The role of compression

Having good compression is an essential component needed for any engine to run properly. To understand how compression works, you need to understand the basics behind the small engine. For one thing, your engine is a pressurized system. A mixture of fuel and air is combusted within this system using that pressure. The result is the rotation of the crankshaft and any external parts attached to it such as the head on your trimmer.


Signs that your engine may be losing compression are loss in power or trouble starting. However, no need to fear. Diagnosing the reason for the loss in compression is the first step to fixing the problem.

Common causes of low compression

Worn or damaged piston rings

Piston rings extend or push outward against the cylinder walls, resulting in a vacuum seal.  This vacuum seal prevents the air and fuel mixer from escaping.  These rings also expand when the piston becomes hot, therefore reducing blowby (escape of air-fuel mixture). Over time a ring may break, become fused, or be damaged, consequently degrading the effectiveness of the vacuum.

Loose or damaged parts

Low compression can sometimes be a result of loose or damaged parts. An engine is constantly vibrating which can result in bolts and other parts becoming loose over an extended period of use. As part of your routine maintenance, it is important to check for cracks in the engine, worn gaskets, loose bolts, and spark plugs. Excessive oil buildup on the other surface of your engine is a good indicator that you may have a damaged gasket or crack in the engine.

Scored piston

A damaged or scored piston will have visible vertical lines or indentations. These lines cause air and fuel mixers to escape. A piston in decent shape will be smooth and free of any major indentations. Pistons may end up damaged for reasons such as: running improper gas fuel mixer, accidentally running straight gas, or normal wear and tear over time. Removing the spark plug, muffler, or carburetor should give you visual access to your piston

Depending on the extent of the damage a simple replacement of the piston rings should restore compression. If the piston is damaged, then there is a good chance that the cylinder wall is also. In this case, rebuilding the entire top end is necessary.

Diagnosing low compression

reason for low compression

A compression tester is one of the most common ways to measure your engine’s PSI (pound per square inch). First, remove the spark plug and install the compression adapter into the spark plug hole. Place the engine in the run position and pull the starter recoil until the compression gauge needle reaches the highest PSI number on the gauge.

Most engines need a PSI of 90 and above to function properly, however, some engines have been known to run at lesser PSI. They are many compression testers on the market, so be sure to shop around for a high-quality set. Alternatively, some auto stores will loan or rent out their compression testers.

Another method of testing is, comparing an engine suspected of low compression with a functioning engine. The starter chord on an engine with low compression will require less force to pull when compared to a functioning engine.