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Staining on top of piston

Old 08-10-2007, 09:53 AM
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Default Staining on top of piston

I have an Evolution 26GT that has 1 gallon of fuel ran through, at the 32:1 mix. Using Yamalube synthetic racing oil (formulated for air cooled dirt bikes). Engine is running great on the ground, around 8700 RPM, but on maiden I had problems attaining full RPM in flight. Bent axle on landing prohibited further experiment, but I am assuming I was rich in the air.

Anyway... I pulled the cowl to inspect possible air leaks adjust needles, etc. When I pulled the plug, while the porcelain was a nice medium brown, the bottom of the threads of the plug showed a great deal of buildup...almost looked like baked oil. Wouldn't come off with fingernail. So I pulled the head off the jug, and the top of the piston, in the area of the plug, has a large black stain similar to the debris on the plug. A rag soaked in gas showed that some of it would come off. The cylinder walls look good, it's just the top of the plug.

Is my oil content too high? Poor performing oil? Normal occurrence? Changed the plug and engine was revived, reaching a higher RPM (+200) without touching the needles. Going to adjust the needles, as I am obviously too rich, but is the stain I see an indicator of something? Thanks.
Old 08-11-2007, 03:18 AM
pe reivers
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Default RE: Staining on top of piston

staining of the piston with carbon deposits is a normal thing, and it also happens in four stroke engines where there is no oil present in the combustion process.
Modern two stroke oils have additives that clean the piston and moving parts like exhaust port valves. Depending on the oil used, there are more or less of these cleaning agents. Racing oils have less, because racing engines are inspected frequently, and slight carbon deposits are of no importance.
Model engines have no exhaust port valves, so a bit of carbon does not hurt at all. Good lubrication properties are far more important. And that is what racing oils are very good at.

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