Beno, you think too much. Not really. But you are missing some important facts. For one thing, you are confusing 2-stroke and 4-stroke basics. But rather than get into all that, I’ll just cut to the chase and answer your basic question: Would a bad reed valve affect a compression TEST? Answer is no. Reason is, on a 2-strike the cylinder is not filled with air by sucking it in through the intake manifold. It is filled with air by a pressure charge in the crankcase, caused by the downstroke of the piston. I think you already understand that and are correct in assuming that the pressure charge depends on the reed valve and everything before that. But let’s take it a step forward and consider what happens at that critical moment when the air charge is admitted to the cylinder. When the piston nears the bottom of its stroke the exhaust ports open first. This allows the previous power stroke’s gasses to escape, unless of course the exhaust outlet is blocked. THEN the intake ports open and a puff of air from the pressurized crankcase enters and part of it blows through the cylinder. This purges the cylinder of the remaining exhaust gasses from the previous stroke. So, what is in the cylinder pressure-wise at this bare moment in time? Zero (atmospheric) psi. Or possibly a mere trace of pressure from the exhaust system. By now, the piston has started back up on the compression stroke compressing that atmospheric air pressure to whatever. and you read the resulting compression test on your gauge at its peak.
So, what happens if no crankcase pressure if a bad reed valve? Nothing. The piston is still starting its upstroke with zero pressure.
