1974 15hp evinrude low compression help...

wolfman77

Cadet
Joined
Sep 23, 2013
Messages
17
motor starts easy and runs good. but has low power and wot rpm. compression is 35psi in both cylinders when spinning motor by hand. I even compression tested both cylinders while running on one and testing the other and still only got 60psi. removed bypass plate, pistons are clean and smooth, rings are not stuck. im stuck on why compression is so low. could the ring grooves be worn to allow blow by? any ideas appreciated.
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,274
Re: 1974 15hp evinrude low compression help...

You say it runs " good " but has low power ????---------Try using another compression tester as your gauge is suspect.-------Could be a problem with the head gasket to.----Exhaust housiing could be plugged as well.
 

nwcove

Admiral
Joined
May 16, 2011
Messages
6,293
Re: 1974 15hp evinrude low compression help...

i really doubt the motor would even start with 35psi, so as mentioned, try another guage. and plz dont do the test while running the motor! thats a good way to fill a tester with fuel and making it useless. id try an aggressive decarb with seafoam or the likes.
 

boobie

Supreme Mariner
Joined
Nov 5, 2009
Messages
20,826
Re: 1974 15hp evinrude low compression help...

Agree............
 

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
Re: 1974 15hp evinrude low compression help...

Inspect your headgasket for blow by failure, inspect the head, then check you have spark in both cylinders.
 
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