Johnson 70 hp 1979 compression

R

RJDAWSON

Seaman
Joined
Sep 4, 2005
Messages
54
Took a compression test today, Top cyl 65psi mid and bottom cyl both 95psi.
Suspect a ring or scored wall in the top.
The book dose not say what the compression should be.
What do you think?
Open for sugestion.
 
JB

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: Johnson 70 hp 1979 compression

You have a 50% psi discrepancy with the top cylinder. The others are okay.

Several things could be the cause. Blown head gasket, carbon stuck rings, broken rings and/or piston.

If it runs I would start with a strong decarb treatment. If that doesn't fix it it is time to pull the head and inspect for damage.
 
R

RJDAWSON

Seaman
Joined
Sep 4, 2005
Messages
54
Re: Johnson 70 hp 1979 compression

Thanks, Fleetwin. I'm sure this will take care of some other problems that I am having. Days with long hours of slow trolling dose bog the motor down and makes it hard to crank.
 
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