So I wanted to dial my WR400 in so it would be spot on for trail riding. After installing a full FMF powercore exhaust I completely lost my over rev, and the bike just felt extremely down on power.
I purchased a JD jet kit, and went about redoing my jets. I also cleaned out my pilot jet very thoroughly, and installed the jets recommended in the kit. After riding it a bit, and seeing some advice on here, I didn't wind up using the main jet included in the kit (170) and tried a jet size larger than the kit, a 172.
Here's video of my two dyno runs:
Here's the dyno sheet:
First run (blue lines):
Big lean spots, and the bike was a bit anemic in a few spots, and at risk for burning up the motor it was so lean in said spots. This definitely called for a larger main jet, and given my main jet was already larger than anything provided in the JD jet kit... I can't recommend buying it. Jet's are cheap ($4-6 each dependign) and $75 for just a needle isn't worth it.
I reinstalled a 178 main jet, and then had my second run.
Second run (red lines):
Peak power did not improve from the first run, however all the excessively lean spots disappeared, the engine really settled in. The engine was smoother, it's response was stronger, and MAN did it deliver! I took it out for a spin before the dyno testing, and sure enough the bike stood right up with some serious gusto! WOOHOO!
So yeah, a 178 main jet is apparently the way to go. I'm still a bit lean, but given the temperature today (55 ish degrees) this leaves room for warmer days, where there will be less air, reducing my air to fuel ratio, and bringing me closer to the target mixture represented by that red line there.
(Popping on deceleration still to be resolved. I'm somewhat convinced it's an air leak now at the intersection of the header and the engine. More on that to come though when I try and suggest solution to the problem)
Edited by Jnicola, May 08, 2011 - 02:55 PM.