Tried to run a used 91 AC for the first time this afternoon and it now has no compression. The engine seemed fine and looked almost new when I got it. According to the previous owner it only had 10-12 runs since a YS performance overhaul. The engine started fine. After letting it warm up for a few seconds I advanced the throttle slowly and at about 1/4 it sputtered and died. It re-started immediately but died after a few seconds. Now the engine has no compression at all. The engine turns over the same both with or without the glow plug in. When the engine died it just died, no strange noises. Anyway, the piston still moves up and down in the bore, and the valves still appear to work properly. I was at the field when I pulled the valve cover so I didn't have a feeler gauge to check them, but I can see light between the rockers and valves so I don't think one of them is sticking and the gap did't look excessive. I haven't torn into the motor yet and hoped you'd have suggestions on what else to look for.

If the engine has been sitting for a long time since those "10 to 12" runs since overhaul, the ring could be stuck depending on what type fuel it was run on and how long it sat.

Even though the valves move, it doesn't mean they are sealing. some oil down the intake and exhaust holes with the piston on the compression stroke then turning it through will reveal lots of bubbles if the valves are not sealing.

Also check the head bolts for tightness.

My guess is the engine has a lot more than "10 to 12" runs since overhaul.