Hello guys!
I'm current fixing up a 1940 9N. Checked the compression and all cylinders were ~ 75 psi. I assume this would indicate a crap gauge and next step would be to try with another gauge? Isn't this supposed to be 90 psi or do I need to heat it all up and then test again? Any insight appreciated. I hope all cylinders didn't all deteriorate equally by some fluke..

Is the 75 lbs the same on all four?

Is that dry or with a squirt of oil in the cylinders?

Hey Bob! I put ATF/acetone mixture in cylinders in advance. Yes, all 4 were at 75 psi.

I’m not sure acetone would be a suitable fluid to use for compression testing, as it is a solvent, and ATF may work, but may also be too thin, as it is 10wt.

As I, and Ed mentioned, do a dry check first, then repeat with some oil. The same oil used in the crankcase, fresh from the container. I use an oilcan filled with fresh oil, a squirt or two is all it takes.

You need to run two tests. Remove all four spark plugs and open the throttle wide open. Do a dry test first and record the readings. Put a tablespoon of oil down each spark plug hole and retest and record. If the readings go up on the wet test, it tells you that the rings may be sticking or are worn.