COMPRESSION TESTS

To conduct a quality compression test:

Remove clutch belt,
Remove air filter,
Ensure battery is fully charged,
Ensure starter belt has correct tension and is not worn causing slippage,
Open choke fully and accelerator baffle plate,
Ensure engine is warm,
Removing one spark plug at a time fit your compression tester to the engine,
Conduct a dry compression test by rotating the engine with the starter until the gauge pressure peaks,
Record the gauge reading,
Conduct a wet compression by pouring 1/2 ounce of 30-weight oil through the spark plug hole,
Repeat the compression test,
Record the gauge reading,
Compare the dry compression test with the wet compression test results.

Note: Higher wet compression test results means poor ring sealing or a leaking cylinder head gasket. If both results are the same then you have an indication of valve sealing problems.

Your engine should increase 20lbs. above the rated PSI for an engine with seated rings. Compression readings that are 20lbs. below the rated PSI for an engine means poor cylinder sealing.

160 psi (1103 kPa):

New Engine

140 psi (965 kPa):

Good Compression

100 psi (690 kPa).

Engine has experienced many hours of use and is starting to wear to low compression. A top end overhaul should be considered for prolonged future use. Engine will start to burn lubricating oil and product black exhaust fumes with a bluish tinge.

50 – 60 psi (345 – 415 kPa).

Piston rings are in terrible condition or valves are leaking. Conduct a wet compression test. If compression increases to 10psi (70kPa) or more then the rings are in bad condition. If no increase in compression is experienced then the valves are leaking. If seal rings are in poor condition engine will be burning oil and producing black exhaust fumes with a bluish tinge.