#1 #2 #3 low compression in L6 250

[c10hevy]

In my 66 GMC I did a compression test today because it would stall out many times when starting in 1st gear. The results:
#1: 90psi
#2: 85 psi
#3: 75 psi
#4: 120psi
#5: 120psi
#6: 120psi

I was thinking the rings are leaking in the first 3 cylinders but after doing some searching ive found that it could possibly be from stuck valves or possibly even flattening of the camshaft.

Any help would be greatly appreciated.

[Fred T]

Best way to tell is to set the cylinder with both valves closed and connect compressed air to the spark plug hole. The listen for the air out the exhaust or crankcase.

Another method is to repeat the test with a small squirt of gear oil in the cylinder. That will usually seal the rings enough to raise the pressure if rings are the problem.