I have a YS 120AC that I purchased new in 1994. Last year was the first time out of the box. From the beginning, it would sag after exactly 2 passes across the field and then die if I did not reduce power for a minute. It was like clockwork every time. Originally, I suspected overheating due to the engine not being broken-in. It has gotten better with the first 4 gallons of fuel, but the needle valve is extremely sensitive. One or two clicks either way is too much, therefore I can't get a flight without it sagging from being to lean or just not having any power from being to rich. These AC engines have been run a great many hours by others, so I suspect that someone has experienced this before. Any suggestions?

Fuel was Sig 15% nitro
Prop APC 15 X 10
OS type F plug
High end needle approx 3/4 turn out
Regulator flush

Thanks in advance
Chad

After sitting for this length of time there are several possible causes. The ring could be stuck and leaking compression. The regulator is nearly guaranteed to be obstructed by debris.
Your best bet is to send this to YS service and let them bug it out for you, unless you have enough YS engine experience to do it yourself.

Might save you a lot of time.