Mercruiser 120 hard to start, stalls....

Discussion in 'Gas Engines' started by chrisffg31, Jul 26, 2009.

  1. chrisffg31
    Joined: Jul 2009
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    Location: Maine

    chrisffg31 New Member

    This is a project boat I picked up at auction for $900. It is a 1986 Starcraft with a Mercruiser 120 engine. When I got it the engine was frozen but I was able to get it loose by soaking the cylinders in Marvel Mystery Oil. It runs now but it's hard to start, idles rough and while it runs ok in my driveway in the water it will only take about 1/3 of the thottle and then it backfires, sputters and stalls. It acts like its not getting fuel. What do you think? Gummed up carburator? Clogged fuel filter? It will run to redline easily in my driveway but not in the water. This is my first boat - definitely open to suggestions!
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    You've bought a lemon for $900 and now you want us to tell why it isn't running?

    Buy a manual for a boat, car, anything with an engine in it and follow the diagnostics program. Compression, oil pressure, carbs, ignition, the works.
    That way you'll discover most of the problems that go with a $900 boat.
    And when you've done all that and there is still something you didn't find, supply us with the correct data and somebody may come up with the right answer.
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Your engine and drive combination are covered in at least two different manuals that I know of, buy one.

    I'd have to side with CDK on this, you had a frozen motor and rather then sort out the various issues, you just let the rusted piston rings slide up and down the rusted cylinders, of course with a fresh coating of oil on them and now wonder why it's not running right.

    There are probably several things wrong with your engine and drive. Dare I ask about impellers, filters and other annually changed parts?
  4. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    Those are classic symptoms of bad points, but I don't know the engine, just the symptoms. If the engine has points, get some fine emery paper and reface them and gap them according to the specs.
  5. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I'll bet you have no compression. It will run to high RPM with no load. You need a rebuild.
  6. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member


    As you can see, we have:
    - One person who says to buy a shop manual, figure out everything that's broken, and if that hasn't scared you off, post everything you find and we might have ideas
    - One person who says to buy a shop manual and pray that it hasn't already been cooked
    - One person who thinks the innards are shot and it needs rebuilding
    - (Sorry Alan, I seriously doubt that such a simple fix as ignition points will help an engine that was seized up and then freed with marvel mystery oil)

    I don't think this bodes well for the future of this motor. By all means, pick up a shop manual and start doing the diagnostics it suggests, but keep in mind that you *might* have some pretty major internal damage on this thing.
  7. Frosty

    Frosty Previous Member

    All of the above,-- but hey no one has a crystal ball.

    If it came free by soaking in oil it was'nt that badly siezed.

    Ripping stuff apart before knowing is stupid. By all means clean the carb and spent a minute or two on the points after all you would look stupid with the engine all apart and then find a cracked distributor cap.

    Check it all out compressions N all.

    Ditto on the why did you buy that when you dont know about stuff like that.
  8. Guest625101138

    Guest625101138 Previous Member

    There are some really good bargains around at the moment particularly at the higher end of town. A work colleague is eyeing off a 52ft Beneteau for under AUD300k.

    So USD900 might get something pretty good in boat and motor at a fire sale. Probably lacked some maintenance if the owner was in strife. Any power boat is junk unless you can afford to put fuel in it.

    I agree with Frosty. It owes him $900. As much as you pay for a good course in engines I expect. Chris will get better experience, good or bad, hands on than sitting in a classroom.

    Chris Ostlind has mentioned the value in second-hand boats right now on another thread.

    Rick W
  9. Frosty

    Frosty Previous Member

    Rick a 52 Beneteau (ford of the boat world) at 300aus is not cheap.

    Unless its last years model.
  10. Guest625101138

    Guest625101138 Previous Member

    I do not know the year but he was impressed with the price. It will be similar to this one:
    It would need to be quite a few years old before you would think AUD300k cheap for something like this.

    I will ask him if it is being advertised.

    Rick W
  11. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    when you free up the pistons in the cylinders, how do you know the rings are free in the pistons?
  12. Frosty

    Frosty Previous Member

    Compression test, wet and dry. Or should I say dry and then wet.

    Diagnosis will save thousands. You would expect your surgeon to know where he was going before he sharpened his knife,--ok scalpel.
  13. powerabout
    Joined: Nov 2007
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    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    ha ha
    good answer!
  14. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    No, though I tried. I was quite proud of my little guess. If you forget about the seize-up, I begin to sound rather clever.

  15. broke_not
    Joined: Jun 2008
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    Location: North Dakota

    broke_not Junior Member

    Compression and/or leakage tests are fine, but you won't save thousands by spending a lot of time doing them on a simple GM four. I don't even see the necessity of dry and wet tests here. If you have a cylinder or two, (or three or four), that tests low with a dry compression test, the engine needs to come apart anyway. You may be able to hazard a guess as to why the compression is lower by doing both dry and wet tests, but the engine still needs to come apart for a definite look inside and subsequent repairs anyway.

    A guy at work a couple of bays away from me will spend hours doing tests to ascertain the mechanical condition of an engine without diving inside. Then once the data is gathered, mini-meetings are held with other workers to see if everyone's opinions about what the data means coincides. Then after half the day is gone, some bolts finally get removed and some bits get taken apart. Often, many of the pre-disassembly guesses are confirmed.....often they are not. More often than not, other "surprises" are found inside that pre-disassembly tests would have never revealed.

    I'm not saying these tests have no place, but I find them most useful in instances like trying to determine whether or not to buy a non-running machine in the first place. If I've already spent the dough and made the purchase though, I'll spend at most maybe a half-hour or so before the wrenches come out.

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