FMF Powercore4 install on a 2001 Yamaha YZ426FN

I recently bought an FMF Powercore4 slip-on with a power bomb header and I was wondering if I need to jet the carburetor to install this pipe on my bike. If I do how will I do that? I still have the original pipe installed and I want to get this one put on by this weekend. Thank you!

At most you would have to adjust your fuel screw, lower your needle clip possibly, and replace the main jet with one size bigger. All are very easy to do. Needle access is through the cap on top of the carb, main jet access through the bottom of bowl the plug unscrews and the fuel mixture screw is in the front bottom of bowl. Try and find an online manual it will have nice pictures of all the procedures. Search on here as well.

What will happen if I install the pipe without jetting or changing needles?

It may be ok, but it also may need slight tuning to get it running to perfection. Or as close to perfect as it can haha

The motor will not get affected by that, what I mean is valves issues or possible breakdown?

Thank you kindly!

If anything it may run slightly lean, but shouldn't be that lean because it isn't much different than the stock YZ setup. They are similar exhaust as for how open they are and the flow. It's not like going from a plugged up WR to the FMF, that would be more of a step up.

But still there is no way for me to tell you that you will absolutely have to rejet. You may have to you may not, it's all about how it runs. Jetting is all trial and error.

Sounds good, thank you very much for all your help and I will install the pipe this weekend and see what happens. Maybe i will just install the Power bomb header with the original pipe and see if it makes a power difference.

I have the power bomb header with stock muffler and it runs great. I have one size smaller pilot jet, power now valve, adjustable zipty mixture screw about 1 and 3/4 turns out. Also added the 450 accelerator pump and it runs awesome. With the power core you can probably run stock jets just bring the mixture screw out a bit. probably not more than two turns. Just play with it til you get the quick snap right off idle then it should run good. These bike deffinately need to breath. Good luck!!!


Also look at the plug after you ride it. Should be fine because these bikes are jetted on the rich side anyways.

Thank you Brandon, I will install the power bomb and the FMF, and turn the mixture screw out and see what happens.

Thanks again!


I installed the entire FMF pipe, the bike idles real well, I did not get to ride it, but it sounds really good. Do you think still need to jet it still?

There is no way for us to know for sure without riding it....even if it seems ok doesn't mean it can't be a little bit better with some fine tuning.

Yes you are right, I will ride it tomorrow and do what you told me and see how it runs.

I rode the bike and runs great! I did not needed to touch the jetting, idles a lot better then before. Thanks for all your help! :thumbsup:

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