Compression check on a marine motor

[rob v]

I was hoping someone could give me the proper procedure for testing the compression on an outboard engine. I'm considering the purchase of a used boat and motor privately - the motor is only 3 years old. Many folks seem to suggest testing the compression on the engine. Short of buying the motor and then having the process done (it's a bit late then) by a qualified technician I'm left to do it myself. Seller is not motivated to have that test done - he's confident of the motor's condition but is happy to let me have the testing done but he's not interested in me taking the boat anywhere without paying in full.. and I don't blame him.

 

I've been told that the motor should be warmed up, then all the plugs removed (3 cylinder) and then each cylinder tested. the main thing is that the variance is not more than 10-15% from one cylinder to the next. The thing I'm a bit concerned about is the issue of disengaging the electrical system. Is just tripping the kill switch sufficient (so the motor doesn't fire) to ensure no harm is done ?

Am I on the right track ?

 

And is a test such as this necessary on a relatively new motor that looks to be in impeccable shape ?

 

Any input would be appreciated. thanks.

[Tomcat]

I did a Google search on "how to do a compression check on an outboard motor". There was all kinds of information available.

[2 tone z71]

WARM COLD IT just dont matter pull the plugs put the plugs back in the boots,ground them off away from the block away from the holes ,screw the tester in the hole and crank it over ,write down your number then continue on if there all close 120 118 122 etc etc etc put the plugs back in and get fishin

its not rocket science and yes you can shut your kill off wont harm any thing,3 yrs old I doubt you have anything to worry about ,besides theres LOTS that can be wrong besides compression

Edited March 26, 2008 by 2 Tone Z71

[Fisherman]

Go to a shop that deals with the motor you plan on buying and ask them, some do the check hot, some cold, that way you'll have a straight answer. All things bright and shiny can hide things on the inside that may look good from far, but far from good lookin. Do it right=peace of mind.

[Bernie]

Fisherman is right. A compression test is only one step of checking out a used outboard. Also if it's a 2 stroke it may look ok but was quality oil used? Personally I think if the guy won't let you get it checked out I would be suspicious of an issue. Ask him to come along with you to have it looked at.

[yeahookk]

dont forget to ask for a leak down test....also very important!!

[POLLIWOGG]

If the motor has been fogged you will need to run it for a while to get a true reading.

 

Buddy's 3cyl yammi took out a piston at 4years.

[boatman]

WARM COLD IT just dont matter pull the plugs put the plugs back in the boots,ground them off away from the block away from the holes ,screw the tester in the hole and crank it over ,write down your number then continue on if there all close 120 118 122 etc etc etc put the plugs back in and get fishin

its not rocket science and yes you can shut your kill off wont harm any thing,3 yrs old I doubt you have anything to worry about ,besides theres LOTS that can be wrong besides compression

 

Follow the test procedure 2 tone posted. Warm or cold. Crank the engine until the gauge tops out. Then move to the next cylinder. All testers are different so don't be too concerned about the actual numbers. You want compression within 10% of all cylinders. 120 psi is good, but anything over 100 psi is fine.

[rob v]

Thanks guys - yes I did the internet search also and got a bunch of different angles to the process. I suppose the main question I have is ensuring I've disabled the electrical system. The easiest way it would appear is to trip the kill switch or pull the coil wire. But several places refer to "grounding the plugs". How exactly is this accomplished ? thanks again.

[2 tone z71]

Listen if you pull the plug caps and let them hang you run the risk of damaging the CDI/POWERPACK/SWITCHBOX whatever term ya wanna use,shut the key OFF ,pull the tether off ,take out the plugs put them BACK in the plug caps let them hang there off to the side and do you test,if ya gotta look this poop up on the internet,bring it to some one who knows whats goin on

[2 tone z71]

dont forget to ask for a leak down test....also very important!!

A leak down test is your best bet and way more comprehensive then a compression test,if the motor start idles smooth and runs to its max WOT without any issues theres no need for either,I only do a compression test if the motors really running like crap after fuel and other issues have been checked