Cylinder Compression Question

iceman918

Recruit
Joined
Sep 6, 2007
Messages
1
I am looking at buying a pontoon with a 1990 johnson 70hp, the VRO as been unhooked, it runs good, i went ahead and did a compression test, got 111# on the top cylinder, 96# on the center cylinder, and 110# on the bottom cylinder. just wandering how many pounds difference is ok to have?

thanks
 

Gary H NC

Fleet Admiral
Joined
Dec 1, 2005
Messages
8,972
Re: Cylinder Compression Question

That 96 is a little out of the 10% range.
Did you test with the engine warmed up? Still 96 is not a bad reading but why just on one cylinder.
I'm sure some of the outboard pros will comment on this..
 

Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: Cylinder Compression Question

That's right at the limit of 15 lb range. The middle cylinder of the 56 cid motors run the hottest. If I were buying, and knowing the 56 cid history, it I'd want to pull the head and take a peek. If it's OK, then you're all set. The most it's going to cost is a gasket.

The alternative would be a decarb job. Hopefully it's just coked up from being lugged and the carbon and sticky rings will go away.
 

HighTrim

Supreme Mariner
Joined
Jun 21, 2007
Messages
10,486
Re: Cylinder Compression Question

Compliments of TD. I think i would invest in a can of seafoam, in a portable tank, mix 3/4 gallons of premix gas, an 3/4 can of seafoam, (assuming you don't have the vro connected). Put the remaining seafoam in a spray bottle, run the motor up to temperture, remove the plugs, one at a time, spray some seafoam into each cylinder and replace plug. let sit 15 minutes, remove the air silencer from the carb while waiting, restart motor, and spray seafoam into each carb, until it start to stall. repeat till seafoam is gone. run out the rest of the gas. it is going to smoke like it is on fire, and drip goo on the ground. replace the plugs with new ones, and then do the compression check again. this procedure burns the carbon out of the motor and been known to free up rings, and increase compression. let me know the results.
 

jtexas

Fleet Admiral
Joined
Oct 13, 2003
Messages
8,646
Re: Cylinder Compression Question

it's enough difference to pass on the motor, unless the price is reduced by the cost of an overhaul. Otherwise that's an excellent motor.

welcome to iboats!
 
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