Doing compression test all 80/79 except cyl 3. 912 is early 80 horse under 1200 hrs. Runs just fine. Cyl 3 was 80/76 last year and similar for the last several years. Plugs all looked good. So I ran the engine for several minutes and retested. NO Joy. Went flying for an hour, retested, NO Joy. On some of the tests I could hear the oil tank bubbling. On others I'm pretty sure I heard air hissing in the exhaust. My wimpy compressor kept cycling on and off so hearing the noises was hit and miss. I believe I need to take the head off next and inspect the valves. Maybe remove the cyl and piston?