Dead cylinder in Merc 165 GM 250 I6, what to do?

spoolin01

Seaman
Joined
Jul 30, 2002
Messages
60
Earlier this summer I had the boat out and suffered what seemed at the time to be fuel starvation. Changed the fuel filter and managed to get the boat back to the harbor by manipulating the carb linkage to keep the motor from stalling when shifting into gear. Along the way there was another "starvation" episode, and at least one occasion of bad mechanical bang from the motor. When back, I pulled the carb to bring home. The carb was NOS self-installed at the end of last season, after which the boat ran nicely for several trips.

Today I went back to check a few things and started with a compression test (motor cold, carb off, plugs out).
1- 140
2- 35; goes to 50 after squirting oil in
3- 110; goes to 140 w/oil
4- 122
5-124
6- 150

The #3 cylinder has been low for ages, since not long after the rebuild. I last checked compression earlier this season, and the #2 cylinder was in line with the rest. Motor is a GM 230 Merc block bored to 250, rebuilt 1100 hrs ago. The boat is a '72 Sportcraft 22, but has a nice Alpha One outdrive adapted to it. Otherwise there's little value to the boat except that it runs and floats.

I'm debating what to do now. I did the oil test on #2 three times - how strong of an indication is this that the problem may be with the head and not the rings? There are no out-of-place noises when spinning the motor, and the plugs all look the same. It's not worth it to me to have the boat fixed by a shop, or to drop in another engine, but if I can pull the head and have it rebuilt or replaced, I'd do that. It would be worth $500 to get another season out of it before moving on to something else.

The alternative is stripping and junking the hull, or selling it as is. I could probably get $1500-$2000 for the trailer, motor, outdrive, and a few of the other fixtures. I doubt I could recoup more than another $1500 tops even with a fresh motor, and as is I think I'd struggle to match the part-out price.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,297
Here is what I would do.

pull your head, most likely you simply need a valve job from running lean.

if you need a whole motor, look under the hood of any 1970-1980 pickup truck and get a running motor (lots of strong running 250's get pulled for V8's and sold on craigslist for under $200). swap over all your marine stuff bolted to the long block and drop in, align and go boating
 

kenny nunez

Captain
Joined
Jun 20, 2017
Messages
3,056
2xs on Scott. The truck engines have the mounting ear on the starboard front side that the auto engines did not. If you find a engine that still has the HEI distributor that would be a plus. You just have to by pass the resistors and replace the core plugs.
If you find a 292 truck engine every thing from your old engine will swap over. The alternator adjusting bracket needs to be lengthened as well as the fuel line and exhaust hose. The flywheel and the drive coupler has to be drilled to 1/2" and 1/2" studs and nuts have to be installed. The 292 is about 1.5" taller and works well with your Alpha drive. I replaced at least a dozen 250s with the 292 with good results.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,297
a lot of old municipal trucks came with 292's. they have as much torque as the 305 V8
 

oldsub86

Petty Officer 1st Class
Joined
Sep 9, 2008
Messages
311
You describe a lean condition. Did the motor overheat? If so, you may have warped the head and have a headgasket leak. Pull the head and check to see how straight it is with a straightedge and feeler gauges. You may need to have the head planed and put in a new gasket.
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
One cylinder down could be a problem in the valve train and certainly worth looking at before launching into "rebuild" mode. Pull the valve cover and check the operation of the valves on that cylinder. Loose rocker, pulled rocker stud, bent push rod, misadjusted rockers and flat cam lobe(s) are all possibilities.
 
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