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How to Check Compression in a Weed Eater

Compression drives the piston in a Weed Eater's cylinder, turns the crankshaft and pushes fuel through its circuit. Without compression, the engine will not run properly and may not start. Low compression generally occurs when an air leak springs somewhere in the internal engine components, such as the gaskets or seals around the crankshaft, piston or carburetor. Symptoms of low compression include hard starts, erratic idling and low power when the engine heats up. Checking the compression on a Weed Eater requires a special tool called a compression gauge.

Unhinge the black rubber boot from the tip of the spark plug, usually on the top or side of most Weed Eaters. Lift the boot off the plug and hook the socket wrench to the end of the spark plug. Unscrew the spark plug and remove it from the cylinder.

Move the choke lever into the closed setting for regular operation. Squeeze the throttle fully into the wide-open position. Pull the starter rope rapidly five to six times to purge any fuel from the cylinder.

Insert the elongated end of the compression gauge into the cylinder hole where the spark plug sat. Pull the starter rope until the compression gauge needle maxes out, or stops moving. Standard, new two-cycle engine compression readings will be 90 psi when hot and 100 psi when cold. Lower readings indicate engine problems.

Check Small Engine Compression

Small engines require a certain amount of compression to drive the piston and turn the crankcase. If air is leaking somewhere in the engine, you will notice a drop in compression. Symptoms of a compression problem can include hard starting, erratic idling, loss of power under load and hard starting when the engine is hot. The only efficient and accurate way to test for compression is with a compression gauge, which resembles a tire gauge. Unscrew and remove the cylinder cover with the screwdriver, if it’s present on your small engine, to gain access to the spark plug. Place the socket wrench over the spark plug. Crank on the starter cord three to four times to purge any fuel remaining in the cylinder or crankcase.


Before rebuilding the engine, remove and pressure test the carburetor as the source of your air leak.

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