After the recommended tune-up, valve adjustment, etc I would do the next step
in a compression test. (Note that the valve adjustment should be done before
this step!) Re-do the compression test for each cylinder, noting the results.
If the compression is normal (140 or so) for all 4 cylinders (or at least
consistant) the the problem was valve adjustment! Otherwise, proceed to the
next step.
Then squirt a modest amount of motor oil into a cylinder and check the
compression. If it stays the same (and is low) the the leakage is either the
valves or the head gasket. The head gasket would be suspected (as others have
noted) where two adjacent cylinders are both low and the oil trick does not
help. If it jumps up to a normal range (140 or so) then the rings are leaking.
The logic of this is that the oil does a temporary sealing job on leaky rings
for a compressiomn test.
Good Luck!
Jim Henson
70 TR6
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