40hp water in cylinder?

[jackle61]

'91 classic forty, 4cyl, 2stroke. Idleing rough with a slight loss of power. Pulled the plugs and found the bottom one had the telltale milky residue of water and oil mixed together. Using a spark tester I get good spark, but there must be water getting in the cylinder keeping it from firing. I also developed a slight knock just before I discovered this.
Two questions, how would the water be getting in there?(other than a cracked block) and could the knock just be the piston "free wheeling" in the cylinder? I do have 120#compression in the questionable cylinder.
Thanks for any assistance.

 

[emckelvy]

Re: 40hp water in cylinder?

Since you only have water in #4, the most likely culprit would be bad crankshaft seals. You'll have to pull the powerhead to check these out. The bottom bearing cap can be removed without splitting the cases but it's easier if you loosen the bottom (2) crankcase fastening bolts and use a harmonic balancer puller or wheel puller to remove the cap via the tapped puller holes in the cap. Definitely don't pry on the "ears" of the cap as they'll break.

2nd possibility would be leaking exhaust manifold internal baffle plate, or gaskets.

Given the age of the motor, and that you'll have to pull the powerhead to fix either problem, might as well pull the exhaust man cover, inspect & renew gaskets & internal plate as required.

The only caveat would be, if this is a Salty Dog, the manifold bolts will likely be frozen due to corrosion. In this case only remove the cover as a last resort (if you don't find the crankshaft seals to be leaking).

Salty Dogs require lots of heat, penetrating fluid, and sometimes drilling the bolt heads to gain access. Not that it wouldn't be a good idea to do service on it anyway, but you might not want to unnecessarily open up a large Can of Worms.

You can look up parts for your motor at www.iboats.com and I also find the following site to be a good resource for diagrams etc:

http://www.mercruiserparts.com/SelectModelType3.asp?class_id=2

HTH..........ed