I have one of those older OS 90 4 strokes which is new by the way and while my 1st flights were great and tuned right, my last 2 flights a day or so back wasnt. I tuned it as usual with my other 4 strokes a little on the rich side and even changed the plug. But while I got it in the air it lost power at least by half. I barely had enough power to keep her in the air. As long as I gradually gained altitude so's that just in case of a deadstick I felt better but I landed and tuned and tried it again and the same thing. SO.. I just ran it REAL rich on the ground and at full power until it ran out of fuel and kept richening it up until it ran out. I havent ran it since but what do you all think might have been the problem? I dont even have a gallon through it yet. Probly not even a half gallon. But from all my 4 strokes I have and had, I havent had this problem without it just being a high end needing adjusted. HELP!

Usually when an engine loses power like that, it is because the engine leaned
out in flight for some reason. Did you balance the prop and spinner? An out
of balance engine can vibrate the low end needle and actually change the
mixture setting.

Check over the line to the tank from the muffler, and the feed line to the carb
as well for a leak.

If the engine had gone rich and lost power....you would have seen a lot of smoke
and heard the change as well. You might want to check the valve clearances too.
You may have lost clearance on a valve, and lost compression and power as a
result.

I have one of those engines....it likes the OS "F" plug, and fuel with castor in it.

FBD.

If you are running a straight pipe like mine you might consider a Perry pump. With no tank pressure it eliminates a lot of headaches.

well unless some gremlins came in the night before it is ok. I flew it twice and no problem but, i havent checked the valves. I dont think they should be out yet but I might check them anyway. I am using the standard muffler. I caint say it ran perfect this time on the ground, it just was running half way decent. As far as the prop goes, I have balanced some and some I havent. This one I aint sure of. I will check that also.

Hello; I have two of these engines, and like them a lot. Your problem sounds like a fuel delivery problem. Maybe vibration is foaming your fuel. Maybe as teh engine heats, the valve clearances close up, not allowing one of the valves to close. It shouldn't be too hard to track. These engines run very well when they're right. Keep trying, it's worth it.