Run a heavy dose of lightweight solvent-type oil additive, maybe Marvel Mystery Oil or something similar to address the possibility of a sticky valve guide. Also be sure the valves are adjusted correctly. A tight valve can have a major influence on compression pressure and general smoothness of operation.
You can also do a "cylinder leakdown test" where air pressure is put into the cylinders one at a time through the spark plug hole with the piston at top dead center and the transmission in gear with the brakes locked or the rear wheels chocked. Listen at the carburetor, tailpipe, and oil filler for air leaks. If a valve is burned or rings are bad, you'll hear a leak at the appropriate place. Lots of engines have run for years with low or even no compression on a cylinder or two- - - -it just drops the power somewhat and makes for a rough idle.
Jerry
