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OS .91 FX Over heating Problem


Old 09-13-2011, 04:08 PM
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Location: Palm Beach u, FL
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Default OS .91 FX Over heating Problem



Any help would be appreciated.

-Engine is mounted in a SkyBolt Bi-plane, on it's right side with remote needle valve pointing straight up
-15x6 APC prop
-Morgan "Omega" 15% fuel
-Needle valve 1/4" above center of tank
-Has slim-Line Pitts style muffler
-Prior history of engine unknown, but engine looks like new but has been run
-It starts, idles and runs without overheating below 75% throttle


Engine runs hot at 75% to 100% throttle, and appears to be running lean. Idle, transition, and mid range are fine.
The high speed adjustment is very broad, without traditional pronounced peak. Richening the high speed jet does not eliminate the problem but causes the mid-range to run poorly.

Engine sags but does not quit when aircraft is pointed vertical. High speed needle adjustment affects but does not eliminate sag.
The needle valve is very tight and difficult to turn. Removed and inspected but looks normal.

It appears like engine is not getting enough fuel. I have eliminated the fuel lines and tank of all possible restrictions.

Additional thoughts:

Needle valve defective?
Morgan fuel has insufficient oil? (17%)
Engine was not properly broken in?
Slim-Line muffler does not supply enough pressure?

Thanking you in advance,
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Old 09-14-2011, 05:47 AM
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Location: Monticello, IL
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Default RE: OS .91 FX Over heating Problem

Try a stock muffler or just an open exhaust. The one you're trying to use may be too restrictive. If the needle's difficult to turn, it's bad. Get a new one. Richen the idle mixture a LOT-you need to get the high-speed needle set before you can get a good idle setting. The idle setting can interfere with high-speed fuel flow.

Put the engine on a test stand. Prop it for about 11,000 (even up to 12,000 or so!)at full throttle when leaned to just rich of peak RPM. That will be a lot louder, but usually results in better running. OS likes large props to help control noise...it also controls power.

Anything you mentioned could be the cause.
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