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1972-1973 80hp vers.85hp

fixitdaily

fixitdaily

Regular Contributor
I have been reading some of the posts and was wondering compression difference between 80hp and 85hp motors.On my 72 80 hp I am lucky to hit 125 psi and on some post I have read that the 85 hp motors are hitting 140/145 psi.Looking at some of the engine specs there isnt much difference between the two motors especially engine block and pistons.Just curious???
 
I agree with Jeff.I use an old mercedes benz compression tester was getting 90psi. Guy with a Merc marine comp. tester was getting 125psi.New rings,honing,bearings etc. and I'm getting 125 on my tester,which is an increase of 35 which would lead me to assume that on the mer/marine tester I would be hitting 160.Is it possible?????? I don't know,but she's running well for a forty one year old:D
 
Then leave her alone!

Automotive testers read low because ONE cylinder of a SB Chevy is bigger than all four cylinders of your 80/ 85 hp Merc! There's so much volume inside the tester that the pressure doesn't come ap as far as it should.

Jeff
 
Okay,I spent all morning doing the seafoam thing.First compression test (cold) without seafoam soak cylinder 1 125 Psi 2 135 3 135 4 135 after seafoam soak the same.After letting her warm up on Muffs cylinder 1 dropped to 115 Psi rest stayed the same.I am thinking about tearing her apart and honing out number 1 and a new set of rings before its too late.I am also guessing my water pump isnt working right due to just a drible at Idle and the water was getting pretty warm.Used to shoot right out no problem.I wounldnt doubt it one bit if she had got a little to warm on the top cylinder at some point and time.
 
Thanks Jeff,I didn't know that,another little bit for the memory bank:)
Steve,If you are going to open her,I would suggest that you do all the rings and hone all the cyls,it would be a pity to need to open her in a month or so to do another cyl.
 
I think your right and good advice.I do however have a question,all I do is cruise around no towing etc. do you guy's think I could keep this on the water for this season and then rip her open in the fall and do all this?I dont do awhole lot of boating anyway.
 
I'd wait until the boating season is over befre tearing her down. Hell, the season's short enough as it is. Those compression readings re not all THAT bad--I've run motors like that with worse.

Jeff
 
Thanks Jeff,
I will follow that advice. I think its time to go boating instead of fixing things.By the way Jeff I came across your Post on your merc. ignition modification that was pretty slick.
 
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