Put a squirt of oil (about a teaspoon) in the low cylinders and rerun the test. If the compression boosts then the rings or cylinder walls have leakage. If no boosts then likely a valvetrain issue and you can move to the next tests.
Don't be as concerned with overall psi if they are all over 100 psi. More important is that they be within 10% of each other.
Thank you for that helpful advise. I will try this week to do a compression test with oil. Do you have recommendations which oil to use? Hopefully it will be a valve issue since I don't like to pull the engine. The car was not driven often in the past 20 years.
Doesn't matter what oil, whatever you are running in the car would be fine.
Next step would be to perform a leak down test. You can usually rent a leak down tester from an auto parts store.
Important that you get each cylinder at top dead center. You will probably have to source a bolt for the center of the crankshaft to allow you to rotate the engine by hand. Unlike many cars, ours have a cork plug in the crankshaft threaded hole rather than a large bolt.
Since you said the car has been sitting a long time, it's possible some of the rings are gummed up and stuck in the ring lands. Might be worth squirting some Marvel Mystery Oil into each cylinder and letting it sit a day or two. Turn the engine over to expel the excess oil out the plug holes and then run the compression test again.
A rust buster type penetrating oil ( PB Blaster-is best ) would be good in the cylinders to help loosen the gummed up rings of an engine not run much in last 20 years ?
Easy to spray into the cylinders
Very good chance the rings are gummed up and can come back to life with some initial lubrication ( in your compression test ) — an engine oil flush - ( to get the lower rings loose from inside the crankcase ) and some new miles on the engine ( doing two early oil changes )
Sure worth the time and effort to hopefully bring the engine compression back to life and improve performance
Marvelous Mystery oil is a thin oil that will get into places a thicker oil cannot reach which helps a lot to loosen parts in tight confined places and increases compression in cylinders
My Dads airplane mechanic used the Mystery oil in my Dads airplane engine to bring up the airplane engines compression to spec —to be passing his the yearly required complete airplane inspection —called an annual —in the airplane world
MMO was recommended by my bike shop when the low end of my Triumph motorcycle engines bearings got so tight it was hard to kick start the bike
Kicking the bikes engine with MMO in the crankcase a good number of times until it loosened up corrected the problem to my wonderful surprise
The lubrication should loosen up the rings and could improve your current compression numbers
I would do the lubrication above before the leak down test because if the rings are gummed up you will not be getting good or better numbers until they are freed up
I always use —MMO in my gas tank ( 50 years ) to lube the fuel system and to oil the valve guides and top of the piston rings which is called a —a Top oil
The only way that any oil gets to the valve guides ( intake mostly ) and top piston rings ( some gasolines have Top Oil in them - Exxon - Marathon Stp )
MMO has been around for 100 years which does speak for itself
Out of curiosity, how much MMO do you add to a full tank of gas?
I know that one of the front bumper bolts will thread into the harmonic balancer bolt hole ( where the caulk plug came originally stock ) —but not sure which one it would be —if that is the way you desire to be turning the engine over
Someone might add to that thought for the bumper bolts location you might need
A good machine shop will help you sort out what you should do and replace on the heads
Your country is know for outstanding mechanical engineers in many fields and
Be especially careful to —-Not put the rocker arms on backwards — as that has happened much to often with professional engine builders with our year Cadillac engines
i would have the heads CCed ( make all the head chamber volume the same size ) and do some smoothing of the fuel and exhaust passages to remove some casting roughness that was noted when they cut up a head on this site a short time ago - but that’s just what I would be doing and not for everyone
Having fresh heads on the engine is a very good thing !
If your rings are gummed up from limited use using an engine flush for that purpose and a few oil changes putting some fresh miles on the car could show some improved engine compression and performance
Time will tell on the last suggestion
Hello from Florida USA !