compression and leak down

dozerII

Admiral
Joined
Oct 25, 2009
Messages
6,527
1981 Mercruiser 140, ser #5962858 I has another thread were I asked for info on the exhaust elbow, I ended up replacing both the elbow and the manifold, as well as the shutters. I did a carb rebuild, plugs, points, rotor, cap, wires. The previously cracked elbow had allowed some water to get into mostly #3&4 cylinders with out any rust forming on the cylinders from what I could see in the video from my bore scope, there is also an old previously welded crack in the block, just below the head on the port side. Today I fired her up and ran it for 30 minutes, to check for water leakage from the weld and possibly into the oil, neither showed any signs of water. After I shut it down I did a compression and leak down test, the results are as follows.
#1 120 #2 125 #3 105 # 118
Leak down was as follows

#1 17% could only hear a very faint leak from the vent in the valve cover.
#2 28% Slightly more of a leak in the vent in the valve cover.
#3 38% quite bad from the valve cover
#4 38% quite bad from the valve cover


Video of the motor running,


Opinions?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
Guess you could run it till it pukes, or pull the head. Since your hearing all your air coming from the valve cover, this would tell me it's rings. You could try a valve job and it is more probable the 38% is part rings and part valves/guides.
 

ziggy

Admiral
Joined
Jun 30, 2004
Messages
7,473
Guess you could run it till it pukes,
that's where i'd be at. meanwhile looking for a new block so can get rid of the freeze crack. fwiw, i been running with worse numbers than you for 7 years or so. my engine has never been real healthy, but it continues to run. i just try not to abuse it (give it hard runs) so it might last me another year. if i had the bread, i'd do something about mine, but i don't, so i keep treating it as kindly as i can. so far so good. maybe you can get lucky too. unless ya got a lot of bread, then ya know what ya need to do. imho, the freeze crack is the issue. it's got to go, only way to do that is another block, the low compression can get cured along w/ block replacement, when that occurs,
 

tpenfield

Moderator
Staff member
Joined
Jul 18, 2011
Messages
17,620
Yea, probably a tired engine, but if it keeps running and has decent power, no need to mess with it.
 

dozerII

Admiral
Joined
Oct 25, 2009
Messages
6,527
Thanks everyone, with the cracked block I don't want to spend a bunch of money on it. Come spring and soft water up here we will run it and see what happens.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Thanks everyone, with the cracked block I don't want to spend a bunch of money on it. Come spring and soft water up here we will run it and see what happens.

And in the meantime keep you eyes open for a replacement... Finding a really cheap, working engine over the winter and building up a good unit would be a nice little project.. ;)
 

dozerII

Admiral
Joined
Oct 25, 2009
Messages
6,527
And in the meantime keep you eyes open for a replacement... Finding a really cheap, working engine over the winter and building up a good unit would be a nice little project.. ;)

Thanks Chris
 

Volphin

Lieutenant
Joined
Jun 5, 2011
Messages
1,405
I'm with Chris for sure. Right now you have a motor that runs pretty well, and you can take your time to find/build another. Plus, you get to have fun on the water while you do it. WIN/WIN. :)
 
Top