SBT

The first thing you have to do is get a good compression gauge, not the one in the bottom of your toolbox that's been sitting around for a year. You can either buy a cheap one that will work for about a week for $20.00 or you can buy an expensive one that will last you for years, but still needs to be calibrated every six months.

With a good compression gauge you must make sure you have the correct adaptor. There are ½ inch and ¾ inch spark plug reaches and you have to have the correct one or your compression reading will be incorrect. Yamaha uses the ½ inch reach, veryone else uses ¾ inch.

When you are sure you have the correct reach, then you need to make sure you have a good battery. A good battery should be fully charged and able to turn the engine at the proper starting speed. You can't check compression on a slow turning engine.
The next thing you need to do is remove all spark plugs. Then you need to ground the spark plug wires. Sometimes the manufacturer will provide you with posts to install the spark plug caps, if you don't have these, hold down the stop button while pressing the start button to disable the ignition while you crank.

Install the compression gauge into any spark plug hole. Open the throttle fully. Press the start button and rotate the engine for 5 to 7 seconds. Read the compression gauge. Refer to the shop manual to determine if your compression is good or not. There is not one compression for all engines. You should perform this procedure at least twice per cylinder.

Most two-cylinder engines should have about 140psi, but not all. Most three-cylinder engines use a lower compression usually around 110psi. The difference in compression between cylinders is important as well. The standard is 10% difference for all but Sea-Doo, which has a 5% tolerance. If you have a Yamaha with 130psi and 140psi, as long as both compressions are with-in spec, the compression is good. If you have 130psi and 150psi even if both compressions are with-in spec, compression is bad because you have more than a 10% difference between cylinders.

dorf

From Groupk---I tried it and it works-leave the other plug(s) in while testing

GETTING AN ACCURATE (as possible) INDICATED COMPRESSION READING

THE GAUGE - Most engine builders have a drawer in their rollaway dedicated to storing all the "cheap compression gauges" that they have used the poor judgment to buy during their career. However in the very front of that drawer is the Snap-On gauge that gets used regularly. This gauge is preferred not just for it's good accuracy and durability, but rather for one simple design feature. The adaptor hoses, of the Snap-On gauge, that screw into the spark plug threads has a Schrader air fitting at the spark plug tip location. That is, the pressure is sealed off at the face of the dome in the cylinder head, which gives the truest representation of the exact combustion chamber volume. Most other automotive gauges have this air seal fitting mounted in the gauge body, at the end of a 16" hose. This means that the air volume inside that hose (usually about 3-4 cc) is added to the combustion chamber volume during a measurement. The end result of his added volume is a reading that is 20 - 35 psi lower than the true reading. For similar reasons, the tapered rubber "hold - on" type gauges are virtually useless. Besides indicating the added 2 cc of the threaded spark plug hole itself, these gauges are notorious for leaking as well.

THE ENGINE - 1) All accurate readings must be taken from a dead cold engine. A warm engine will yield slightly lower numbers. 2) The exhaust pipe and carburetor must be installed. The restrictions in the carb throat and the back pressures of the exhaust system can affect the readings. 3) Have a full charge on the battery. The speed that the engine is spun, has a significant affect on the indicated reading. Having the charger connected during the test insures maximum starter motor RPM's. 4) Leave a spark plug in the cylinder not being measured. Contrary to what you may think, the engine will turn over slightly faster with the opposing spark plug installed.

THE TEST - 1) Make sure both spark plug caps have spark plugs mounted in them, and those plugs are grounded to the cylinder or head. 2) Hold the throttle wide open to admit the maximum amount of air. 3) Hold the start button down until the needle on the compression gauge is no longer rising. 4) Test both cylinders.

DO NOT squirt any oil into the cylinders to improve ring sealing for the test. The presence of added oil can cause readings 20 - 30 psi above the accurate "normal oil presence" reading.

Ideally, there should be no more than 10 psi difference between cylinders. If the difference is greater than 20 psi, you should consider removing the cylinder head for inspection. If the difference is greater than 30 psi, you should consider removing the cylinder and pistons for inspection.

SBT

We recommend taking reading with all plugs out for two reasons - that is the OEM method, and it's our method - so it keeps things on an even playing field for comparison. That being said, it's only a couple PSI difference, if the starter & charging system is in proper condition. Leaving plugs in makes it more sensative to other problems.

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Ed M

And before putting attaching that new expensive compression gage to the motor clear the cylinders first after removing the plugs and grounding the caps by hitting the start button for a few seconds to ensure the cylinders aren't full of water, oil, or gas.

99spxman

I want to buy that Snap-on Compression tester. What Snap-on model is it they make more than one?