Compression test help

[jacobmorin]

I have recenlty purchased a 72' 65hp merc. I am about to do a compression test on the motor, however I just want to make sure I am doing some of the basic steps right:

1) take out all the plugs
2) screw the compression tester onto one of the empty plug holes
3) To test the compression do I just try to start my electric start merc with the controls?? Meaning do I just turn the key and try to start the motor to get the reading? I am asking this becuase I have seen sights that say you have to start with the rope, and my motor doesn't have one.
4) What should the compression tests show for a result, and how far off should each cylinder be off from each other to be considered a good test?
Thanks

 

[anthony115merc]

Re: Compression test help

it should have an emergancy pull start rope which goes onto your fly wheel

 

[Windykid]

Re: Compression test help

Take all plugs out.

Install guage.

Crank motor with starter or rope about 6 revolutions.

Each cylinder should be within 10% of each other.

 

[tashasdaddy]

Re: Compression test help

within 10% and over 100psi.

 

[Chris1956]

Re: Compression test help

Jacob, After I install the compression gauge into a plug hole, I jumper the starter solenoid (battery cable to small terminal with yellow wire)to spin the motor. I leave the ignition key in the off position.

 

[Big Keepers]

Re: Compression test help

Here is a question I have. I did a compression test on my boat with a battery that wasn't really at 100%. Its an old battery and I had it on a trickle charge for about two days. The battery couldn't hold the load very long and by the time I got to the last two cylinders (its an Evinrude 175 2 stroke) the battery wasn't cranking the motor very fast but it was cranking. I got poor results on the last two cylinders. Is this a result (partially) of the poor battery condition?

The reason I ask is this. I got 0% compression on the last cylinder. The gaskets are in good condition, the piston has no hole in it, the walls are not scorched or scoured, the rings are good, (it was rebuilt fairly recently) and I can't find a crack in the block that I can see. What would my next step be? I'm thinking new head gaskets, new water jacket cover gaskets and a fresh battery to re-test the compression.

 

[tashasdaddy]

Re: Compression test help

yes it's possible. with all the plugs out the weakest battery should turn it over. it only has compression on the cylinder you are testing.

 

[Big Keepers]

Re: Compression test help

I know that part but what I'm asking is if the battery is so weak that the motor cranks slowly, but still cranks, should I get some compression reading? It was explained to me that since the 2 stroke is a high compression motor, you need a good battery with a 100% charge to get the proper reading or else all compression will leak back around the rings and into the ports. It won't have enough compression to force past the schraeder valve and into the gauge.

If I still get crappy compression or no reading, how can I find the crack in the block? At this point that is the only thing it could be if a good battery doesn't improve the compression.

 

[Windykid]

Re: Compression test help

Why dont you recharge the battery and replace and retest!

 

[ddennis]

Re: Compression test help

Or Jump it from your car/truck!

 

[rick56]

Re: Compression test help

And be sure to ground all the sparkplug leads to prevent arcing. Could get exciting quick if gas fumes are present and do damage to your ignition system!