Motorcycle Repair: KX 125, mixture ratio, air screw

Question
Hi, i have a KX 125 2002 2 stroke motocross bike. I have had some problems before with it for fouling plugs, i think it was my mixture ratio of being to rich, because after every ride there was oil running down my silencer. But i switched my oil mixture to 32:1 and no more oil running down my exhaust! I have put in a good NGK irodium spark plug and after 4 rides it was no good, or fouled up. I have put a new one and drove about 3 hours with it in 1 drive and it seems to work good. The only change i made was i put out the air screw to 1 3/4. Do you think that is good, or have any idea about the fouling plugs problem? Thanks for your future answer, and look forward to your reply.

Answer
Josh, you have a myriad of issues with jetting 2 strokes....

Too much oil can foul plugs, but also can lean out the mixture as it thickens up the fuel mix in the jets, which slows down delivery.

Less oil means less lubrication, although most synthetic 2 stroke oils can be run all the way out to 50:1 in some applications. Less oil will also enrichen the mixture for the same reasons as stated previously.

The mixture screw that you adjusted only affects idle and initial part-throttle metering, then it is out of the circuit. Obviously, when you chop the throttle it will drop back to the idle circuit for a moment. You probably want to keep the idle mix as lean as possible, as the deceleration conditions when you chop the throttle back, also creates a high vacuum at the intake tract, which wants to pull fuel in from any available jet.

Fouled plugs can come from oil fouling or fuel fouling or a combination of both.  Remember these are race bikes, not trail bikes, so expecting them to run all day on a plug may be beyond their capability, even when tuned to the maximum efficiency.

You have to make very small changes to fuel-mix, jets and needle settings in order to get it dialed in to the best performance. Do one small step, try it and see how it goes, then change something else. Two-strokes like lots of oil for lubrication, but you can always overdo a good thing.

Have a chat with these guys...

http://www.kdxgarage.com/

Bill Silver