Tom, there is no CORRECT compression testing procedure for all conditions. [it's a diagnostic tool that tells you different things depending what you are looking for, how you do the testing, & how you interpret the results].~Most published compression specifications are given for a hot engine with the carb & throttle wide open. Some compression specs are even given for a specific cranking RPM. ~The carb & throttle being wide open are necessary because what can't get in can't be compressed. A restricted air intake either due to closed choke plate, closed throttle plate, or plugged air cleaner can have a big effect on the compression reading.
~The cranking speed can also be a big factor as leaking rings will leak off more compression the slower an engine is cranked.
~The number of puffs that it takes to reach maximum PSI can also tell you a lot [especially on a hand start tractor]. If you have an engine that reaches 80 psi on the 4th puff but only 20 PSI on the first puff it would probably start ok by pull starting or with an electric starter but would be hard to start by hand because on most hand start tractors you only get one time over per try. ~Doing the testing hot or cold can also tell you a lot. If you are looking for a smoke or blow-by problem then hot compression testing would be the way to test. If you are looking for how your engine compares to the published specs then it would be hot again. If you are looking for a hand crank starting problem then testing the compression at cold starting temperature would apply. ~The compression variance side to side can also tell you a lot as both sides should be within about 10 psi of each other & take about the same number of puffs to reach the same value. ~Testing with the spark plugs shorted out just keeps it from starting if not all the plugs are pulled at once. On a distributor equipped tractor with by-pass starting, or on a magneto equipped tractor it can still fire the spark plugs even with the ignition switch turned off. In any case the plug wires should actually be shorted to ground to protect the ignition coil from internal arcing, but most people don�t take the time to do that. ~Something that is a better diagnostic tool [& something I personally use a lot] is called a leak down test. All that involves is bringing the piston to TDC or just after TDC [both valves closed] then replacing the spark plugs with an air hose adapter [made from an old spark plug] , then placing the transmission in high gear, setting both brakes, & blocking the rear wheels. Then about 60-80 psi of air pressure is put in the cylinder with an air hose. With the cylinder pressurized you will be able to hear where the air is leaking out from & if an exhaust valve is leaking there will be air coming out the exhaust, the same with the other valve or rings. One thing a leak down test is good at is telling you multiple leaks in the same cylinder & also pointing to the actual problem area. The leak down test should be used in conjunction with a preliminary compression test. ~I don�t know of a single book to tell you how to interpret all the compression readings & variables but I have over 40 years of experience with compression testing & engine diagnostics still learn new things every year. You have to use a compression test AS A TOOL & not an absolute measure of an engine�s condition�.
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