Need help with compression reading - replace engines or keep ???

erich said:

The direction for taking compression in my 1990 Crusader manual says to operate the engine until it has reached proper operating temperature. This may have changed in more recent years.
Erich

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I don't have a strong opinion on hot, warm, cold for doing compression testing. I can see a case for hot if the engine uses forged pistons, that tend to expand when warm, more than cast. We are generally all running cast pistons. Also, another down side when cold is the oil is more viscous, making the starter a bit slower, more labored and maybe providing a better piston ring oil seal, than when running at temp.
More important is to use the same process for each cylinder, since its the relative psi delta that's relavent. In florida right now, it's 90 degrees, so it's not likely this particular engine is laboring with "thick" oil .

Having either the choke closed or the throttle plates closed will cause a pressure drop across the carb. Pressure drop is what it sounds like, a change in pressure between the inlet (at atmospheric pressure) and the intake manifold (much less than atmospheric if either restriction exists). Any pressure drop will reduce the density of the cylinders' inlet charge and will adversely impact compression tests' accuracy. ( the volume swept during the piston cycle is fixed.) A purest would also open the air valve on a factory configured engine. That said, have you ever seen that step in a factory compression test description?

Also, 'pressure drop' is usually characterised by a vacuum gauge so it isn't always a bad thing to experience.