correct method of compression test?

[z-man]

could someone tell me the CORRECT way to do a compression test. i have the tools just need the know how.

 

[cclemon]

Re: correct method of compression test?

z-man<br /><br />Keep the compression can vary depending on if the engine is cold or warm.<br /><br />With that said, I like to test an engine cold. <br /><br />1)Remove all spark plugs.<br />2)Connect the gauge to a cylinder.<br />3)Most manufactures suggest setting the throttle wide open for the test. <br />4)Crank the engine while watching the gauge. The gauge should peak out in just a few seconds.<br />5)Record the cylinder # and pressure.<br />6)Repeat steps 1-5 for the remaining cylinders.<br /><br />Compression varies wit engine type, but if you have more than 110 psi on each cylinder that is usually enough for the engine to run well.<br /><br />You also want to make sure all cylinders are within 10% of each other.<br /><br />After the test if you choose you can swirt a couple of shoots of oil from an oil can into each cylinder one at a time and retest. It the presures increase dramatically, it can indicat the engine is worn.

 

[z-man]

Re: correct method of compression test?

thanks!!!!!!!!!

 

[Bob_VT]

Re: correct method of compression test?

You should also make sure the carbs are open w/o choke to get proper reading.<br /><br />Bob

 

[z-man]

Re: correct method of compression test?

when he said throttle wide-open, im assuming that the boat is cranked?? if so this will be a hard test to perform if im going to try and check out a motor before i buy it. is there any reliable way to check a used motor out before purchase?

 

[Yepblaze]

Re: correct method of compression test?

Manual compression test:<br /><br />Pull starter rope through all compression revolutions.<br /><br />Listen and feel force required<br /><br />1)Thump<br />2)Thump<br />3)Whoosh<br />4)Thump<br /><br />That third one failed

 

[SeaDawg]

Re: correct method of compression test?

Z-man, you can open the throttle (to "wide open throttle") on a motor all you want, if it has no spark plugs in it, and it will not start.<br /><br />You may have to bypass the in-gear safety switches, by using a remote start switch connected directly to the start from the battery.

 

[Franki]

Re: correct method of compression test?

Also, 110psi is not "required" to run..<br /><br />many OMC engines had only 100psi from factory.. <br /><br />and a motor with 90 would still run.<br /><br />Its the difference between cylinders that counts.. not the compression itself, (unless all your bores are clagged.)<br /><br />regards<br /><br />Frank

 

[grandx]

Re: correct method of compression test?

cold or hot, the difference in compression of all cylinders is what is important to determine if something is wrong with one or more cylinders. To get the actual compression, the engine should be warmed to operating temp and the gage calibrated (most are not accurate). Throttle wide open so no restrictions. If all read low but equal, the engine is worn evenly.