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Measuring Compression On Kohler K-Series Engines

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dkirk

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David Kirk
Testing the compression on an engine is an excellent way to determine its general internal condition. If piston rings are not sealing properly and/or valves are leaking, it will not develop full power and possibly use excessive amounts of oil. The most common way to measure compression pressure is to use a pressure gauge screwed into the spark plug hole, then crank the engine with the starter. Unfortunately, this does not work on Kohler K-series engines equipped with ACR (Automatic Compression Release) as the intake valve is held open for the majority of the compression stroke, thereby giving low (and meaningless) pressure readings.

The proper way to measure compression on a Kohler engine is to use a Leakdown Pressure Tester, sometimes referred to as a Differential Pressure Tester. This device measures the rate of air leakage across an internal calibrated orifice located between two pressure gauges. An air regulator on the tool sets the air pressure such that the downstream gauge reads in percent of leak rate. If the engine cylinder were perfectly sealed (an impossibility on a real engine) the gauge would read 0%, meaning no air was leaking internally. Conversely, if a valve were to be open, the gauge would read 100%, meaning that all the air supplied was leaking out of the cylinder.

Using the Leakdown Pressure Tester on the Kohler engine is straightforward. First, rotate the engine such that piston is at TDC (Top Dead Center) on the compression/power stroke. Following the supplied tester instructions, the gauge pressure is calibrated using the regulator with the tester hooked up to a steady air source; a compressor with 100 psi in the air tank is suitable. The adaptor is then threaded into the spark plug hole and attached to the tester. At this point, you may find that applying air pressure will cause the crankshaft to rotate. Detach the air supply and reposition the crankshaft at TDC. (Note – as in my case, the “T” on the flywheel is only an approximation of TDC and is actually a few degrees off. It takes me about 4 – 6 attempts at finding TDC by feel and this is the hardest part of the whole job). When hooking up the air supply, keep an eye on the crankshaft to see which direction it rotates…this makes it easier to reposition again.

When air is applied and crank does not rotate, you know you’ve found TDC and the gauge reading can be viewed. It is normally recommended that the engine be run for awhile and be hot for this test. I prefer doing it on a cold engine which will usually indicate a slightly greater leak rate. My engine has a little over 500 hours and the leak rate is 18%, about where it was after the first 5 hours of operation. While air pressure is being applied, pull the dip stick tube and you’ll hear air leaking past the end gaps on the piston rings – this is perfectly normal. Then listen at the exhaust, then the carburetor intake for any air leaks…ideally, you should hear nothing. Any leakdown 40% or less is considered acceptable.

These testers are available at most automotive supply outlets and are not that expensive. The first photo shows the tester with hoses and adaptors. The second photo shows the tool in action on my K301. This is a must-have instrument for the serious engine-guys tool box.
 

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