Mercury Newbie....Help

[wreckless]

What should compression be on a 1975 70hp Merc? I am looking at purchasing an older Merc to put on my crabbing boat and I don't know squat about Mercs. I have always owned OMC outboards. The guy who owns the 70 Merc said it ran about a year ago and only wants $100 for it but I would like to check the comp before I get it. Is there anything else on these engines I should look for? Is there a web site for Merc specs like OMCs have?<br /><br />Thanks...

 

[SeaKaye12]

Re: Mercury Newbie....Help

Hi Wreckless...and may I welcome you to a great forum here!<br /><br />I have an older Merc like yours (1978 85hp). I think that the pros here will tell you that an exact number for compression isn't as important as all cylinders being within about 10% of each other. My 85 pumps about 125 lbs all around...and I think that is considered normal.<br /><br />I'd also take a good look at the wiring under the cowl...look for corrosion and insulation that is flaking off.<br /><br />Look at oldmercs.com...yours is just about **OLD** enough for them to list it there.<br /><br />Good Luck...Chuck

 

[Texasmark]

Re: Mercury Newbie....Help

Normal compression is 150 with not more than 15% variance between cylinders. Min is 120....outta da Merc Service manual.<br /><br />Mark

 

[Clams Canino]

Re: Mercury Newbie....Help

Originally posted by Texasmark:<br /> Normal compression is 150 with not more than 15% variance between cylinders. Min is 120....outta da Merc Service manual.<br /><br />Mark

I dissagree with the manual and go with the poster before this. I've only seen one direct charge motor read over 145 on my guage. Average is about 120-130. <br /><br />-W

 

[Texasmark]

Re: Mercury Newbie....Help

Well Clams, you just said something I overlooked. My manual is for 3 cyl loopers that came along after the direct charge. I figured we were talking about apples and apples. Looks like there are some oranges to be considered. Grin.<br /><br />Sorry for the erroneous information. Since we're talking about the manual, I have always heard of and used the 10% theory, but they seem to think 15 is ok. Oh well.<br /><br />Mark