The leakdown test will tell you for sure. Usually you go with your highest compression cylinder and compare that with your lowest. If it is around 30 or less...there is an issue (mechanically). I would look into possibly pulling the head off or doing the leakdown test. The leakdown test is easy to do...make sure its at TDC on the compression stroke (both valves closed) and then listen at your Throttle body (with the butterfly open), your oil filler cap (with the cap off), and at your exhaust tip. You will have to listen hard but if you hear a hissing, that is where your problem is. Intake/Throttle Body, Exhaust/Exhaust Tip, Rings/Hole in piston/Oil Filler
Hope that helps
-Drew