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Stuck Rings

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mstull

R.I.P.
Joined
Jun 23, 2005
Messages
1,263
Location
West Texas
I always wondered why Rotax recommends running their 2-stroke engines so rich. I assumed it was their way of being super conservative, and to keep the CHT under control for those who run their engines real hard. Running rich seemed to waste fuel and pollute the atmosphere unnecessarily. So I've been running my engines clean and lean.

But I've had constant problems with piston rings sticking in their grooves. I think some types of rings and some engines might be more susceptible to that than others. And some oils might be better than others. After just 50 hours, the upper rings in my Kawasaki 440 were completely stuck. I couldn't even pry them out. I was using Castrol TTS, and had my EGT adjusted to burn the plugs light tan.

The oil in the ring grooves must cook and scorch from the heat, turning thick, dark, and gummy. When the rings are no longer free, combustion gasses blow by, and carbon mixes with the gum, turning it into a solid. Eventually the lower ring will gum up, and power will decrease. Fortunately this problem usually doesn't cause a catastrophic failure nor expensive engine damage.

I'm not sure how rich you have to run a 2-stroke engine to prevent this. I've heard all the usual things, like coffee with cream, for the spark plug color. Some people assume the richer the better. I don't buy into that logic. Fouled plugs probably cause as many engine problems in 2-strokes as anything else. I'll tune for a light brown, coffee with cream color, and see how it goes. Lesson learned.
 
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