Hello,

I have a YS91 AC.

I've had the engine in a few airplanes, and it's never really run right. The idle has always been perfect, and it starts easily. The problem is that no matter how rich I set the high end needle on the ground, it seems to lean out in flight.

The engine has been sitting on a shelf for a few months, and I'm getting ready to put it in another plane, so I've got it on a test stand.

I've got a 14x10 prop on it, and am running 15% Powermaster fuel. It's got an OS F plug. As it always has, it starts easily and idles great. When I run it up to full throttle, it winds up to about 8800 rpm, and puts out a steady stream of smoke. After about 5 or 10 seconds, the smoke stops and it runs really rough (rpms bounce between 4500 to 7000 rpm.)

At the moment, I've got the high end needle 4 turns out, although the symptom is the same, even if I turn the needle in to about 1.5 turns. The regulator screw is flush with the housing.

Any ideas would be appreciated.

Thank you,
-Wade

It needs some work. This engine was made long before the gas port mod in the piston and it has the symptoms of an engine that leaks past the ring and messes up the regulator function.

I think a trip to service is in order.

YS Service
1295 Industrial Court
Gardnerville, NV 89410

Phone 775-782-4562

Based on my experience with this motor, it is over propped. I have run mine with anything from a 15x6 wood (which it destroyed) to a 14x6 M.A.. It seemed happiest with an APC 14x7. It really cranks and has given no particular trouble in years. Just a thought.

We usually run them with either a 14x10 or 15x8, BUT this is with YS 20/20 fuel or 30% heli fuel. When you run anything with lower nitro, reduced prop load is the way to go.

This may help, but the gas port mod to the piston will make the problems go away in many cases where nothing else works.

Ok.

If you can let me know what's involved in "sending it in", I can do so.

In the time since I originally posted, before I read your reply, I decided to check the condition of the diaphragm. It looked to be in good condition, so I cleaned it off and put it back together. Now it doesn't want to run at all. It looks like it's not drawing fuel any more, at least not past the regulator. It's pressurizing the tank when I flip it over, and fuel is getting from the tank to the engine, but not from the front of the engine to the needle valve.

At this point, if you say that there is a modification that corrects this [running lean] problem, then that sounds great to me. I love the engine when it runs, but I haven't ever been able to keep it running properly for any period of time.

Thanks again,
-Wade

Send it to the above address with your name, address,and a note detailing the problem. Include a daytime phone number, and request an estimate of charges to be given to you prior to repair so there won't be any surprises.

Dave