I have a YS 91 that up until last fall ran great. It is in a UC3D and would hover at less than 1/2 throttle. Last fall it began losing power when hovering. It would run fine the rest of the time. I run it very rich, so it isn't leaning out. I have returned it to YS twice since last fall and it is still having the same problem. YS replaced more parts than I can list including crankcase, piston, two valves, rear bearing and misc other parts. Still having the problem however. Any suggestions anyone???

Thanks

Sounds like a fuel draw problem to me. Have you tried to have someone hold the plane vertical while you run it up? In other words, can you duplicate the problem on the ground?

It runs fine on the ground, even when you hold it up vertical. The YS "pressure system" seems to be working fine also. When I land, the system still has pressure on it. Based on that, I did not think that it was a fuel flow problem.

Ok so how about overheating? There's no airflow except from the prop while hovering.

Someone else mentioned that today. I took the cowl off tonight and hope to get a chance to try it again tomorrow. I had not considered overheating as a problem since I had been flying it for almost a year when I began having this problem. Let you know how it turns out after I get a chance to try it again.

Oops, I forgot you mentioned that it was running fine before. Anyway I hope that hovering without the cowling helps.

Considering it ran good before , have you checked the bearings?

I have not checked the bearings, but YS replaced the rear bearing. I assume that they would have replaced any bearing that had a problem. I did fly it today without the cowl and it did run a little better. At least it did not lose power like it had been doing while in a hover; it still will not pull out of the hover like it use to...maybe it's just worn out??? I may try some 30% heli fuel and see what that does instead of the 20% Pro Pattern that I normally use.

Im having the same problem with my .32 size heli only mine was made in 1987.Im considering a high output cooling fan. Im figuring that when the motor warms up the cylinder is expanding and Im losing all my compression.[:@]