Merc inline 1150 Rough idle and water intrusion

[kswiatk]

1983 Merc 1150 inline serial number 641xxx.

Bought it last year with 120psi on all cylinders, ran almost 50 mph if you feathered the throttle at start up, but would constantly die at idle and in gear idle.

I figured that it was a link/sync or carb issue. Ignition checks out all good after replacing plugs and a switchbox.

I decided to rerun compression after not being able to make any difference with carbs or sync. #4 and #6 plugs looked concerning, and #4 now showed 90 psi, the rest 120psi.

I purchased a cheap boroscope and the issue is obvious now. #4 is steam cleaned, and I believe #6 may be showing issues too? The pics are attached below and label with cylinder number.

Plugs #6 on left, #1 on right

This is #4

#5

#6 Is this wash okay?

Now is this indicative of a inner water jacket leak? The exhaust manifold has new gaskets from previous owner, is it just possible that they did not machine the inner plate flat?

What are my options for the engine? Is is easier to replace just #4 piston and rings? Or will I have to pull the crank and all pistons anyways?
Is it worth all the work or would a different powerhead be a better option?

 

[Chris1956]

Plugs 3,4 and 6 look a bit too clean, like water infiltration is occurring. It is hard to judge the cylinder pictures.

Removal of the transfer covers on the starboard side of the motor will allow you to see the rings. You will need to replace the rings, and likely the piston, if the rings look bad. Consider boring the affected cylinder. There is no way to get the piston out w/o removing the crankshaft.

I would replace the inner water jacket cover gasket and the outer water jacket cover gasket, after making sure the exhaust baffle is flat. Also, replace the lower crankshaft seals.

 

[kswiatk]

I was able to get the transfer cover off number 3 and 4. #3 was fine, the pictures are of number four. Definitely looks like an oversized piston is in order..


Is there any shop or person that does cylinder boring or have ring compressors for these motors in the Lower Lake Michigan area ( Illinois , Indiana, Wisconsin, MI)?