Hey Dave,

I have a YS91 that I sent in last Summer for a tune up. All gaskets were replaced etc. It's had about 2 gallons of fuel run in it and it's been working great until recently and I can not figure out what the problem is.

It holds pressure fine. I've changed the glow plug (twice) just to be sure (OS F type). I use 20/20 Power Master fuel and am swinging a 15 x 6 prop. The engine is mounted inverted on a UCD3D.

It idles fine somewhere around 2100 to 2400. The transition from low to high is quick and powerful. At full throttle I have a fairly thick stream of smoke coming from the engine. A tach shows the engine turning around 9500. In the air it runs great at low to mid-throttle... but if I do an extended up line at full throttle it loses significant power. Unable to keep the plane even in a hover... when I back off the throttle and let it coast back down I can hear the engine surging. Sometimes I can work the throttle stick and get it to come back to life, but most of the time it will die.

I've tried richening the top-end a bit, but then it gurgles when I'm testing it on the ground. I've also tried leaning the top end, and the engine died during level flight.

As odd as it sounds, the bad behavior of the engine seems linked to the up-line more than the throttle. I've done some tests flying it level at full throttle and it seemed ok(?). I think I could fly the tank empty with the plane level... but that would defeat the purpose of flying a UCD3D! lol!

My needle settings are: top end - slightly less than 1 1/2 out; low end - 1 1/2 out; regulator - flush with top.

As a last observation, I did notice that if the engine is idling and I raise the nose of the plane, the engine will die. Again, the nose up attitude caused probs.

Thanks for your help Dave,

Bobby