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  1. #1

    Default 1976 merc 20 hp motor turns easily till I put in the spark plugs

    Got a 1976 model 200, 20 hp not running. All the wiring was burned up so I replaced the switch module and fixed the trigger. Now I had spark. Figured that was the problem so I reassembled enough to see if I could get it to run. Motor turns freely until I put in the spark plugs. Then I can barely pull the starter rope. feels like 200 pound of compression. The bottom gear case has been removed to replace the water pump, so that can't be the problem.
    Anyone have some thoughts? Could there be a compression release to make using the pull starter easier that's not working?

  2. #2
    Join Date
    Nov 2011
    Location
    tennessee
    Posts
    4,767

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    No compression release
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  3. #3
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    16,976

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    Does it also do this with the plug wires off the plugs? If not, I suspect the timing is way advanced and is fighting you.

    Finally, I had one that did this exact same thing to me. Its problem was bad coils. Not sure why that was, but new coils fixed it.

    Jeff

  4. #4

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    plug wires are off.

  5. #5
    Join Date
    Sep 2010
    Posts
    20,700

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    Where you able to start it OK last year ?-----

  6. #6

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    bought it not running. bottom was off and when I saw the burned electronics I figured that was why it was taken out of service. I was told that it hadn't been run for a while.

  7. #7

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    Check the compression to see what it is, I have a '68 20 with the padded cylinders that I'm working on and its 135 psi on each. If the compression checks out, just put a bigger boy on the end of the starter rope

  8. #8

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    The recommended plugs are surface gap L78V plugs if you put regular plugs in it the pistons could hit the side electrode.

  9. #9

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    Quote Originally Posted by fitz73222 View Post
    Check the compression to see what it is, I have a '68 20 with the padded cylinders that I'm working on and its 135 psi on each. If the compression checks out, just put a bigger boy on the end of the starter rope
    A bigger boy is not an option. Few people could spin this fast enough to start it. The motor I'm using now is an Evinrude 15 hp 2 stroke. I have no trouble pulling that.
    Could there be too much carbon build up on the head? That would make push up compression.

  10. #10

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    Quote Originally Posted by Quicksilver View Post
    The recommended plugs are surface gap L78V plugs if you put regular plugs in it the pistons could hit the side electrode.
    the plugs are L78V. I thought of that and the first thing I tried was screwing in the plugs only half way. No difference.

  11. #11

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    Can you turn the engine over by rotating the flywheel by hand with the plugs in? It would take a huge amount of carbon on the piston tops to make it not turn over. I'm wondering if the starter is binding somehow under load.

  12. #12
    Join Date
    Sep 2010
    Posts
    20,700

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    We will not get anywhere with guessing.-------Answers are needed.------What are the compression numbers ?----Any sign that the motor was taken apart and not assembled correctly ?----Could be bad bearings binding.----Remove bypass cover and look at pistons and rings.

  13. #13

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    Motor does turn over. Can feel compression build and then release. Feels almost normal with top plug in place and bottom plug removed. Still hard though. With bottom plug installed and top plug removed it is almost as hard to pull as when both plugs are in place. Looks like I have some work to do. Could stuck reed valves do this?

  14. #14
    Join Date
    Sep 2010
    Posts
    20,700

    Default Re: 1976 merc 20 hp motor turns easily till I put in the spark plugs

    A short answer ----NO

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