Low Compression in Small Engines
In This Guide
When fuel, spark, and air all check out but the engine still will not run right, compression is the next thing to test. A low compression small engine has lost its ability to seal the cylinder and squeeze the fuel mix, so it struggles for power or will not fire at all. Here is what causes it and how to decide the fix.
What Compression Does
Compression is the squeeze. As the piston rises, it compresses the fuel and air mix so the spark can ignite it with force. If the cylinder cannot seal, that squeeze leaks away and the engine loses power or will not start.
The Common Causes
- Worn piston rings. High hours or poor maintenance wear the rings that seal the piston.
- Leaking or stuck valves. Carbon buildup or a bent valve stops a clean seal.
- Blown head gasket. Lets compression escape between the head and cylinder.
- A worn cylinder bore. The most serious, usually from age and hours.
How to Test It
A compression test takes a few minutes with an inexpensive gauge.
Remove the spark plug and thread the compression gauge into the plug hole.
Pull the starter cord briskly several times and read the peak pressure.
Compare it to your engine's specification. Many small engines read healthy above about 90 psi.
Repair or Replace?
Some low compression causes are worth fixing. Decarbonizing stuck valves or a valve adjustment can restore a seal cheaply. Worn rings or a scored cylinder are a different story, since that repair often costs more than the machine is worth.
Weigh it against the numbers in the repair cost guide, and use the half price rule to decide.
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