1973 mercury 500 50hp compression test

basstracker1970

Petty Officer 2nd Class
Joined
Feb 4, 2009
Messages
147
Ok guys here is what I have. I bought a 1973 Mercury 500 50hp outboard motor for 50.00 bucks that came with controls also. I panned on putting this motor on my bass tracker next spring. The motor has been setting a long time from the looks of it. I have the motor stripped down and got around to checking compression. The number 1 cylinder has 110 psi number 2 has 111 psi number 3 has 90 psi and number 4 has 110 psi. I know the number 3 cylinder is low but im thinking its not from damage of the piston or cylinder but just stuck rings maybe. I can put a little oil in the cylinder and it will shoot up to 110 psi fast. What do you think about this? I need all your opinons on this one please.
 

hotrod53

Chief Petty Officer
Joined
Oct 16, 2009
Messages
508
Re: 1973 mercury 500 50hp compression test

I don't think that putting oil in the cylinder gives you a realistic compression reading. I know for a fact that a 2 stroke lawnmower with bad rings can be started by putting oil in the cylinder, it seals up the rings and therefore raises compression.

IMO, and correct me guys if I'm wrong, you need to run that motor on the intended fuel to get that load of oil out of there, check the compression, and it is what it is. You may however have gummed up rings, maybe run some Seafoam thru it to decarbon it.
 
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