Should the compression test be done with engine warmed up or cold? Also merc manual says to ground the Dist ignition coil wire to the block . Is that a must?

Chris

Warm and with throttle open. Coil high tension wire grounded to block to prevent engine starting or you getting shocked. I just put a philips screwdriver in the dist end of the coil and lay it on or near a ground.

I would just disconnect power from the ignition coil.

Cold is fine, just be sure to crack the throttle blades open while doing the test. Also, if its a carbed engine, I prefer to remove all the spark plugs, and crank it over for a little. Sometimes, esp with a blower, there could be a little leftover fuel sitting in the cylinder. the little bit of liquid in there can raise the number you see on the gauge.

It is a carbed motor. So how do you keep the choke from engaging when you open the throttle and its cold.?

Chris

You could hold the choke plate open with a screwdriver

I have to agree with all that. It's best with engine warm but I've tested plenty of engines cold and they will usually just be about 5 psi lower across the board. Probably best to disable the coil completely to eliminate any sparks in an enclosed boat engine room and, yes, just put a screwdriver in the choke housing. And pull all the plugs so it cranks faster and since you have to take them all out anyway.

pull your boats lanyard

^^^ What he said,

The "correct" way is to have engine warm, throttle open, ect, bla, bla,bla... I have laways done it with a cold engine even on my snowmobiles. All your looking for is that each hole has pretty much the same compression, (with in 5% of eachother is what I go by). if you have 125psi on 7 holes and 75psi in 1 hole, you have a problem.