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  1. #1

    Default Korky in MN - Mercruiser miss

    1986 (approx) Four Winns
    Mercruiser MCM 140 #A676736 (from engine placard), 4 cylinder
    3.0L (assumed)

    PROBLEM
    After cruising for a short time, the engine develops a “hardmiss” which is persistent once the engine is hot. The tach confirms RPMfluctuation. Must throttle back and can only idle after the miss hasdeveloped. Once the engine is turned off and has cooled, then it’spossible to cruise again for a short time before the engine warms up and the“miss” returns.

    Ignition problem? Fuel problem?

    The following were changed May, 2017:
    34235 breaker
    33662 condenser
    18-5418 rotor
    18-5385 cap
    RV9YC plugs

    Ignition?
    Replace the ignition coil and spark plug wires?

    Fuel?
    Replace fuel filter in fuel pump? Replace any otherfilters or screens in the fuel line and carburetor? Gaskets?

    I don't have a lot of high tech testingequipment. Can you help me out with an educated guess on this problem?

  2. #2
    Join Date
    Jun 2006
    Location
    Spotswood, NJ, USA
    Posts
    3,813

    Default Re: Korky in MN - Mercruiser miss

    what do the spark plug tips look like ?

  3. #3

    Default Re: Korky in MN - Mercruiser miss

    Excellent. New plugs installed. Problem continued. Re-installed old plugs. Problem continued.
    Dont think the plugs are suspect.

    I plan to change the ignition coil and plug wires next.

    Thoughts?

  4. #4
    Join Date
    Jun 2006
    Location
    Spotswood, NJ, USA
    Posts
    3,813

    Default Re: Korky in MN - Mercruiser miss

    What color are the tips? They can tell if its a lean condition, or a too rich condition, or just right condition.
    Blister white could mean water ingestion . If color is correct then suspect the wires or the coil

  5. #5

    Default Re: Korky in MN - Mercruiser miss

    Quote Originally Posted by Bt Doctur View Post
    What color are the tips? They can tell if its a lean condition, or a too rich condition, or just right condition.
    Blister white could mean water ingestion . If color is correct then suspect the wires or the coil
    THANK YOU. That's kinda what I figured to do next.

  6. #6
    Join Date
    Jul 2008
    Location
    Northboro, Ma, USA
    Posts
    4,793

    Default Re: Korky in MN - Mercruiser miss

    Also I would remove distributor cap and see if shaft moves side to side.

    One issue of this design (also the inline six cylinder) is the distributor shaft bushings wear egg shaped due to distributor being on angle.

    Also confirm mechanical advance mechanism under point plate is moving freely and fully. Not all rusted or frozen.
    Jack
    Northborough Ma
    Helping others sometimes with an attitude....haha..

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