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Stumped -- Fuel Pump Problem, 25 HP Merc w/integral pump

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  • Stumped -- Fuel Pump Problem, 25 HP Merc w/integral pump

    25 HP Merc outboard, circa 1989, non-oil injected, with Walbro WMC carb (integral fuel pump), "external" reed block design.Engine runs fine if you keep pushing fuel to the carb via the priming bulb, but runs out of fuel otherwise.Here's what's been done so far:- complete fuel line replaced, including fittings and primer bulb; tank pickup and vent checked. Fuel filter checked -- clean and clear.- carb cleaned and carefully rebuilt, Merc rebuild kit, including fuel pump diaphram. Check valves look fine. No varnish or other crud observed.- Pulled reed block. All reeds undamaged, lightly seated against block. Replaced U-shaped reed block seal, which was pretty crudded up with what I guess was varnish. Replaced reed block-to-block, "carb adapter plate" and carb gaskets- Compression check: 120 psig in both cylinders- If I pull the fuel line between the filter and the pump, I feel no vacuum. I understand the pump should draw enough suction to pull fuel up two feet, so I should feel some suction, I surmise!Engine runs like a top -- trolled all day by squeezing the primer bulb every 1-2 minutes!- If I remove the carb, and blow on the air port at the downstream flange end of the carb, which provides the pressure pulses to move the pump diaphram, I can hear the diaphram doing its thing.Seems to me that the pressure in the intake cavity between the reed block and carb is not fluctuating like it needs to, but I'm stumped as to why. Book says reeds shouldn't be seated on reed plate, but stood off up no more than .007". Mine are just touching. Also, there are two small vacuum-like hoses that come off the bottom of the "carb adapter plate". Wondering if these might be players, though I think I read that these are just for cycling unvaporized fuel back into the cylinders.This has me stumped, but I think Ancient Mariner and others may have some ideas. I'd be grateful for any advice/experience. I'm sure someone out there has been through this before!

  • #2
    Re: Stumped -- Fuel Pump Problem, 25 HP Merc w/integral pump

    You say "Check valves look fine" Replace them and give it a try.


    • #3
      Re: Stumped -- Fuel Pump Problem, 25 HP Merc w/integral pump

      I had the same problem with an Evinrude, I rebuilt the carb and fuel pump and still had the same problem, I was having to squeeze the bulb to keep it running. Finally I said to heck with it and bought a new fuel pump and all my problems went away.


      • #4
        Re: Stumped -- Fuel Pump Problem, 25 HP Merc w/integral pump

        It could be on the exaust end too. If the exaust ports are clogged, it cannot properly expell what it draws-in, thus not drawing anymore in.But also, the two 'diaphram-gaskets' in this carb, (one poly-fiber,of some kind ,the other rubber),when transposed will not pump fuel,even tho they are shaped identicaly. So check that out first (exaust port plates are like head gaskets, and torque/sequnce specs are strict, and why go there, if you do not need to).
        "Did you hear about the guy whose whole left side was cut off? He's all
        right now."


        • #5
          Re: Stumped -- Fuel Pump Problem, 25 HP Merc w/integral pump

          Katoom, look on the outside cover of the fuel pump...is there a 1/8" hole in the cover? early wmc carbs had a service bulletin on this problem, dealing with poor fuel delivery...you had to drill the hole.....


          • #6
            Re: Stumped -- Fuel Pump Problem, 25 HP Merc w/integral pump

            more about fuel pump and check valves http://marinemechanic.com/2000/images/ob_fuelpump.htm


            • #7
              Re: Stumped -- Fuel Pump Problem, 25 HP Merc w/integral pump

              Thanks to all for the speedy replies and ideas...YEPBLAZE AND BEAR, given your inputs, I'm thinking the check valves are a good possibility. They do look unaltered and well-seated, however. I just verified that they aren't sticking.ODDD1, I just checked to verify that there is, indeed, a thru-hole in the fuel pump cover. I use a small drill bit, and was able to touch the diaphram.NATHANY, perhaps unlike your Evinrude unit, the fuel pump is a sub-component of the carb, and cannot be replaced as a unit. I purchased the rebuild kit, which has gaskets and diaphram, but no check valve plate assy. That is purchased separately.12FOOTER, two things --- This Walbro carb only has one gasket-like diaphram (as initially installed, and as provided with the rebuild kit). The check valves are part of a plastic plate which fits between the carb and the diaphram. This part and the diaphram are non-reversible.- I'm thinking the exhaust plugging is also a good theory, as this motor has spent most of its recent life trolling at low rpm, with inferior gas, at 7000ft elevation. These factors would tend to form carbon deposits in the exhaust. The motor does troll down nicely, however, when I keep honking on the primer bulb.I'd like to get more information on the exhaust port plugging. Also, does anyone know what the two lines in the "carb adapter plate" (sandwiched between the carb and the reed plate) do? They are about 3/16" OD, and connect to the block near the cylinder ports. Are they just for pushing unvaporized fuel (which collects in front of the reed plate) up into the cylinders to be burned?Wondering if I can pull the carb, and test for an alternating pressure (which drives the fuel pump diaphram) as I crank the engine. I imagine that "intake manifold" pressure doesn't vary a great deal from stroke to stroke, but it need to be enough to tickle the diaphram...I really appreciate all the input. I'm sure I'll figure this out with your help!


              • #8
                Re: Stumped -- Fuel Pump Problem, 25 HP Merc w/integral pump

                Mystery solved --I can't believe I didn't catch this, but the carb gasket the dealer gave me didn't have the hole in it through which the diaphram gets its driving pressure! There was a hole in the gasket, but it was shifted up about 1/4". So, of course the fuel pump wasn't pumpin!You don't know how many hours I've burned scratching my head on this. I really appreciate all the replies; got me in the troubleshooting mood, and finally...!Thanks to all for your input. Hope the thread may someday help someone else.The motor purrs like a kitten now.


                • #9
                  Sign up today
                  Re: Stumped -- Fuel Pump Problem, 25 HP Merc w/integral pump

                  I know it's been a long time since katoom began this thread. Katoom posted his final entry and his resolution on this subject on Feb 2, 2003. Here it is July 2011, and Mercury still delivers the same problem gasket in its carb kit!

                  I have to thank katoom for his posting of the solution, because after finding this posting, it was the first thing I checked on my motor. It was the problem in my case as well.

                  I just had the same, identical problem with a carb rebuild on a 2001 25 hp Mercury. The damn hole in the carb gasket doesn't line up with the hole between the motor and the carb, and as a result the carb would not pump fuel. This gasket set was direct from the Mercury dealer, and I used the serial and model info off the motor when I ordered them. The carb repair kit Mercury part number is 1395-9761--1.

                  So, if you do a carb overhaul on a Walbro WMC carb on a 25 HP merc, compare the old and new gaskets that go between the carb and and the motor. Make sure they are the same. If they are not, you can probably cut a hole in the appropriate place to correct the matter. If you don't the motor will run fine until it empties the carb bowl of gas. Then, either you pump the bulb on the fuel line, or the motor stalls............

                  Last edited by skinorth; July 6th, 2011, 09:41 PM. Reason: correction