 158, & 160. I am still having this problem even when I change the mains around. Has anyone had this problem before?
The exhaust is the DMC Alien & I also removed the choke.

 

are you sure you arent just hitting the rev limiter? because i thought i had the same problem as you, but i was only reving out to fast

 

Have you been reading your plug and then making your jetting adjustments, or are you tryin to do this by feel?

 

I believe that you are hitting the rev limiter. Your engine now has no restrictions in airflow in the intake or the exhaust and you don't notice but you reach you rev limit quicker. There is a rev kit from White Brothers that will give you the added rpm so you don't hit the rev limit before you shift. Are you hitting it right before you go to shift? You could teach yourself to shift a little sooner which would be cheaper.

 

Before going out and getting any of the "name brand" rev-boxes, you can get the xr250 stock rev box. It is the same box that White Brothers sells. You can get them new from Service Honda for $72. From what I've heard, most people put a rev box on, don't like it, and then take it back off. If you're revving that much, I wouldn't want to raise the limit. You'll just burn up your motor.

You may still need to bump up your main jet a bit. Try a 165. Four strokes can be a little finicky, and what works for one guy's bike with all the same mods won't work on your bike. Are these the stock jets and needle or dynojet (k&N)??

You'll definitely know if it's your rev limiter. When you hit that it doesn't sputter (backfire). It kind of goes into a repeated hi-lo rev (hard to describe, but it sounds like "wah....wah....wah....wah" ....something like that) I hit mine climbing a big dune a few months ago, and it sounded pretty bad, but nothing like a jetting problem.

 

Well I know I am not hitting the rev limiter because I know what that feels like. This sputter is coming on at about 6000 RPM. I changed my main jet out to a 165, 168, & 170 and I am still getting the same results. I was wonder if their is an air fuel mixture screw on this carb that I could try adjusting?

 

is your choke on?

 

check for a loose wire, i had the same problem on my 250x and found out it was a loose wire that came from the stator.