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1985 70hp problem

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  • 1985 70hp problem

    Got one that I just can not figure out, need help, please. I have done a lot of work to this and spent a ton of money with no positive reactions.
    it starts fine, it runs only on the top 2 cylinders, the bottom cylinder will not fire, I have REPLACED the carburetors with new, all coils, base timer, reed valves, plugs, fuel pump, fuel lines, fuel tank.

    Yes this was all done at a shop and 1400 dollars later and this thing still does not run and a huge dumb look of I don't know what is wrong with this motor.
    I had then ran out of money...so it has sat since last Sept.... Im trying to get this going again now. so Im trying to do this my self.

    I have taken the carbs and swap them from cylinder to cylinder to see if it was a fuel issue, and see if the problem would move to another cylinder, it did not move.

    Compression on all three cylinders is 130 to 138.
    I can pull the plug wire off of 1 and have an rpm drop, pull off 2 and it will kill the motor.
    pull off 3 and it does nothing. So I know this cylinder is not doing anything

    I have spark, have checked it with a spark gap gauge, plenty of nice blue arc, all three cylinders.



    questions I have first is, can the air/fuel mixture be getting pulled into cylinder 2 in the lower crankcase, starving cylinder 3? And that is why when I pull the plug on cylinder 2 it will kill the engine? Cylinder 2 seems to be the power cylinder.

    If anybody can give me some help please, The only thing that has not been changed on this motor is the block, crank, and pistons.
    Everything else has been replaced, and nobody can find what is wrong with this.

    Thanks in advance.

    If this is happening, Why?

  • #2
    Do you have a timing light you can clip on to the plug wires, just for seeing if any difference in flashes? If bottom/ all cylinders are firing the same, I'd look at what idle mix screws are set at for each carb. Count the turns as you close each till lightly seated. Write it down for each carb. also, look at each plug and note if wet, color- black/brown or lighter color for each of those carbs/cylinders.

    Post back when able. Ill check back later. Good luck

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