May 3, 2001
I have a 92 KDX 250 with a major jetting issue. I have changed all jets except the needle jet and am still experiencing fouled plugs. This happens during idling when first started and when riding below 1/2 throttle for extended periods of time. I am pretty sure i need to go to a leaner jet needle but am cunfused by all the conversions for Keihin carbs. If anybody could provide me with a size I would appreciate it. I believe stock is a 1368.

My current setup is #40 slow jet, 160 main jet and air screw 2 out with 40:1 gas

dirt bike dave

Sponsoring Member
May 3, 2000
The higher needle # is a fatter needle which will restrict more fuel, leaning out the bike. In your manual I think there is a list optional parts - go for the highest # on the needle.

Have you filed your slide down or replaced witha KX250 slide? It is a common mod to remove 1mm from the bottom of the slide to lean out the low end and improve throttle response.

You should also try a #38 slow speed jet as you are two turns out on the airscrew.

What plug are you using? If you are using a B9ES, that is part of the problem. A hotter B8ES or even a B7ES works fine in that bike.

Depending on your weather, you might also want to try a 158 main.

Good luck getting it sorted out.

Canadian Dave

Super Power AssClown
Apr 28, 1999
Have you tried adjusting the needle's clip position yet? You don't go into much detail but it sounds like lowering the needle would be a good place to start it you haven't already.

Its not uncommon for KDX250 owner to go with a leaner slide/throttle valve or have the stock slide recut.

Have you optimize throttle response using the air screw? It could be the air screw is set too rich.

Be sure you've eliminated the possibility of any mechanical problems like worn crank seals, poorly adjust float level etc etc before jetting or it quickly turns into a nightmare.

Check out the carb tuning section on the JustKDX home page for more details on jetting. Use the link below.

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