I have a 33 Generac giving some fits. Just replaced the cylinder heads, valve train, and head gasket. Compression on both cylinders is down around ~50 psi. These engines spec at 185+ ish. Here's the kicker, I can pass a leak down with less than 15% leakage at TDC using 90 psi. I get *some* blow by into the crank case, but nothing out of the ordinary. I can hear the air from the dipstick tube, but can't feel much air at all. Slight air past the valves, I can feel a little air on the back of my hand with the valve cover off.
Started to think the cylinder wall was warped or out of spec, but what's the chances of both cylinders being equally bad at the same time? My next step is to run the leak down with rocker arms disconnected to ensure valves are closed and then rotate engine to get a leak down reading from TDC to BDC.
My first guess would be automatic compression release, if you are spinning engine with starter it won't turn fast enough to get a good compression test, try turning engine by hand backwards and see if you can get a better reading.
Only thing I'm not sure of if you can get your hands on screen of cooling fan, only Generac I have is on a small generator.
