Excessive blowby is almost always a failure of a piston, and/or rings. It's possibly just a coincidence that the compression is low on companion cylinders, but the simplest and most effective diagnosis short of a teardown will be pressurizing the cylinders. I'd suggest locking the crankshaft in some way, like putting the vehicle in top gear and setting the parking brake and/or chocking wheels in both directions, and then loosening the rocker arm adjusters until you're sure both valves are closed. Then adapt an air hose quick connect fitting to a spark plug shell and apply air pressure to the cylinders. Listen for leakage at the carburetor, exhaust, and oil filler cap. I'm going to guess most, or possibly all of the leak will be into the crankcase. Good luck!
Jerry
