Jetting stock /w/ aftermarket pipe????

I called my dealer and he says that my 02' yz426 has an 162m 42p clip on 4th from the top on needle and the air/fuel screw 1 3/8 out.

the person that I bought it from knew little about bikes and the screws on the carb are not mauled and still look factory, so I'm thinking that he left it stock jetting and put the new pipe on there to make it legal at all the ohv areas.

It has an FMF powercore Q (quiet) on it. You can see it in my garrage,

or here:

HERES THE QUESTION: what do you recomend I change it to encrease hp?? the jetting cant be right, I havent pulled the plug yet but I do plan to.....



Good question. I am in exactly the same situation with the same bike/pipe setup.

"Engines 101" states that if you open up the intake (free flow filter) and exhaust of a motor, you can richen the jetting and basically get more air/fuel mixture into the head resulting in more power.. My 01 XR400 was like a new bike when I did this..

With the 426, it comes with good breathing so it is a judgement call if we are getting better breathing with the new pipe or not. I also put on a NoToil air filter cause they are so easy to clean, and feel that this filter probably opened up the breathing somewhat - but again, I am convinced the stock unit wasn't bad.

So I am running stock jetting with my set up (the FMF pipe and air cleaner) and it runs well. Richened the mixture screw to reduce the popping on decel, and that helped..

Saw the jetting chart, and for my setup, it actually called for a leaner main and needle adjustment.. With my mods, I will not likely be going that way.

The most reliable way is the old fashion way and read the plug.. But much harder on these bikes (compared to my old XR). Idea here is that you basically do a power run (3/4 to full throttle with a relatively new plug) and kill the motor. Then check the color of the plug.

Bottom line, I will likely just check the plug periodically and keep the stock jetting..

Hope this helped.


it did, thanks, soon I'm gonna get a no toil also,

thanks alot,


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