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  1. #1

    Default 96 Mercury 40hp C40 fuel system overhaul

    This is the two stroke, carbed prop model. I've been using this motor one week a year for the last few years. I only put 90+ octane E0 through it but I can't speak for previous owners.

    Anyway, last year it didn't want to start at the start of vacation week and the mechanic got it going by squeezing the fuel bulb really hard. To avoid it again, he advised we run the engine fuel system dry before storing it, which we did.

    This year it won't start again. One mechanic suggested the carb needles are sticking - but it seems unlikely that both carbs are stuck equally.

    So rather than get deep into diagnostics and kick the reliability can further down the road, I'm inclined to overhaul the fuel system. It's 20+ years old already. Fresh fuel pump kit including lines, a fuel filter, check valve, new tank to motor fuel line assembly, and carb rebuilds. The tank fuel line is only two years old but I might replace it anyway and keep one as a spare.

    So I have a few questions:
    - Do the Merc carb rebuild kits contain a float and needle? I'm told they do not, and I am advised I should buy them separately.
    - If the carb needle(s) are stuck, should I just take the boat out for a drive on the trailer and shake them loose?
    - Anything else I should look into?

  2. #2
    Join Date
    Apr 2016
    Location
    barnegat NJ
    Posts
    3,141

    Default Re: 96 Mercury 40hp C40 fuel system overhaul

    pull a bowl off one of the carbs. see if fuel is there

  3. #3

    Default Re: 96 Mercury 40hp C40 fuel system overhaul

    The upper definitely has fuel in the bowl. Did not remove the lower.

    Pulled the main fuel line to the upper. Got plenty of fuel when I pumped the primer bulb. I have the fuel pump, starter, and oil tank off, so I'm pretty much ready to replace all of this once my parts arrive.

  4. #4
    Join Date
    Jun 2013
    Location
    Duluth Mn
    Posts
    618

    Default Re: 96 Mercury 40hp C40 fuel system overhaul

    Just did a Mercury carbs overhaul, and the float and needle are not included. You will need these.......sounds like ethanol messed up the needle tips causing them to become soft and sticky.

  5. #5

    Default Re: 96 Mercury 40hp C40 fuel system overhaul

    I'm in it now, the 9650 carb kits contain a needle but no float. The floats on mine look good. The new needles are solid steel.

    Bought the full Merc fuel pump kit too. It has almost all new hoses - if you buy that kit, it won't include the run of fuel line between the carbs. The local shop had some.

  6. #6
    Join Date
    Jun 2013
    Location
    Duluth Mn
    Posts
    618

    Default Re: 96 Mercury 40hp C40 fuel system overhaul

    Do the original needles have viton tips? The 1954 Johnson 5.5 hp that I rebuilt last night has an original solid stainless needle. It was perfect condition, so I reused it. Maybe the engineers are looking to the past in order to make this new stuff more reliable. The floats back then are cork, I reused that too. Just did a 17.5 hp riding mower carb......what a joke, ethanol dissolved this tiny o-ring which then allows the main jet to fall out of its place and bounce around in the chamber. The plastic float is not even adjustable. The floats made of plastic can fail too, Just give it a good inspection and if in doubt, replace. I have a box of new ones here but they are already about 10 years old. Plastic, of course, has a shelf life.......it slowly deteriorates in time.

  7. #7

    Default Re: 96 Mercury 40hp C40 fuel system overhaul

    Quote Originally Posted by timguy View Post
    Do the original needles have viton tips? The 1954 Johnson 5.5 hp that I rebuilt last night has an original solid stainless needle. It was perfect condition, so I reused it. Maybe the engineers are looking to the past in order to make this new stuff more reliable. The floats back then are cork, I reused that too. Just did a 17.5 hp riding mower carb......what a joke, ethanol dissolved this tiny o-ring which then allows the main jet to fall out of its place and bounce around in the chamber. The plastic float is not even adjustable. The floats made of plastic can fail too, Just give it a good inspection and if in doubt, replace. I have a box of new ones here but they are already about 10 years old. Plastic, of course, has a shelf life.......it slowly deteriorates in time.
    Original needles had viton tips.Anyone happen to know why this motor used to start very easily when it was warm, and a few years later it needs fast idle to get running, basically in all conditions?Thanks,David

  8. #8
    Join Date
    Jun 2013
    Location
    Duluth Mn
    Posts
    618

    Default Re: 96 Mercury 40hp C40 fuel system overhaul

    Maybe the compression is lower than before. Another reason could be that it really needs this carb overhaul, with fuel pump kits.

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