compression

bluejim115

Cadet
Joined
Dec 18, 2006
Messages
13
I recently was given a 1958 5.5 H.P. motor. I'm not sure if it runs or not. I did a compression test and the results were 77PSI & 80 PSI. That seems to be a little low to me. I would like the opinion of others. Thank you.
Jim P.S. Would decarb help?
 
Last edited:

HighTrim

Supreme Mariner
Joined
Jun 21, 2007
Messages
10,486
Re: compression

The main thing is the comparison between the two, they say 10 percent, which you're in. There are many variables that determine the actual reading, the type of gauge being one.
 

wbeaton

Commander
Joined
Jul 30, 2006
Messages
2,332
Re: compression

That's lots of compression. I have motors from that era in the +60 psi range.

Check your coils. If they are original they need to be replaced. Also do the impeller and check the lower crankcase seal while you are in there. Here is a great website that outlines exactly what to do to tune-up a motor just like yours.

http://www.outboard-boat-motor-repair.com/
 

HighTrim

Supreme Mariner
Joined
Jun 21, 2007
Messages
10,486
Re: compression

(Compliments of Tashasdaddy)

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.
 

floatingwoody2006

Chief Petty Officer
Joined
Feb 2, 2006
Messages
500
Re: compression

that sounds just fine to me.. A decarb will probably bring it up a bit also. Got spark?
 

bluejim115

Cadet
Joined
Dec 18, 2006
Messages
13
Re: compression

By the way, the motor is an Evinrude. Have'nt checked any thing but the compession. Thanks for the input, I'll keep ya posted. Thanks again!!! Jim
 

bluejim115

Cadet
Joined
Dec 18, 2006
Messages
13
Re: compression

Yes, It has good spark. Lower unit oil is clean. No shavings. Gonna put new lines on tank tommorrow and see if she'll start. Jim
 

wbeaton

Commander
Joined
Jul 30, 2006
Messages
2,332
Re: compression

Check your coils first. No sense doing anything if those coils are shot. You can't tell if they are ok by the spark. You have to look at them - HAVE TO.
 

Walker

Captain
Joined
Jun 15, 2002
Messages
3,085
Re: compression

Rope start motor right??
Its very hard to get a true compression reading by pulling the rope. You numbers sound good to me.
 
Top