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OS 200FS lost compression

Old 05-13-2010, 04:04 AM
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Default OS 200FS lost compression

Hiya All,

I was just wondering what the causes of lost compression would be. I've checked for stuck valves and glow seal and with the head off and my hand covering the barrel the piston does "suck" well. Visually the valves look seated ok but I've not taken them off yet to check the seats properly.

I was thinking that no compression must be either Valves, Ring, or Glow. And if I get suction with my hand over the barrel then the ring should be ok.

Am I missing something?
Old 05-13-2010, 09:05 AM
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Default RE: OS 200FS lost compression

No, you're not. You may want to put the head back on with the valve train and see if the valves are moving correctly. Are they closing? If you turn the engine over quickly, do you hear any noise of air passing the valves?

With a four-stroke engine, you get low compression when the ring doesn't seal to the cylinder liner, when a valve leaks, or if the cylinder head isn't making a proper seal to the cylinder liner. That's assuming that there's nothing physical, like a hole in the piston or a great, big gouge in the liner.

You can get some sucking when you place your palm against the top of the cylinder even if the ring isn't sealing well. Check to see how well the ring is seating against the cylinder. You should be able to feel some "drag" when the liner is dry of oil and you gently move the piston up and down. Don't do that too much, or you can damage the ring and/or liner. If you dribble some oil into the cylinder and the compression leaps up a lot, then we'd suspect a bad ring-to-liner fit.

The engine may not have anything wrong, other than the ring has not been properly bedded-in to the groove in the piston and against the liner. You can try to start the engine with the throttle opened to 1/4 to 1/3...or even more. Trying to start at an idle setting when you don't know if the engine's adjusted correctly can be a trial.
Old 05-14-2010, 02:31 AM
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Default RE: OS 200FS lost compression

Thanks Bax.

The valves looked ok but I cleaned them up a bit and now I've got great compression. Must have been some carbon under the valve, although as I said, visually they looked ok.

Anyway, one more question. I looked at the exploded view and thought the valve springs were the same part number but I was looking at the wrong manual. The 200 springs are different parts and one is indeed longer than the other. Could you please tell me which is which? Should the longer one go on the intake or exhaust valve.

Also, I note a heap of metal in the crank oil, plus the big end is rough like it's been hit by something. I got this engine second hand with a plane so I don't know the full history. Is it normal for some metal to be produced during the run-in? If not are there any usual suspects for what could have been ground up? I guess it's probably worth it to order a set of bearings and clean out the existing "metallic" oil with some methanol or alcohol.

Thanks for your continued awesome support via this forum.

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