Compression Test Myth

On single cylinder minibike engines we think of good compression, not in terms of pounds per square inch, but in terms of horse power output. If the engine produced enough horse power for which it was designed, we believe the compression must be good. It is extremely difficult to make an accurate compression test on a small single cylinder engine without expensive equipment. The reason for this is lack of a starter to crank the engine at a constant speed and the small displacement of the cylinder. Therefore you will not find compression pressure figures published by most small engine manufacturers. As specified by most manufacturers, as a simple compression test for all small engines, give the flywheel a quick spin counterclockwise. A sharp rebound on the compression stroke indicates good compression. Slight rebound means weak compression. No rebound means poor compression and a rebuild is necessary. This test is less dramatic on newer easy spin starting engines as they have a camshaft with a longer duration. This longer duration provides a slight leak down of compression to accommodate the ease of starting. It is claimed by the manufacturers that this leak down is overcome at higher RPM and does not affect engine performance. I'm sure if you polled a few old time engine builders that they would disagree on that issue, me included. If considering purchasing a used engine that is claimed to have a specific amount of compression, you must consider the test inaccurate. This does not mean the seller is dishonest. The information provided in this article is in accordance with the manufacturer's specifications but it is still not a well known fact even to experienced mechanics.

Happy Trails, Chuck at Valley Farms

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