Questions on a 2F engine compression test??

[CHCP]

1. Does it matter whether or not the air injection ports are open when taking cylinder compression tests?
2. What is the optimum rpm driven by the starter for a good compression test?
3. Will a bad valve adjustment affect a compression test?

I'm testing a 2F motor from a 1977 FJ40LV-KCJA, it's been sitting for about 20 years. Drained the oil, no water present, replaced with 5w30. 1st test had bad ground, corrected and also replaced tired starter.
Valve train function looks good. Put some MMO in the cylinders 5 days ago. So the following results could be a wet test? #1 60 psi, #2 115 psi, #3 80 psi, #4 100 psi, #5 90 psi, and #6 90 psi. I haven't taken a huge amount of compression tests but a fair number and usually the numbers are closer to each other even with a bad engine, do these numbers indicate something I'm not seeing?? Thanks for any help!

 

[Nostradamus]

1. Does it matter whether or not the air injection ports are open when taking cylinder compression tests?

-- if you're referring to the air injection in the exhaust manifold, it won't matter for a compression test. The CT will be looking at the cylinder pressure(s) built up over a few compression cycles (usually compression testers have a "hold" function that keep the highest pressure readings encountered when the engine is turned over a few times).

2. What is the optimum rpm driven by the starter for a good compression test?

-- not sure here, but a healthy battery and starter would be ideal. Slow cranking will allow some air to bleed around rings and valves, but it's hard to say how much

3. Will a bad valve adjustment affect a compression test?

-- absolutely. If valve timing is off some and/or valves don't fully seat (because out of adjustment, or carbon buildup on the valves/seats), you'll get a lower reading

-- have you been able to start the engine? If the rings are sticking some, you may get some weird / inconsistent numbers until they get some cylinder combustion pressure behind them. Also, doing a compression test at operating temp is generally considered better than when cold. Did you have the throttle wide open and all of the spark plugs out when you did the test?

I'm testing a 2F motor from a 1977 FJ40LV-KCJA, it's been sitting for about 20 years. Drained the oil, no water present, replaced with 5w30. 1st test had bad ground, corrected and also replaced tired starter.
Valve train function looks good. Put some MMO in the cylinders 5 days ago. So the following results could be a wet test? #1 60 psi, #2 115 psi, #3 80 psi, #4 100 psi, #5 90 psi, and #6 90 psi. I haven't taken a huge amount of compression tests but a fair number and usually the numbers are closer to each other even with a bad engine, do these numbers indicate something I'm not seeing?? Thanks for any help!

 

[CHCP]

-- if you're referring to the air injection in the exhaust manifold, it won't matter for a compression test. The CT will be looking at the cylinder pressure(s) built up over a few compression cycles (usually compression testers have a "hold" function that keep the highest pressure readings encountered when the engine is turned over a few times).

1. I'm using a Snap-on compression tester with the hold function.

-- not sure here, but a healthy battery and starter would be ideal. Slow cranking will allow some air to bleed around rings and valves, but it's hard to say how much

2. The battery is good but small, 24 series I think, voltage was 12.65 at the beginning of the test.

-- absolutely. If valve timing is off some and/or valves don't fully seat (because out of adjustment, or carbon buildup on the valves/seats), you'll get a lower reading

3. This needs to be done

-- have you been able to start the engine? If the rings are sticking some, you may get some weird / inconsistent numbers until they get some cylinder combustion pressure behind them. Also, doing a compression test at operating temp is generally considered better than when cold. Did you have the throttle wide open and all of the spark plugs out when you did the test?

4. The carburetor is missing so the intake manifold is open to atmosphere, no radiator or gas tank, I might get it to fire with some jury rigging but wouldn't be able to run it to temp. I have some Marvel Mystery oil in the cylinders maybe Sea Foam would be better. Also have a friend that has an endoscope, maybe a look-see would be good. All Spark plugs were removed for test.

 

[SteveH]

I have personally seen cases on older engines where removing the plugs (to test the compression) knocked loose some carbon bits on the plugs that then lodged in a valve seat, and created the appearance of crummy compression.

Ideally, you'd get it running to get the engine warm (even without a cooling system - run it for 5 min.) and adjust the valves before doing so. Then do a wet/dry with a teaspoon or so of light oil (MMO is fine). Don't put in a lot of oil, or you will jump the compression values artificially. I have a pump-squirt oil gun I keep filled with ATF, and I use 2 pumps/cyl. which is a regulated amount so it's consistent among cylinders.