3\4 Throttle stumble

[sprayedgt]

My '08 400EX is having a little miss\stumble in the 3\4 throttle range. It seems to only happen with the engine under a load (uphill) and I'm in 3rd or 4th gear. Runs perfect on flat ground in every gear right on up to the limiter. I had a 155 main in and switched it out for a 152 thinking it was a problem with the main jet circuit activating. With just the mods in the signature, do I need to adjust the needle or the air\fuel screw?

[NacsMXer]

Well the needle and the main jet circuits overlap each other, so it could be one or the other. Sounds like a rich stumble.

Try raising the clip on the needle one notch. If that doesn't help much put the needle back where it was originally and try a 150 main in it.

[MtnEX]

Originally posted by sprayedgt
My '08 400EX is having a little miss\stumble in the 3\4 throttle range. It seems to only happen with the engine under a load (uphill) and I'm in 3rd or 4th gear. Runs perfect on flat ground in every gear right on up to the limiter. I had a 155 main in and switched it out for a 152 thinking it was a problem with the main jet circuit activating. With just the mods in the signature, do I need to adjust the needle or the air\fuel screw?

OK, I have the full Yoshi exhaust + K&N filter and have been tinkering with mine at 1200 ft elevation.

They put a 155 main in mine and set the needle e-clip in the 3rd position.

I had a rich runner, but once I went in it I figured out my needle holder/needle jet was not turned in to seated. So I fixed that and raised the needle clip one position.

Afterwards, mine ran well, except it had a real bad lag/bogg at 3/4 throttle pulling up hill.

There are a few things that can cause this... (in my opinion)

Rich on the main or needle clip position...
Lean on the main or needle clip position...
Carb synchronozation out of whack (slide - butterfly timing)


Myself I set the carb timing and dropped the needle back down to the 3rd and that cured it. Not sure which did it.

I wrote some instructions in another thread on how to set the carb.

[sprayedgt]

Well the needle on mine is not adjustable so in order to adjust it I'd have to get one for a -'05 model. I'm really too scared to take it down to a 150 main since it came stock with a 148. Just one size main jet bigger IMO is not enough fuel since they are jetted lean from the factory anyway. When I had the carb off I looked at the butterfly/slide sync and it looked perfect. Do you guys think that I could throw in a 158 main to simulate moving the clip down?

[04TRX400EX]

You'd be surprised to know how different 400EX's run from the factory. We have three and they all feel different. I run similar mods and went from a stock 148 main to a 150 and it runs great. At the same time, I also moved the needle clip down (to richen it up slightly) and adjusted the air/fuel screw out to also richen the mixture.

I think your 3/4 throttle stumble is a sign of running rich, not lean, so switching to 158 will only make the problem worse. If your needle is fixed for whatever reason, pick up a newer one from your dealer.

BTW, from what I understand, Honda's like most manufacturers, jet their quads a bit rich from the factory to avoid any issues as it is better to run rich than lean. The only company that seems to jet lean is KTM, and probably Polaris since they use the same engines.

[NacsMXer]

Originally posted by sprayedgt
Well the needle on mine is not adjustable so in order to adjust it I'd have to get one for a -'05 model. I'm really too scared to take it down to a 150 main since it came stock with a 148. Just one size main jet bigger IMO is not enough fuel since they are jetted lean from the factory anyway. When I had the carb off I looked at the butterfly/slide sync and it looked perfect. Do you guys think that I could throw in a 158 main to simulate moving the clip down?

You'll never know if you don't try ;)

It's not gonna blow up if you run a 150 in it. Try it out and see if it cleans up, then you'll know if it was the problem or not. Go ahead and try the 158 too, but i'd be willing to bet it will make things worse. The point is, you will never know where you are at until you experiment a little bit. Get a couple spark plugs and make some runs with different combos.