Compression on a Merc

[NathanY]

Checked the compression on my 20 HP Merc this morning and it said 90 lbs on both cylinders. That was just with one pull of the rope. Is that correct or do I need to pull the rope more? Seems like that would not be accurate with one pull since it takes a little air to fill the hose and guage on the tester? Can anyone tell me?

 

[Kenny Bush]

Re: Compression on a Merc

Nathan, you need to run the engine and warm it up, unplug the wires or kill switch, put the throttle wide open, and then pull a few times(to reach max comp.). Make sure your wires are disco'd and you may want to run the gas out to be sure of "not firing up". As long as you cylinders are within 15% of each other, powerhead will asperate OK. At operating temperature, you should see readings at about 120 psi. Goodluck......BushCat

 

[NathanY]

Re: Compression on a Merc

Well In not able to start it yet, but cold compression it 119 on both. that is doing it like you said. Is that good?

 

[Harry Blanchard]

Re: Compression on a Merc

That should be fine, high enough and not more than 10% difference.

 

[NathanY]

Re: Compression on a Merc

Is compression higher or lower on a cold engine?

 

[Kenny Bush]

Re: Compression on a Merc

Nathan, your motor compression looks to be perfect, even if its cold......Sometimes the rings seat better when the motor is hot. I think your gonna be OK..........BushCat

 

[NathanY]

Re: Compression on a Merc

Ok, I went back and re did it again with the fitting with the o-ring on it that seal around the hole, and this is what I got on a cold motor<br /><br />Top 125 PSI<br />Bottom 126 PSI<br /><br />What kind of compression can I expect with a warm engine? will it go up or down?

 

[Kenny Bush]

Re: Compression on a Merc

Nathan, run it............compression looks great......I guess that O ring got you a few more pounds......anyway compression looks normal so your powerhead looks good.......BushCat