YZ426 Jetting and running problems

A mate of mine has a 01 YZ426 he wants me to get running properly for him.

Its hard to start, it bogs down REALLY bad if the engine is lugged AT ALL, and is just generally very flat, little top end and it just feels slow. The engine is in A1 conditon, has just had new rings installed and is in excellent condition. It has a white bros E-series muffler on it and what I think is the stock header.

We ride at sea level, the bike is stock except for the muffler.

I assume that its running lean?

its only marginally faster than an XR400 the way it is so something is deffinalty up, its run this way the whole time he has owned it so it is nothing he has done.

Could someone please give me some jetting specs to try out on it?

Any help appreciated :mad:

You say lean, but it really sounds rich to me. Probably all across the board. In order to help much you may want to get all of the current jetting settings and post them. At a minimum you will need to get us the main and pilot jet sizes. Nice to know also what needle and position it has installed, and the fuel screw setting. Sorry, not much help just yet, but if you get this information you will get much more. Hope this helps.


Something to investigate might be the condition of the air filter.

Just a thought.

Hopefully Ill be able to get his bike over here on the weekend and check the specs but until then, could someone please reccomend me a ballpark jetting specs so I know what it should be.


Well, I am running a 165 main, 42 pilot, fuel screw at about 1.25-2 turns depending on the day. My needle is a JD blue at #3 I believe. I am at about 1200 ft ASL. Hope this helps.


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