Feb 22, 2001
I have a stock KDX 250 '94 model and I am Going to put Boysen power reeds and either a FMF Power core or pro circuit silencer. I am keeping the stock pipe as I have heard that it is as equal to the after market pipes, My bike also has been ported. My question is will I need to rejet my carb to obtain max performance and if so what would be the best combination.

Thank-you in advance.


dirt bike dave

Sponsoring Member
May 3, 2000
The fact that your bike has been ported makes it very difficult for anyone to recommend jetting.

In the USA, the KDX 250 was jetted too rich; I don't have mine anymore but I know I leaned out the main and the pilot several steps (i think to a 160 main, and a 38 pilot) as well has modified the slide from a #6 to a #7 cutout. With an FMF pipe, aftermarket silencer, KX250 head gasket, Boyesen reeds & rad valve and slightly opened up KIPS valves, the bike ran pretty good. Most guys also run a B8ES plug instead of the stock B9ES. Jetted pretty good for 4,000+- aboe sea level.

BTW, the stock dual wall pipe weighs a ton; you can save a lot of weight with an aftermarket pipe. With an FMF pipe and a aftermarket silencer, the power will smooth out and rev higher, and the bike should lose its hard mid rpm hit.
Last edited:


Feb 24, 2001
Just a detail or two. The stock cutaway on the stock slide is a #5. You can get a #6 for about $69 through Sudco (look in motorcycle mags for add). My 91-250 runs on a #41 pilot, #55 main, the longest needle option, stock taper, on the #2 slot from the top. FMF fatty/turbin core, opened airbox, boysen reeds. All this works for above 5000 feet and very low humidity. I'm adding a kx head gasket and divider this week and I'll post my thoughts. The fatty pipe was worth the cash. Just more power everywhere wth the balance in the mid and top end.
Good luck.
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