RE: compression testing
I often repair used yard machines, and of course, convert engines for airplanes. When I convert them for airplanes I just automatically use a new Frank Bowman ring because I want max power, easy starting, etc., and what's 11 bucks for a new ring in the grand scheme of things??
As for the yard machines, I cheat a little. I shoot some WD-40 into the spark plug hole, and then turn them over several times. Then I check the compression. In my experience, anything below 90 psi is probably not worth fixing. Anything below 60 or 70 lbs. probably won't even start. 100 or over still has life in it. This has proved out over numerous years of repairing and selling the machines.
The compression reading will vary according to the original compression ratio of the engine, the exhaust port timing, the amount of carbon build up on the piston and head, and of course ring wear. I figure that my WD-40 treatment leaves more or less the same amount of oil to help seal the rings as a normally just run engine would have for the compression test. The injection of premix (oil/gas) and then turning it over numerous times before taking the reading has merit as well.
As mentioned, be sure to block the carburetor wide open when checking the compression.
AV8TOR