Could be the valve adjustment screws are worn; easy enough to replace; part number is in the sticky here. I take it you've read this thread

Do a leakdown and compression test on it to see how things are; magic numbers are 142-171 psi, +/- 4% Valve condition will show up during the leakdown, which is how you check out the top end. <img src="/forums/images/graemlins/kewl.gif" alt="" />

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