Where are you gonna get air to compress if the throttle is closed?
Unhook the coil wire so the car doesn't start and have someone crank it
while the compression tester is tightly held in each plug hole.
Readings should be about the same and hopefully over 100 psig, maybe as much
as 150. If it is appreciably lower on one or more cylinders, put about
a tablespoon full of motor oil in each cylinder and do it again.
If the pressure is much higher it indicates that the rings were worn.
If the pressure is still low, the loss is probably in one or more valves.
You may be able to tell if it is intake or exhaust by taking off the air
cleaners and listening. If it is intake you probably will hear some air
leaking. If not, it would probably be exhaust or, if you have other symptoms,
maybe a gasket.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma