Should I pull head to see what I can find?
Jim I AM A BEGINNER but I am now repairing an engine with very low compression that would not run (20 PSI measured) or less
First I assume the engine will not run
Yes after measuring the compression on each cylinder I did pull the head no blow by on the head gasket (good)
When I pulled the pistons, I found a very large gap in the worn piston rings (1/2 inch or more)
I did measure the bores, checked and pulled each valve and relapped the valve face in the head
installed new rings being careful to check the proper gap offset by 120 degrees
I hope to fire it up tomorrow but it has a tremendous improvement in compression
Don't pull the head until you check compression dry and wet. Do it dry first. Make a note of the reading for each cylinder. Then squirt a little oil (One or two pumps from a pump type oil can will do) into each spark plug hole and do your compression test again. If the readings come up, your rings are the problem. If they stay about the same, your problem is either valves or head gasket. Rigging an air hose (Turn the pressure down low) to the spark plug hole with the piston at TDC can also be helpful. If you hear air escaping through the tail pipe, you have an exhaust valve problem. Hear air at the carb? Intake valve problem. Hear air at the breather? Piston ring problem. Bubbles in the radiator? Headgasket or a crack somewhere.
So no, don't pull the head just yet.
Uh......See the thread titled "Cranking".
Could it be a damaged cam gear?
How long has the engine been sitting ?
are the valves moving ?
remove the cover and look at this first if the engine has been sitting
You can see the valves moving thru the spark plug holes. I completely agree with Hal, do some measurement and write down the result, both dry and wet. If you do the air pressure test, make sure the piston is at Top Dead Center as you do each cylinder.
I never start by removing the head, you can tell so much from the compression test, it is really an essential first step. Let us know the results.
Re; "No compression on any cylinders. Two cylinders will push thumb up. Any thoughts?"
How can you have 2 cylinders pushing your thumb up and yet no compression on any cylinders?
Maybe a little more info in the body of the question instead of just a title would help.
While you might end up pulling the head anyway it certainly isn't the place to start. I'm amazed at how many guys start with this then ask here after it's off. You actually lose the opportunity to diagnose that's wrong with the head off because sight alone isn't good enough. The dry & wet compression tests mentioned are the way to go. It will tell you exactly why your compression is low. Valves or rings you'll know when you've done it. As Hal suggests check out the post titled Cranking.
When I first got Barney, who had been sitting in a barn for at least 50 years, I found mud dauber nests in the intake, and in the valve galley. Once removed and given the ATF in the cylinder treatment overnight, she started with the hand crank!
I did mosquito abate the area for about 5 minutes though after the first start--ATF makes a fine smoke screen! With the valve galley cover off, you can check for valve movement/sticking too.
It is much easier to pull the side cover plates off before pulling the head. Then you can easily set valve clearances and see if all the valves are actually going up and down! Sometimes they get stuck up and no clearance can cause them to sit high, leaving little or no compression. I had a car once which the valves would always stick if it had not been used for awhile. a little lube and a few turns and it was always good to go.