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?

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    When doing compression tests we would remove most of the air inlet tract (filter, airbox, pipes etc etc) and lift the pistons in the carbs...
    – Solar Mike
    Commented Feb 23, 2018 at 23:31
  • Thanks. Well I don't suppose all that was done for fun. Is that not half my question answered. @solar mike Commented Feb 23, 2018 at 23:39
  • I 2nd that it's best to remove any restriction in the inlet, to get a transparent result, although the piston and air filter won't have a dramatic effect. The air flow is minimal during cranking, so only the throttle forms a serious restriction. Piston and filter may make a difference of a PSI or two at most. It's far more important to make sure the engine is fully warmed up. Push the engine hard for a few miles to get rid of any deposits on the (exhaust) valves.
    – Bart
    Commented Feb 24, 2018 at 15:55

1 Answer 1


We would also do a modified Morse test when testing engines in the vehicle as we did not have a dynamometer... Another reference morse test info

With the engine warm and running at 1500 or 2000rpm, each cylinder would be prevented from running in turn and the drop in rpm noted - this would effectively show which cylinder was contributing the most or least to the power delivery.

On a dyno, all the cylinders can be stopped from firing and the pumping losses determined by rotating the engine with the dyno...

When doing these tests we would also test if the filter was causing too much restriction - although it has to cause some... We would consider removing most of the air inlet tract (filter, airbox, pipes etc etc) as applicable to which test Morse or plain compression.

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