Learning how to do a compression check on an engine is an important skill. When you buy a boat and cannot get it in the water for a test drive, at the very least a compression check will help you determine the condition on the engine. When a boat you own is not running well, performing a compression test will determine if you have a serious engine problem or if you need to look elsewhere for your trouble. Doing a compression check on an engine is not difficult and one person can easily do it.
Prepare the Engine
Operate the engine for at least 30 minutes. If you can get the boat in water to put the engine under a "load," it improves the chances of an accurate reading. If you are on land, keep the engine at 1,200 rpms. This gets the choke opened up. Pay attention to the temperature gauge. Keep the engine running until it is at normal operating temperature.
Remove and Check Spark Plugs
Turn off the engine and take out all of the spark plugs. While the plugs are out, it is a great time to inspect them for burns. The spark plug's correct color (after use) is tan and it should not have any burn marks, or charcoal coloring. If one or more sparkplugs is wet, shiny and black it is fouled by oil, gas, or both.
Attach the Compression Tester
Attach the good sparkplugs back on their wire. Lay them on the engine. Set throttle and choke open. Screw in the compression tester into one of the spark plug holes. Spin the engine throttle and read the tester. It will give you a readout of PSI (pounds of pressure per square inch). Do the same for each spark plug cylinder and write down the number for each.
On two-cycle engines, the readouts should all be within a few psi of each other--no more than 15 psi difference between any of the cylinders. On four-cycle engines, the lowest number should be 75 percent of the highest reading.
Low battery power, a weak starter, or an engine that cools down before the testing is complete can corrupt the readings you get from a compression tester. A compression test is a useful gauge as a part of your engine diagnostics, but not as a single method of troubleshooting.
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