1980 Mercury 7.5 HP running on one cylinder

ianferron

Cadet
Joined
Aug 20, 2011
Messages
23
I just got that motor and was running on one cylinder so I decided to take it apart. I found a hole in the baffle plate so water was allowed to enter into the cylinders. I also check the compression and came up with 60psi in both cylinder. I took the piston out and found the lower rings both broken on both pistons. I put new rings and inspected the cylinder but after putting everything back together with a new baffle plate my pressure is still 60psi. It does start on the first pull tho. I know reeds vavle can lower the compression a little but don't think that motor has any. It used port. Why is my pressure still 60 psi??? I don't even know why it's still starts that good with that pressure.
 

emckelvy

Commander
Joined
Jan 16, 2004
Messages
2,506
Re: 1980 Mercury 7.5 HP running on one cylinder

As long as compression readings are even, you're probably OK. I'd suggest that your compression gage might be off; 60 lbs is rather low, if the motor really had that it wouldn't run very well.

You can still have deceptively good compression readings, even with a broken ring. As long as the ring remains in the piston groove and hasn't busted out yet, it'll seal somewhat.

At any rate, sounds like you found the problem and she's running well, enjoy!

HTH..........edx
 
Top