1969 Mercury 4 hp Model 40

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  • archangel


    Replies: 118
    Topics: 21
    #4557

    I have a 1969 Mercury Model 40, 4 hp that I got running tickety boo last summer. I rebuilt the carb, rebuilt the fuel pump, replaced the fuel hoses, replaced the impeller and a couple of lower unit oil seals and it ran like a new motor. Last week I took it out for the first ride of the year for that particular motor and it was running fine at WOT for 10 minutes or so. Then I throttled down to idle for 30 seconds or so to adjust some gear in the boat. When I cranked the throttle again it wouldn’t run at full RPMs. I limped back to my dock and I have now tried the motor in a barrel.

    What I found running it in the barrel is:

    The plug was fouled. Cleaned it and reinstalled to start the motor.

    The motor starts and runs relatively normally up to 1/2 or 2/3 throttle, then sort of bogs. I say relatively because there does seem to be a slight surging to the way it runs (the gas line ball is hard at all times and squeezing it does not impact the way the motor runs, by the way). My hunch is that there is an ignition problem of some sort, as it smokes when it bogs, too. It seems like the ignition is not correctly advancing with the carb throttle after 1/2 to 2/3 throttle. The smoking, rich, and unburned gas is the cause of the plug fouling I assume. I haven’t yet had the flywheel off, but that is my next step once my puller is returned from being lent out.

    Does anyone have any insight for me? Any tips on what direction I show proceed to diagnose this?


    billw

    US Member - 2 Years
    Replies: 2071
    Topics: 66
    #38721

    Make sure the high speed jet in the carb didn’t come loose. I have seen that happen more than once, which COULD cause your exact symptom….

    Long live American manufacturing!


    archangel


    Replies: 118
    Topics: 21
    #38725

    I’ll check that. Thanks!


    westwind

    Canada Member - 1 Year (includes $3 online payment fee)
    Replies: 374
    Topics: 10
    #38728

    New sparkplug maybe?


    archangel


    Replies: 118
    Topics: 21
    #38732

    I’ll need to pick a plug up tomorrow and give that a try, too. Thanks, Ron.


    billw

    US Member - 2 Years
    Replies: 2071
    Topics: 66
    #38744

    Thinking about this some more, can you SEE the throttle valve open up at full magneto advance? You should be able to see the throttle cam follower working on the cam and it should go nearly to the end of the cam.

    If my first two ideas are a bust, then maybe the points closed up, from wear, during the run and need to be reset. That can do weird things, too. My money is still on the jet coming loose, though…..

    Long live American manufacturing!

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