OS FS 70 Surpass compression loss? - RC Groups
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Oct 21, 2010, 10:18 AM
Registered User

OS FS 70 Surpass compression loss?

Hi, I have an os 70 surpass which seems to of lost compression.
It was always easy to hand start, just prime/choke, and a reverse flick and it would fire up, but sadly not any longer.

It seems to have little compression and will no longer fire by hand, it will fire using a starter and run at about 9.5k on a 13x7 prop (maybe 10k but a few clicks of peak is about 9.5k). It usually idles ok maybe a bit higher than normal but sometimes the revs will just splutter/drop a bit and it stops.
I could feel a bit pressure on the inlet side during the compression stroke so assumed the inlet valve is leaking a bit.

I removed the head to inspect, the inlet valve looked clean, cleaned up both valves, the exhaust had a bit of carbon on it but not to bad and put it all back together, couldn't tell much difference in compression still feel pressure in the inlet, thou fuel draw is better so think the exhaust valve is sealing better.

Still runs the same thou and wont fire by hand.

Bottom end looks perfect.

Reading the forums i'm reluctant to try lapping the valve, and I think its to old to find a new head.

Do you think it would be worth trying a new valve or is it more likely the seat will wear first?

Valve clearances were fine both before and after cleanup.
Last edited by krazy2; Oct 21, 2010 at 11:05 AM.
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Oct 21, 2010, 11:15 AM
Registered User
Put some soapy water around the head and see if it's leaking there. Do the same with the plug (and use a new gasket and plug). Are you sure the valve springs are good? Have you turned it over with the head cover off to see if it's leaking around the valves?
You did try a new plug, new fuel, etc. I'm sure.

Any strange noises that would indicate bad bearings? Did you fly in a dusty area?

You can always try to find a new head, either dealer or in the for sale forums.

Oct 21, 2010, 01:22 PM
Registered User
You get pressure coming out of the carb on the compression stroke, there is no leaks anywhere else.
Valve springs are good.
No strange noises, no dusty areas, any yes plug, fuel etc was tried first.
Oct 21, 2010, 01:31 PM
Registered User
Broken Wings's Avatar
You could run a little more oil in your fuel and get better ring seal. I use 18-20% oil.
Oct 21, 2010, 10:35 PM
jeffie8696's Avatar
If you turn the engine over to where the compression should start then pour some fuel or whatever in the intake/exhaust port then pull it through the rest of the compression stroke if it bubbles up you have leaky valves.
New springs may help as they do loose tension with use and it is most noticeable at the seat.

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