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With a 40 psi difference between the lowest and highest reading there is something going on---could be a tight valve, burned valve or seat, broken piston ring, etc. You should adjust the valves first thing and then take another dry compression test. If the readings don't even out with that, the next step would be a "wet" compression test--a few squirts of motor oil in each cylinder as you test them should show an increase--10 or 15 psi change would be normal--a dramatic change could indicate worn cylinders/rings. After that test a "leak down" test could be in order. This requires compressed air. Parts stores sell a simple fitting that screws in to each spark plug hole and an air hose coupler fitting screws onto the tool. Rotate the motor for each cylinder so the piston is at the bottom of the stroke with both valves in the closed position (or remove the rocker arm assembly so all the valves are closed--if you do this you still should rotate the crank for the piston to be at the bottom--so the motor doesn't rotate with the air pressure thereby spinning the fan--can hurt if your hand is in the way). The leak down test will tell you where the loss of compression is--out the exhaust, out the intake, out the crankcase. -- Dave
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