Compression Testing Procedure

Satisfactory engine performance depends upon a mechanically sound engine. In many cases, unsatisfactory performance is caused by combustion chamber leakage. A compression test can help determine the source of cylinder leakage. Use a compression tester such as the Sun model UTC-48 that has a screw-in type adapter.

A proper compression test should be performed with the engine at normal operating temperature when possible. Proceed as follows:

1. Disconnect spark plug wires, clean around plug base and remove plugs.

2. Connect compression tester to front cylinder per manufacturer's instructions.

3. With choke and carburetor throttle plates in wide open position, crank engine continuously until 5 to 7 full compression strokes are completed.

CAUTION — Make sure that throttle plate is in the closed position before starting engine after test.

4. Note ga uge readings at the end of the first and last compression strokes. Record test results.

5. Repeat steps 2 through 4 on rear cylinder.

6. If the final readings are 90 psi or more and if the final readings do not indicate more than a 30 psi variance between cylinders, compression is considered normal. If compression does not meet specifications, see diagnostic chart below.

7. Inject approximately 1/2 oz. of SAE 30 oil into each cylinder and repeat the compression tests on both cylinders. Readings that are considerably higher during the second test indicate worn piston rings.


Test Results

Ring Trouble

Compression low on first stroke, tends to build up on following strokes but does not reach normal. Improves considerably when oil is added to cylinder.

Valve Trouble

Compression low on first stroke, does not build up much on following strokes. Does not improve considerably with the addition of oil. Check tappet adjustment.

Head Gasket Leak

Same reaction as valve trouble.

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