Taking the Guesswork Out of Leak Testing

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One of the most popular uses of a vacuum tester is the testing of valves and valve seats. After resurfacing valve and seats to ensure a gas-tight seal, we can test that seal with vacuum.

Assemble the cylinder head(s) completely with springs, retainers, keepers and spark plugs. Then cover the chamber you plan to test with a non-porous foam covered plate with a port for applying vacuum. Connect a vacuum source with the ability to produce and hold between 20 and 24 pounds of vacuum to the plate. Turn on the vacuum pump until maximum vacuum is achieved. This should equal the 20 to 24 pounds you achieved before connecting the test plate. If it doesn’t repeat this value or leaks down excessively, you have a leak that must be corrected. Usually this means you will need to re-machine the affected components.

Now here’s the really cool part. To determine where the leak is occurring, use a spray bottle filled with a bubble solution to lightly spray the combustion chamber. Spray the valves, valve seat area and the spark plug area too. Reinstall the closure plate and apply vacuum. Let it stand for a few seconds then release the vacuum and remove the closure plate. Note where the bubbles are located. This is your vacuum/air leak. No guessing required.