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

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

    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.

  • #2
    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
    1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - http://forums.iboats.com/forum/owner...988-rogue-2420

    Past Boats
    1970 Wooster Hellion - Merc 9.8
    2002 SeaRay 190BR - 5.0 - A1G2 - "Cheasheads in Paradise"
    1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

    What’s behind you doesn’t matter.Enzo Ferrari

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    • #3
      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.
      Kenner, Louisiana 1980 Donzi 18'. 2+3 6.2 MPI Alpha 1

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      • #4
        a lot of old municipal trucks came with 292's. they have as much torque as the 305 V8
        1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - http://forums.iboats.com/forum/owner...988-rogue-2420

        Past Boats
        1970 Wooster Hellion - Merc 9.8
        2002 SeaRay 190BR - 5.0 - A1G2 - "Cheasheads in Paradise"
        1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

        What’s behind you doesn’t matter.Enzo Ferrari

        Comment


        • #5
          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.

          Comment


          • #6
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            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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