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Thread: 5.0L Mercruiser hard to start/will not start

  1. #1
    Senior Member easttnboater's Avatar
    Join Date
    Nov 2002

    5.0L Mercruiser hard to start/will not start

    I thought I would share the saga with my port engine:

    1. Last weekend I decided to fire up the engines for the first time this year.
    2. Starboard engine fired up after a few cranks and some pumps on the throttle.
    3. Generator engine fired up after giving it a shot of gas down the carb.
    4. Port engine cranked and cranked and cranked, .... Finally caught and would run at 3,000 RPM but spit and backfire through the carb at anything lower.
    5. This weekend I went to work on it.
    6. Checked the two year old distributor cap, rotor button, wires, .... Cleaned it all even though it looked good. Cranked, but no start.
    7. Swapped the carbs between the engines. Starboard engine started fine, port engine cranked, but no start.
    8. Swapped the Thunderbolt IV ignition amplifiers between the engines. Starboard engine started fine, port engine cranked, but no start.
    9. Swapped the distributor sensor between the engines. The port one looked like crap. But, you guessed it - starboard engine started fine, port engine cranked, but no start.
    10. I am kind of hacked off at this point, so I pull all the plugs on the port engine and check the compression. 165 to 175 on all cylinders.
    11. I test all of the two year old plugs by using their respective wires and grounding them on the block. I had three dead NGK plugs - no spark at all. Replaced them with an old set of AC Delcos and the port engine started right up.

    I would have never thought to check the plugs. They looked fine. I just did it as I was putting them back in after the compression test.

    So, I have 16 new AC MR43T plugs on the way and already have my 34 year old Sun timing light loaded in the basket to go to the boat this weekend. I am going to replace them all, check the timing, and enjoy my two well running engines.

  2. #2
    Senior Member 42gibson's Avatar
    Join Date
    Nov 2010
    i had problems with ngk plugs in snowmobiles and my old MGB. like you, i always use ac delco in the boat.thats what they came with on all of mine.
    Last edited by 42gibson; 04-15-2013 at 12:17 PM.
    44 gibson executive
    on the muskingum river & ohio river

  3. #3
    Senior Member Endurance's Avatar
    Join Date
    Nov 2005
    Lake Powell, Utah
    I'm glad it came together for you. I remember a time when it was almost always something other than spark plugs. Any more, everything else has gotten so good that it seems like spark plugs seem to be the problem more and more often.

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