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OS91 FX Inverted

Old 11-25-2006, 08:32 PM
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ghofferbert
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Default OS91 FX Inverted

I have a new OS91FX mounted inverted with a Pitts muffler and a 14X8 Prop in Stinson Station Wagon. I concede that the center of the fuel tank is 3/8" higher than the center of the carb. It did successfully run throught the break in as recomended in the manual, however it would never run a good 2 cycle and after about 3 minutes of flying would lose power and die within seconds. Subsequent tuning efforts on the ground produced an engine that would not run well at any power setting and got hot, even though there is enough fuel present to "drip" from the carb when not running. On recomendations from locals I screwed up the low end setting, so I need help with two things. First how do I "reset" the low end to a factory setting and two what can I do to make this reliable? Some considerations are a glow plug with an idle bar, a pump, relocating the fuel tank, relocating the needle valve and smaller prop. I have limited experience, but have other OS engines that run great....wish this one did. Or is it such a thing that this engine just isn't suited for this application and I need to get something else?
Old 11-28-2006, 12:01 PM
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Bax
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Default RE: OS91 FX Inverted

How does the engine run when it's on a test fixture with everything in good positions, relative to each other? Mount it upright with the fuel tank's center at the same height at the spraybar in the carb. Do you have the needle that has the plastic housing?

If the engine gives no problems on the test stand, then the difficulty is in the airframe/engine combination. Have to start with the engine isolated, though.
Old 11-30-2006, 02:47 PM
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ghofferbert
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Default RE: OS91 FX Inverted

I never tried to run it right side up, and it does have a plastic needle housing. I can't test it right side up now until I learn how to reset the low end needle. It got screwed in so far as to come out of the threads, I did get it threaded back in but now have no idea as to where it's set. So if I can not learn how to "baseline" it, I need to send it in for repair. Then I can test run it right side up. I do plan on removing it from this plane and installing it in one that runs right side up....just seems like that might save a lot of trouble.
Maybe you can answer another question for me, I've been looking at this FS120SIII but I need to know what size the exhaust header is....it should be like 13MM with 13X1 threads or something like that. To be honest I have a saito pipe that fits my plane that I would like to run on it if possible.
Old 12-01-2006, 11:24 AM
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Bax
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Default RE: OS91 FX Inverted

The instructions would tell you how to "ballpark" the idle mixture. O.S. calls it the "mixture control screw".

The Saito exhaust header won't fit onto an O.S. engine. They use different threads. The cylinder head has a 15mm thread diameter. The thread pitch is about 0.75 or so.
Old 12-01-2006, 02:25 PM
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ghofferbert
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Default RE: OS91 FX Inverted

hhmm..I looked for that in the book and didn't see it. I'll look again. Thanks for your advice!

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