I have just rebuilt my 400EX after sucking down some sand this summer at a race. I went 2mm bigger JE piston with 11:1 comp, and a TC racing cam. After finally putting everything together and getting it to start, it won't run good. idle to 1/8 or maybe 1/4 throtle is good, but after that, it is rough. Then after about 3/4 throtle, it gets a little bit better, but not much. Then I pulled it back in the garage and the plug was completely black! Correct me if I'm wrong, but doesn't that mean that it is rich. Should I try to rejet it or did I mess up the timing a little bit. please help. Did anyone else have this problem? Did yours work with your stock carb and stock muffler? help as soon as possible!

 

Sounds like you jetted extremely rich (black plug, and same symtoms as what mines like with the choke open).

 

It sounds like a jetting problem doesn't it. But with the mods I've done, I didn't think it should be running rich. That's what makes me believe that the timing might be a little bit off or something.

 

First go back into the head and look at the cam chain, make sure it's straight and everything. Then set your bottom timing mark on T. Then set the cam timing mark straight up. Put 'er back together and if she's still running bad, it's probably a jetting problem.

 

If you upped the compression (I believe stock is about 9.2:1) to 11:1, then you are probably running too rich. Increasing the compression causes the piston to be driven down at a faster rate than stock. This increases the amount of fuel that is pulled into the cylinder, hence the rich condition.

The correct method to jet a four stroke is to increase the main jet size up until it cuts out due to a too rich condition and then start backing the main jet size down until it clears up.

Regards,

Rog

 

I have a 142 main that came with my dynojet kit in the 400 right now. That should be lean with these new mods, right? That is one size down from the stock setting. Since it's already running rich, I wouldn't want to go any bigger, right?

 

The numbers on Dynojet jets aren't comparable to keihin (stock) jets. 142 Dynojet might actually be 132 keihin(just as an example).

 

Id call White Brothers. They have a 416 kit and im sure that they would know thier way around one pretty good.just a thought

 

i would agree with the guy above.
make sure your timing is right.