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1970 mercury thunderbolt 65 hp outboard

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  • 1970 mercury thunderbolt 65 hp outboard

    I just purchased first boat, it has a 1970 Mercury Thunderbolt 65 hp on it. It takes 5 to 10 key turns to start it and ran ok if not a bit rough and then just stalled. Pumped fuel bulb cranked a few times and would restate for a bit and then die. Seems to run a bit linger on full throttle vs low speed. Low speed engine dies pretty quick. When engine does restart you can see a bit of gas floating on the water.
    With cover off, the carbs look damp at the air intake - it has two carbs joined by plastic rod.
    Could it be dirty carbs - if yes how to clean and when people say turn the screw to adjust mixture, which screw?
    The carb on top has a plastic piece that looks adjustable to the throttle, the plastic link rod has adjustment between the two carbs and then there's a screw to adjust air to fuel ratio?
    Does anyone know what to do to set etc
    How to check spark plug for lean vs not mix etc
    How to check have power to each spark plug etc

    Very new to boat motors

  • #2
    Re: 1970 mercury thunderbolt 65 hp outboard

    Before messing with much more I'd really suggest locating a manual. You have a number of tasks potentially before you, including but not limited to a carb cleaning, link, and timing adjustment. Messing these up can result in fuel starvation which can cook your new outboard.


    • #3
      Re: 1970 mercury thunderbolt 65 hp outboard

      It probably warrants cleaning and rebuilding the carbs and fuel pumps. The carb kits are only $6 and the fuel pump kits are $7. Be VERY careful removing the carbs as that plastic link between the carbs will break very easy. It is still available, just had to replace mine, about $16. Once you remove those blue inlet pieces on the carbs, you will never get them to stop leaking unless you replace those too, again still available and only about $6 each. Being dome shaped and tight foir years, the dome deforms and keeps the gasket from sealing.

      Replace your float needles with new ethanol compliant needles, they will stick closed and run you on 1 or no carbs, the ethanol swells the rubber tips. I would suggest pulling your fuel line and running the motor out of fuel after each use, this ethanol really is bad stuff. The carbs are pretty simplistic but there are some parts that are a little tougher. If you do a "complete" rebuild, there are plugs that need to be broken out and new ones glued back in, same with the idle circuitry. I was fortunate and didn't need to do that with mine. If you have ever worked on motorcycle carbs, you'll be right at home.

      Here is some advice that I learned when I did mine.. (1) remove the power plug and then the starter first, you will need the room (2) you have to carefully twist the choke linkage to get it off, but it is pretty tough (3) always remove the top carb first (4) careful with the plastic piece between the carbs (5) replace the inlet blue pieces, there is nothing that you can do to keep them from leaking (6) try multiple wrenches to get the nut off of the bettom carb closest to the starter, not all wrenches can get in there
      '78 Sylvan Sea Master
      '77 Merc 500, 4 cylinder Thunderbolt
      55# Minn Kota V2 Powerdrive

      Three-fourths of the Earth's surface is water, and one-fourth is land. It is quite clear that the good Lord intended us to spend triple the amount of time on the water than taking care of the lawn.


      • #4
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        Re: 1970 mercury thunderbolt 65 hp outboard

        Thank you for info
        Did get marina to check out they said cyl comp good and spark ok. They changed impeller and pump which I saw was melted got so hot. They did not do the carbs will do myself or wife will kill me ref another big bill
        Where can I get original mercury manual and the carb parts you mentioned

        Thanks again for writing back boat now runs but still stalls enough to be a nuisance