New to me mercury 25hp low compression on #2

Trickle56

Seaman Apprentice
Joined
May 4, 2010
Messages
43
Have an opportunity to buy a '99 mercury 25hp from a buddy. I checked compression on #1 it's at 115psi #2 was at 90psi. My question is it worth the buy? My first thought is head gasket or lower crank shaft seal. He says it runs good and I trust this guy. Says its a little hard to start but when it does it runs fine just doesn't push the boat as fast as it should. He said it will do 22mph on a GPS. Motor is on a 1440 mod v aluminum boat. With a 13 pitch prop. Would like y'all's two cents as well. Any help is greatly appreciated.
 

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: New to me mercury 25hp low compression on #2

Do a decarb and repeat compression test.
 

Trickle56

Seaman Apprentice
Joined
May 4, 2010
Messages
43
Re: New to me mercury 25hp low compression on #2

Ok thanks JB. Lets just pray its not a repeat of my '85 25hp evinrude. After decarb I had to replace just about every gasket imaginable on it.
 

Trickle56

Seaman Apprentice
Joined
May 4, 2010
Messages
43
Re: New to me mercury 25hp low compression on #2

So I did the decarb and redid compression test. Top went up to about 122-123psi, bottom went up to almost 100psi. I noticed that bottom spark plug is clean not much oil at all. I I haven't checked spark on it yet. I'm guessing water is leaking into this bottom cylinder. I was going to remove the exhaust cover for a closer look, but I can't get the bottom two bolts. The bottom cowling wot let bolts pull out. Do I have to rove he powerhead to remove the exhaust cover? Or does the bottom cowling come apart somewhere? I'm I looking at a powerhead rebuild here?
 
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