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Mercury 500 50 HP Power Problem

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  • Mercury 500 50 HP Power Problem

    Hi,I have a Mercury 500 50 HP 1976-1979 model with the symptons as follows:Starts OK, runs OK for anywhere from 5mins to 20 mins then the motor cuts out while running fast (ie. 1/2 to full throttle). I can then start the motor and it idles OK but cannot apply any throttle beyond just over idle otherwise it cuts out. After leaving it a while it seems to be OK then can apply more throttle and continues to run OK until the symptom appears again.Any help appreciated and thanks in advance.


  • #2
    Re: Mercury 500 50 HP Power Problem

    Sounds like fuel starvation. Have a passenger squeeze the Primer bulb during this condition to force the extra fuel reqd. to the pump, if that works then consider the following:Try installing a short piece of clear pvc line between the fuel pump and your main fuel line, this will allow you to monitor the fuel flow at high speed, which is of course when you need the most fuel. Do this on the water, unless you have a test wheel and a large tank. Its very hard on these motors to be reved high with out a load placed on the engine. By doing the above, you are confirming adequate fuel to the carbs. It could be that you have a air leak somewhere between the tank and the inlet side of the fuel pump (which would give more air than fuel) or your fuel pump could be worn out. I wonder if this is the original pump? You may want to rebuild it if it is 8 years or older regardless.Yes, you can have air leakage at a fuel connection and still not have a fuel leak. Fuel is more dense than air. Good Luck, Mike

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    • #3
      Re: Mercury 500 50 HP Power Problem

      I concur with Evin300. But I wanted to add while underway ,with motor giving you grief, remove the gas cap. If it takes off like a drag boat, it's a venting problem. The tank must get air to replace the fuel the motor burns.
      "Did you hear about the guy whose whole left side was cut off? He's all
      right now."

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      • #4
        Re: Mercury 500 50 HP Power Problem

        Good point 12Footer, Could it also be a faulty check valve in the Bulb Primer? Or what if valve doesnt work at all? stuck open?

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        • #5
          Re: Mercury 500 50 HP Power Problem

          If the check ball doesn't close,the bulb wont pump up hard, Mike.
          "Did you hear about the guy whose whole left side was cut off? He's all
          right now."

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