NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Compression testing                     
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Compression testing

Special compression gauges are available at auto parts stores or small engine parts suppliers. These will catch and hold the highest pressure reached so you don't need to be in two places at once.

Note that this procedure may always yield a very low reading if there is a compression release mechanism on your engine - which is very likely. In this case, the crankshaft must be spun in the opposite from normal direction by the flywheel (counterclockwise when viewed from the flywheel end, with the starter removed).

For Briggs & Stratton engines, the manufacturer simply recommends spinning the crankshaft by the flywheel in the opposite from the normal direction with the normal spark plug installed. A sharp rebound (as it compresses the trapped air since the compression release mechanism is not active in reverse) indicates good compression. Little or no rebound indicates low compression and need for service.

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