I have a YS 91 in a Fury Extreme that has started acting rather funny. I can fly one flight OK, but in subsequent flights the engine stops very suddenly, as if it spit out the glow plug (which has happened).
I get great auto practice, but would like to know what is happening.
Engine starts easliy and idles great; I am running 30% fuel with an OS 8 plug.
Any thoughts?

Which 91? How old is it and how much fuel has been through it? What are your needle settings and what is the temp of the engine? Blowing a plug is usually a lean run or the engine wearing and the bits of metal poisoning the plug.

It's a 91 ST. It has less than 2 gallons thru it; idles well, transitions great. High end is open about 1-1 1/4 turns - plenty of smoke when flying.
I run a governor, but it has quit on normal throttle curves as well as in governor modes. It as quit when under high blade loads and while hovering.
The plugs have deformed pretty quickly, which I always have thought meant too much compression or nitro. I thought a head gasket might help.
I have been running YS for years and this is hte first one that has me stumped. I have run the same brand of fuel for 15 years. I run the same fuel in
all my enignes - and my YS 4 strokes love it.

If it is deformed you're lean on one of your needles. You're high needle is at 1 to 1.25 turns where are the others?

There was a guide I used to have on glow plugs that if I can find I'll send to you. PM me your e-mail address.