Dave,

I have a YS 91AC which was sitting on my shelf since 1998 before it found a home in my latest mini pattern plane only recently. It is my first YS and nobody at the club has any experience with YS either. Power is GREAT and the throttle response is simply awesome. There is just one point I wanted to check. Even with 2 -3 clicks off the peak I have to set the needle valve only at 3/4 open. I have plenty of exhaust smoke with this setting. The needle adjustment between very rich to (too) lean seems somewhat critical at this point. However, as I said, I have no comparism what is normal for this engine. Since the transition from mid range to peak goes without much of hesitation I have not fiddled with the regulator (yet). The regulator bolt is flush with the casing. I am still stunned about the acceleration anyway which is no comparism to any other engine I had before.

Are you aware that the needle valve design has been changed over the years to a more gradual taper?

The fuel, I am using is a fresh mix of 25% nitro and 23% sythetic.

Thanks!

It could be that the needle valve seat is worn out, along with the needle itself.

If you screw the needle in all the way until it stops, does the needle valve body bottom out on the hex portion of the spray bar? If it does, it is a sure sign that the NV assembly is worn out.

Dave, there is plenty of room left (about 2 mm) between the needle body and the hex when the needle is fully closed in.

Sounds OK. Since you are not used to YS engines, you will find that they typically do not have much needle range. From full lean to very rich is 1/4 turn on the needle. One click is usually 200 to 300 rpm. Your engine maybe completely normal. They also do not like being set very rich. One or two clicks off the peak is usually perfect.

You'll get used to it. Enjoy the horsepower, and if you have any other questions, let me know.

Good Flying!