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Why a Primer Bulb?

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  • Why a Primer Bulb?

    I have a basic theory question of why my 175 Sport Jet (or most outboards for that matter) need a primer bulb. My previous boat (merc 140 i/o) didn't have one, but seems like all outboards do.Why can't they just get rid of it, since it seems like a source of a lot of problems?Don't the floats in the carb hold enough fuel to get the engine running, if it's been sitting for a while? Thanks,Tom

  • #2
    Re: Why a Primer Bulb?

    your previous boat was an inboard..... diefferent animal, chief


    • #3
      Re: Why a Primer Bulb?

      Part of the problem is that most outboards are up above the fuel tanks. I would guess that most inboards are more or less at the same level and it would be easier for the fuel pump to get a prime going. I would also guess that in addition to normal evaporation, tilting the outboard probably dumps some of the gas out of the carbs.


      • #4
        Re: Why a Primer Bulb?

        Thanks Phatmanmike.I guess my question could have read, why do outboards (and jet drives) need a primer bulb and inboards (and SUVs and cars) don't?It can't be because of gravity, since most cars and inboards I know have the gas tank below the carbs. I know most new cars have the fuel pump in the tank, but older cars didn't, and yet they didn't have a primer bulb either. Regards,Tom


        • #5
          Re: Why a Primer Bulb?

          jimd, Our postings crossed in the web. Good point though about the outboard tilting - I hadn't thought of that. I'm also wondering if there is a difference in fuel pumps since 2 cycle motors don't have a cam to drive a displacement pump. It's been a while since I've worked on 2 strokes - What drives their fuel pumps and could that be the reason?Cheers,Tom


          • #6
            Re: Why a Primer Bulb?

            The reason is this:Car engines and inboard boat engines have much larger fuel pumps, they can fill the carbs in just a few cranks of the engine. Outboards on the other hand have very small fuel pumps and would need much longer cranking times to fill up to six carbs.. Pumping the bulb saves your starter, battery and time.


            • #7
              Re: Why a Primer Bulb?

              Thanks, that makes sense to me!Tom


              • #8
                Re: Why a Primer Bulb?

                Moving to General Outboard.


                • #9
                  Re: Why a Primer Bulb?

                  What?? Your car doesn't have a primer bulb! Automotive fuel pumps (electric or mechanical) are far more powerful than the pulse type on outboards. Plus, as mentioned, it's there to save batteries and make for quick starts.


                  • #10
                    Re: Why a Primer Bulb?

                    Tbsmith, the outboard fuel pump uses the alternating vacuum and pressure pulse in the crankcase/transfer port to suck and push the gas using a rubber diaphragm and a couple of one-way check valves.