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cant get 80's merc 75 hp to idle correctly... to high and dies when put in gear

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  • cant get 80's merc 75 hp to idle correctly... to high and dies when put in gear

    I have been using this Forum for years and thank everyone for the great input that is given here. This is probably my first post because i usually find my answers just searching and reading but I am at a loss
    I have a 84 to 86 4cyl 75 hp merc. serial number 0B107213, compression 115. 120.120, 120. Carbs cleaned 3 times soaked overnight twice, All passages cleared with wire. carb cleaner and compressed air. float level set to spec no leaks when ball is pumped.Good hole shot great high speed 5500 rpms smooth running. new plugs spark is blue and jumps a 7/16 gap very sharply on all cly

    all adjustments done in water

    I cant get the Idle correct. timing is set to 7 to 9 BTDC just as cam is touching per manual I am at 12 to 1300 rpms. If I set timing below that (Closer to 0 ) to reduce idle to anywhere below 1000 the engine dies when put in gear. I can adjust carbs richer (very rich) and bring down the idle with the timing set per manual . i can then get it into gear by playing with the throttle as soon as i get it into gear. I can then adjust carbs for best results and use the idle adjustment to get everything just right boat runs good out of the hole and wot with no problems but idle in gear is not great or very consistent and then when I take out of gear Im back to 1500 rpms. I have started from scratch a number of times with same results
    Any help would be greatly appreciated

  • #2
    You donít set the timing for idle. You set the timing for maximum advance, then doing the link and sync set the pickup point((thatís where the linkage first makes contact with the carbs to start opening them) Your idle speed is determined by how far the spark is retarded by the throttle linkage, and that can vary by different props

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    • #3
      Thank you for the reply. I am sorry after re reading my post I did not do a very good job so let me try again

      Boat in water in gear, cam (pick up point) just touching, held throttle steady at that point and set the primary timing 7 to 9 btdc. locked it in. Used idle adjustment to adjust Idle down to about 700rpms. then adjusted carbs by going lean until stumble then back 1/8 to 1/4 turn (both carbs) re adjusted idle, re adjusted carbs.. several times through this. once done motor idles ok at 700 rpms in gear. Out of hole good and top end is great. But. once I take it out of gear it goes to 12 to 1300 rpms. if I adjust idle down out of gear to 1000 rpms when i put in gear it dies. the other stuff i rambled about was what i had tried to make it work so forget that but this procedure was done 4 to 5 times with same results.

      thanks for your input

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      • #4
        Is it firing on all cylinders?

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        • #5
          yes pulled each spark plug wire and motor stumbled with each.

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          • #6
            Are both carbs fully closed at idle?

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            • #7
              yes both carbs fully closed at idle and not against their stops at wot

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              • #8
                When you are adjusting the idle speed, are you releasing the barrel bolt on the throttle cable from its anchor point, and after setting the idle speed, adjusting it so there is just a small preload against the idle stop?

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                • #9
                  Yes and after I have the barrel back in place and locked in I have verified it rest on the idle stop when I return throttle from in gear to neutral and no mechanical binding exist In the linkage

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                  • #10
                    Sign up today
                    Thanks for your help so far Jimmbo. if there is any thing else you or anyone else can think of I would love to hear it.

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