compression

Pull the plug on that cyl and squirt a tablespoon or so of oil on the piston top. Turn over a couple of times and then check your compression again if it is up substantially then your rings aren't sealing
 
More information needed.

  • What was the reason for the valve job?
  • What did you do? Did you lap the valves or completely change out every component?
  • What style valves are you using? Original or modern?
  • Do you have adjustable lifters? What gaps did you use?
  • Are you getting bubbles in the radiator water or other evidence of a compression leak through the head gasket or a crack?
The more information you supply the better the Barn Forum will be able to help you.
 
Trying to help a friend out with this post. He changed out the head for more power. Before tear down, the compression was. #1=52.#2=55.#3=52.#4=58. After only doing a lap in of the valves, the compression was.#1=70. #2=94. #3-95. #4=115. The head he installed was a 6-1. Trying to figure out what is up with #1 and #4. Engine shows signs of blow by with light oil residue on side of block below oil filler. Checked compression with two different gauges. Both of them read the same. He does not know exactly what the compression should be on a 6-1 head. Valves are set at .013 intake. .015 exhaust. About all I know to tell you.
 
Checking compression only tells you/us that there’s a problem, a differential or leak down test will tell you/us what the problem is. The test you put a measured amount of air into the cylinder and a holding amount of air should be 10% or less. With the pistons at TDC you can listen for air leaking out of the tail pipe ( exhaust valve leaking) air leaking out of the carburetor ( intake valve leaking) air leaking out of the oil filler ( rings on the pistons are leaking)
 
How long ago was the engine overhauled? Has your friend run the engine. He may want to run it for a while and then recheck the compression.

Is the engine smoking? If so, is the color of the smoke white, grey, or black. You said that there is signs of blowby, so the rings could be bad. The higher compression head will cause any blowby to be worse because of the increased pressure. Do what Old Sargent, Post #2, said to do. And/or do what the Big Hammer, Post #5, said to do. The leak down test is a specific type of test, but just putting air in the cylinder you can listen for where any leaks are, as the Big Hammer said. Do the test with each piston at or near the top. Any wear will be worse at the top of the cylinder.

#4 cylinder probably has close to the correct pressure. I will let others chime in with their experience.
 
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