I've had this YS .91AC for over 10 years in a Goldberg Extra (several of those years just sitting). It had been running ok, but down on power slightly, I thought. Anyway I flew it the other day and it ran fine (except I did have to speed up the idle slightly from normal), then for the 2nd flight of the day would barely start and would keep dying. Now it won't start at idle and it may run at 1/2 or full throttle for a while, but will run slightly erratic. Once I throttle down it dies. One odd thing, there is fuel dripping from around the throttle arm.

I went back and changed all the settings to stock (high speed, low speed and pump) with no luck (running new Powermaster 20/20 fuel, YS glow plug and APC 15x8 prop).

I checked the valves and they are ok, changed the glow plug, changed the check valve. I took it out of the plane and checked all bolts, they were tight. It has pressure when I unhook the check valve, so don't see the tank as being an issue. I also changed the fuel lines on the engine. All of this with no help.

((2 days later))
Well, I ordered replacement gaskets and a diaphragm and installed them, but no luck. It will run better than before, so 1/4 throttle to full throttle runs fairly well. Once you get down around 2,000-2,500 rpm it will idle for anywhere from 0-15 seconds, then die. About ready to toss it and get a 1.10 (plus the .91 just doesn't have the umph to pull a Goldberg Extra). (The 15x8 APC only turns around 8,300)

Guess the bearing are the next thing to checck, but just not sure it is worth all this aggrevation.

Any thoughts?

First off I would say the 15-8 is way too much load for a 91 on only 20% nitro.

However it sounds to me like this engine is past due for some service work. Probably a ring

You may have an air leak somewhere but I'm not sure. Since the engine is pretty old I would send it in for service. Richard can fix you up. If there are worn out parts that need attention he can solve it for you. On engines that are old like this with lost of run time or just been sitting likely there is something going on.

I would say it might be rusty bearings. But rather than throw parts at it. I would let YS Parts and Service work it over. In the end they will be able to find if something else is wrong as well. Rusty bearings could so the signs you have with the tough to start and poor idle. bad bearings can kill a glow plug on the first try to start.

www.yspartsandservice.com

Troy Newman
Team YS

Thanks Troy.

I'll box it up and send it forth!

Trevor

Just an update on my .91AC. I sent it in and got the dreaded call. The bearings must have rusted and came apart which scored the piston, liner, etc, etc. It could be fixed, but would be close to the cost of a new engine. Needless to say, I now have a 1.10S.

The 1.10S is amazing. I have trouble getting the thing to stop running. It will tick over at 1,400 rpm and still won't stop. I'm impressed. the idle at 2,000 is VERY reliable. The top end with an APC 16x7 flies my Goldberg Extra much better.