That's one of those things that depends. (I hate it when somebody
tells me that.) I'd guess that anything much under 100 psi overall
would raise an eyebrow, but if it's above that, you'll be interested
in whether the readings are similar to each other--less than maybe 20%
Among the things it depends on is whether you knocked any carbon loose
when you took the plugs out and a chip ended up holding a valve open
just a little. That'll lower the reading for that cylinder, of
course, but will cease being a problem the first time that cylinder
fires and spits out the chip.
Some of the old manuals used to suggest taking the readings once, then
squirting a little light oil or kerosene into each cylinder to see
whether that improved the seal between the rings and the cylinder
walls and raised the compression. The idea is that it tells you
whether you have worn rings. If you do this, watch out for pouring in
enough oil that it takes up any serious part of the volume of the
compression space--the air space when the piston is at the top of its
travel. That'll raise the compression without doing anything good for
Whether the throttle is wide open or at idle setting may make some
difference, though it likely won't be much at cranking speed.
Be sure you take the hot lead loose from the coil, or you could get
bit by one of the loose spark plug wires.
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