Compression testing

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).

• If the compression gauge has a screw thread, install it in the spark plug hole so it snug - it doesn't need to be really tight. If it just has a rubber boot, have a buddy hold it in place in the spark plug hole as you perform the following tests.

• Pull the starter cord several times or use the electric starter in the normal manner.

• New Tecumseh engines should have a compression reading of at least 80 psi. If your reading is at least 60 psi (or the specification found in your engine manual), then compression is acceptable.

• If it is less than expected, squirt a small amount of engine oil in through the spark plug hole so it coats all around the edge of the piston and cylinder.

• Repeat the compression test.

• If the pressure reading is now acceptable, then the rings need replacement or the piston/cylinder are excessively worn. If there is little change, there is a valve problem.

• If the measurement is very low under both conditions, there may be a blown head gasket or damaged (punctured) cylinder or head. (Or your engine has a compression release mechanism which is reducing the reading - see the note above).

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.