Compression test question

[Mad Max (TX)]

I recently did a dry compression test on my newly acquired TO 20 because it doesn't run quite rite and it has 1 oil fouled plug. The readings where 100 to 105 on 3 cylinders. The reading on the oil fouled plug cylinder was 60 to 65. I added a little oil to the cylinder and the reading jumped to 70, 75. My question is how do I determine if it has a bad valve or bad rings? It smokes very little. The oil pressure runs about 10 to 20 when hot. Thanks for any advice.

 

[Bruce(OR)]

Adding oil raised the readings. That is a good indicator of weak rings.
You failed to mention if this is 6V or 12V ??
warm engine or cold?
Warm gets better numbers. Ground the coil wire or disconnect the juice to the coil.
Wide open throttle for better air flow.
Wanna try again?
You can also run the piston to TDC compression and plug compressed air into the cylinder and with the oil fill cap removed, listen for air at the oil fill going past the rings. How about the exhaust for air passing the exhaust valve?
Final check is the carb throat for air past the intake valve.

 

[Jerry/MT]

First of all did you run the compression tests with the engine at operating temperture, with the thottle open and the air cleaner disconnected. That"s the correct way to run a compression test. If you didn"t run it that way, then repeat the test correctly.

I"ll assume your numbers are correct.The wet test raised the pressure in the oil fouled cylinder but it"s still significantly lower than the other cylinders. If you had a burn"t valve, the pressure would not have changed. It changed a little bit but that"s not to say that the cylinder doesn"t also have some valve leakage.

However an oil fouled plug results from loss of oil control by the rings so my call is that your rings are the problem. You"ll have to pull the head to fix it unless your try soaking that cylinder with ATF or MMO, etc and it works. If you pull the head ,check the valves. they may just need to be cleaned up and lapped.