checking compression ?

[whisker]

hi,
i want to give my outboard a compression check but its not on boat or hooked up to wiring harness on boat. can i just jump the starter to get a compression check? thanks for help

 

[hkeiner]

Re: checking compression ?

A compression test is done with the spark plugs removed and the motor cranked with the starter. The motor does not have to be on a hose or in the water. The actual readings on the gauge are less important than the readings between cylinders being close to each other, say no more than 10% between high and low reading. It is also often suggested to ground the spark plug wires while cranking to avoid potential damage to the ignition system components.

 

[whisker]

Re: checking compression ?

thanks,
so i can just jump the starter with 12 volts to turn over the engine,since i have no switch or harness on it right now. whats best way to ground plug.

 

[northernmerc]

Re: checking compression ?

You don't say which motor you have. But a word of warning if it uses the surface discharge type spark plugs (with no projecting electrode).

The piston may come very close to the spark plug hole. Some compression testers (mine for example) may reach far enough into the compression chamber to hit the piston when one cranks the motor. So use an adapter or find some way to prevent that from happening.