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Plugs fouling (oil)


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I gave my son his 2002 KX250 for Christmas this AM. 3 times in the last 3 hours it has cut off. I pull the plug and it is covered with oil. I get spark at the wire but not thru the plug. I put a new plug in it (twice), it had spark thru the plug and started right up. He would ride it for an hour or so and then it would cut out again. Crank bearing leaking? Stator breaking down? Plugs burning out? When it runs, it runs great. He hasn't been lugging it, he has been riding at high RPMs. Help please. Trying to learn.

Edited by Mtnboy1
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What size jets are in there now and what is your elevation?  These things come rich from the factory and thats at sea level.  I went from a 55 pilot to a 35 to get mine running crisp.  I run the pilot lean and the main and needle a little rich.  You can lug around on trails or rip around wide open.  When it needs fuel at larger throttle openings the needle and main provide it.  Modern two strokes should not foul plugs if the jetting is even close.  Crank seals would be the next suspect, but if it burning trans oil it will be smoking like hell.  A cleanly jetted two stroke won't smoke at all once warmed up.

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I'm using NGKBR8ES. He's definitely not lugging it. I purchased the bike near here and the PO had just had the carb cleaned and tuned. What ever it is, it is burning up the plugs. I just tried ohming the plugs and they won't ohm. I had asked before about the jetting and the opinion was that since it had been ridden in the same general area (air temp, elevation) that the jetting was probably OK but I might have to make adjustments to the air screw for temp changes.

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Fwiw, my KX125 would foul plugs in 30 seconds once my right side crank seal got bad enough. The weeks before it got that bad, I leaned it and had to be on the pipe all the time to not foul. At the time I was a newer rider and on trails, so it was tough to manage.

But doesn't the 250 have to have the cases split to replace the seals? That would make me think the PO maybe didn't replace them.

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Is the transmission / clutch oil level going down?

I thought it was, but maybe I just hadn't given it enough time to drain back after the engine stopped. The last time it did it, the oil was in the site glass. That was why I was asking about the crank seal. I'm gonna try building a leak down tester later today. It really acts like something to do with the fire. Like it kills the plugs and that why it gets covered in oil. I'm gonna read a little deeper into the electrical system. After the magneto or generator (on the flywheel) is there a voltage regulator type thing that could be going bad? Like allowing too much voltage thru and burning up the plug? Like I said before, when it dies, my son said it doesn't backfire or pop itr just dies. I spoke to the PO yesterday and he said he maybe fouled 2 plugs in over a year. I'm starting to think it's not 'fouling' the plug, it's burning it out. Again, please forgive my ignorance. I'm OK mechanically but this is a new world for me.

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On mine, when it got real bad I would foul them up in the mtns and run out of spares and try to use a lighter to dry off the wet ones. But what I noticed is that when the trans oil bakes on the plug, it's very hard to remove. It kind of gets crusty and charred looking. On previous bikes and friend's bikes, we've been able to take a lighter and just dry off the plug and put it back in sometimes when it was just rich on fuel.

If you Google some images of fouled plugs you'll probably find some charts with all the different ways they foul. Mine did not pop or back fire or anything like that either, it would just lose power and if I didn't get on the gas hard in a split second it was too late...needed a new one. I'm not that good with ignition systems, but it seems like it would be hard for a dirt bike ignition to over-spark a plug. I was under the impression that they were weak compared to auto systems.

But yeah, building a simple pressure tester is something very useful for all 2strokes so I would do that. It will tell you a lot. No jetting or modifications for performance will fix what a leaking crank seal will create. And we made one for about $30.

For example, my buddy's bike started to pop and backfire like it was lean. We changed the needle clip and it helped some. But after putting the pressure tester on it we found out that it was leaking air though the reed/intake boot seal.

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OK so, I got some 93 octane gas, super M oil, and a new plug. Changed the ratio from 32:1 to 40:1. Drained all the gas out of the bike. Filled it back up. He has ridden 3 hours without the first problem. After he gets finished riding today, I will pull the plug and see what it looks like. Thanks y'all! One happy young man. (And his Dad is happy for him)

Edited by Mtnboy1
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