I just recently got 150 PSI on my 1979 Leyland Sherpa 1.7l petrol Van with SU carb and in my limited experience thought that it sounded too high for such an old vehicle.
When you do a compression test you are supposed to have the throttle wide open at the same time. I guess this reduces the vacuum that would otherwise thwart free movement of the pistons for the test. Also it would provide a level playing field to compare with other engines and their compression test. (please put me right and I will edit)
Then I got to thinking in the case of an SU Carburettor - the piston of the dashpot which damps down air flow (to enrichen mixture serving as an alternative to an accelerator pump) would restrict the air flow negating the effect of Wide Open Throttle.
So would a dashpot piston sitting down on the venturi bridge effect compression results? And to what degree?