HD25 compression?

R

rvanhout

Cadet
Joined
Nov 5, 2003
Messages
6
What should the compression be for an old Johnson HD25? I'm getting a reading of 60psi on both cylinders, should I go ahead and put in some new rings?
 
Chinewalker

Chinewalker

Fleet Admiral
Joined
Aug 19, 2001
Messages
8,902
Re: HD25 compression?

Actually, for that motor, 60PSI isn't too bad. Before you tear into it, I would get it running and do a full decarbon on it, as the oil ratios that that motor has been run on (or should have been run on) have likely resulted in lots of carbon inside. A good decarbon might just bring compression on a bit, and won't hurt to boot...<br />- Scott
 
M

Mark R Groves

Seaman Apprentice
Joined
Dec 5, 2001
Messages
45
Re: HD25 compression?

I put new rings in my HD25 a couple of years ago assuming the compression was too low at about 60 psi. After the rings/rebuild the compression came up to about 65psi. I think approximately 60 PSI must be about the max for these engines. Does anyone have an explanation why these smaller engines have lower compression? I would think the basic compression for all general purpose (non-high performance) outboards would be in the same ballpark.
 
Chinewalker

Chinewalker

Fleet Admiral
Joined
Aug 19, 2001
Messages
8,902
Re: HD25 compression?

Hi McG,<br /> I am sure part of it is simple physics meets economics. A smaller volume would take much more energy to build up high compression. In a small fishing motor, high compression isn't necessary. Higher compression requires higher strength parts, which cost more money, which weigh more, etc. It also can put more stress on the motor and shorten the lifespan. Most fisherman are far more likely to buy a motor that will get them out and back reliably than one that might go a bit faster, but is more likely to break down or wear out quickly.<br />- Scott
 
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