KX Guru Racing

Post Author Update:

Problem:
Oem kx500 carb (Keihin pwk 39mm) running rich, having to reduce jet sizes to compensate.

Cause:
The cause maybe a worn out needle jet.

Determination:
You know you have a worn out needle jet if your carb noticeably runs rich. Reducing the jet sizes may not cure ...

More from author of post:

PWK NEEDLE JET WEARING OUT DISCUSSION- FIX

If the needle IS RUBBING the needle jet you may have a bent needle or needle clip. If there is a flat spot the damage is done. It's an old carb and yes carbs wear out.

KTM OWNER (they also use the pwk carbs) COMMENTS:
Try a ...

So much will depend on your altitude, fuel type, and engine set up.

Former Kawasaki Factory Engineer Doug Haas recommends the following:

Main Jet: 170
Pilot Jet: 55 or 58 if hard starting
Needle: N82M
Clip: 2
Slide: Modified to be 7.0.
Spark plug: B9EVX
Air Screw: 2.0 turns out
Race this setting ...

My oem stock 39mm on my 91 500 needle jet wore oblong. Tried to punch it out and replace from the jet block, didn't work out so well. Ended up crushing the chamber ruining the jet block.

Went craigs list shopping and found a like kind 39mm short body with the understanding it was off a 03 ktm 300 ...