YS91SR went lean in the air and stopped, where to look?

[maddog7]

Hi all,
My YS91SR, which has been an excellent engine since I was kindly gifted it suddenly went lean in the air and stopped. The big bus it's in autos forever so all came down happily.
I've got the engine out of the airframe and am looking to see what went wrong, there was plenty of fuel left in the tank, I've pressure tested the tank/lines and they're fine, The piston does not have a hole in it Both bearings look/feel fine, compression seems good for a ringed engine.
It felt very much to me like fuel starvation so I've got the carb/pump assy in bits. I bought a new diaphragm just in case but old one looks absolutley fine.
This happened a few weeks ago so I'm not exactly sure but I think there was no pressure in the tank when I removed the fuel line.
The one way valve seems to function perfectly so I don't think it's that.
I make the uninformed assumption that fuel pressure in this engine is a two part process, crankcase pumping pressure and the one way pressurise the tank and the little diaphragm pump thingy pumps fuel to the carb?
Can anyone enlighten me as to how the carb pump thing works exactly?

Any insights?
Cheers
D

[trillian]

I had a 91SR that ran amazingly well for about 6 months and then started, seemingly at random, doing that same thing of suddenly going lean and cutting in flight.

I tried everything I could think of, the tank always had good pressure, I replaced the one-way but the old one still worked fine as far as I can tell. Just when I thought it was OK it went lean again and this time the piston and liner were ruined.

Only much later did I discover that the high needle had a very severe notch that I can only assume must have happened once when it was turned all the way in and then turned too hard the wrong way. So my theory is that somehow the notch was letting it be slobbering rich until it reached a certain amount of fuel flow and then making it go lean. (either that or something in the regulator but it seems too much of a coincidence)

[maddog7]

That's very useful to know Sheryl. Both needles/seals look in very good condition. I've back blown the carb body/barrel/ways with compressed air and cleaned the pump assy as best I can. There's nothing left of the glow plug element and the piston top and plug buttom are a bit blackened (as opposed to brown from extreme heat) if that means anything.
I'll maybe just re-assemble it and see if it'll start....
Cheers
D

[Grumpy]

Sounds like the piston/ring and/or liner might be worn out. It's the classic symptom of a worn engine that it will run ok when cold, but as it warms up and the cylinder expands the clearances open up and the motor conks out due to lack of compression.

A gummed up piston ring could have a similar effect so worth giving it a good check over.