I am having a problem with an OS 91 FS it runs rough and hot and the high speed jet does not have much effect, we have checked the valves, cleaned the carb, new plug , its on test stand so the fuel supply is not a problem any ideas?? MIke

You don't say how old or what fuel you run.. bearings?

We are using fresh 15% Cool Power, the recommended prop, the engine is mounted upright in a test stand,the tank is mounted at the correct height, there does not appear to be any fuel draw, if you set the jets to factory specs the engine will not even run, we have to open them way up to even get it to run at all, we have stripped the carb , we have a lot of four stroke engines and this one has us beat Mike

Mulimike,
Having run O.S.4 strokes exclusivly, I've seen bad O rings on the needle valve cause similar problems. Hope this helps.
Karl

Thanks guys problem maybe solved, the OS Carb uses one brass tube sliding over each other with a metering slot , if you screw the low speed jet in too far blocks the slot ,the jet is actually screwed about half way out before the carb will work, I guess who ever had owned the engine before had the jet screwed all the way in Mike

Mike,
The O.S. manuals state that the neutral position for the idle screw is with the scew head turned in flush with the adjustment body, then turned out exactly one turn.
The very same thing happened to my O.S. 81a upon receiving it.
Karl

We flew it tonight and it ran like a dream thanks everyone Mike