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Originally Posted by christuna
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Thanks for that info, I stand corrected!
Compression values depend on proper operation of the valves. Loose valves can prevent a full charge of air (lower compression) and tight valves can allow some of the air charge to escape (lower compression). The test values in the FSM were established on the assumption that the valve clearances were properly set, so if the valve clearances are off, the test results are not valid. A stretched timing chain can also affect compression, that's why a good practice is to check the camshaft timing when the valve cover is off to perform the valve adjustment.
If there is carbon deposited on the valve face or seat, after an adjustment and some vigorous driving, those deposits will probably reduce or disappear, changing the valve clearance. Sometimes a neglected engine will need two or three valve adjustments in a shorter maintenance cycle, for instance 1000 miles between adjustments, until such deposits are gone and the clearances will stabilize a bit.