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 inch and inch spark plug reaches and you have to have the correct one or your compression reading will be incorrect. Yamaha uses the inch reach, veryone else uses 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.

[ April 21, 2004, 12:00 PM: Message edited by: Technical Support ]