I have a 06 400ex. Just put an hmf slip on exhaust on, put on a uni filter with the air box lid, 42 pilot jet, new spark plug, and started by trying a 160 main. Ran it and the plug looked a little black but it ran fine. So I dropped down to a 155 and didn't notice a difference in the performance. What should I do?

 

I mean how is it running in general. It's a little black so It's a little rich, but I say If it running good leave it. You could try lowering the needle down one notch and see how it runs then and what the plug looks like after. You kinda want the plug a nice brown color. Black is too rich, and white is to lean.

You said it runs fine, so I'm not sure what your trying to do :P