questions about fuel pump rebuild

RRitt

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Mar 30, 2006
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after reading comments on this forum I decided to replace the fuel pump diaphram. I had never done this and the port engine is hard to restart if I don't pump the bulb. After reading comments I figured I must be on borrowed time and did them both.<br /><br />the old diaphrams had definite bulging. the new ones were flat. after replacement my fuel pumping at idle is not very strong. Is this normal? <br /><br />The local mercury dealer was no help. When I asked him what parts I needed he said that he was "just a parts counter, you have to tell me what you want". So I said I wanted a kit to rebuild fuel pump. He said there was no such thing. I said I needed whatever parts would constitute such a kit. Then he repeated his "only a parts counter" response.<br /><br />So ... I walked out with two diaphrams and two gaskets. I won't darken their doorstep again. Why should I pay him full list instead of ordering off iboats? Anyway, I didn't do anything other than open it up and put in a new diaphram/gasket. Did I leave out a part? Did I forget to check or replace something? Is it normal for a new diaphram to deliver less fuel until it "loosens up"?
 

roscoe

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Oct 30, 2002
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Re: questions about fuel pump rebuild

No, this is not normal.<br /><br />What motor do you have?<br /><br />If you have the square fuel pump, the kit part number is 21-857005A1 <br /><br />
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<br /><br /><br /> <br /><br />If you have the older "oval" two stage pump, there is no kit from Mercury, and you need to order the individual parts.<br /><br /><br />
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RRitt

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Re: questions about fuel pump rebuild

roscoe. thanks for fast response. I have the oval type. twin '88 85hp with 25" shaft. <br /><br />just checked compression since manual says that fuel pump runs off #1. 100/100/100 and 98/101/100. seems oddly low, even for dead cold engine. when I checked it last year its was 142 to 137.
 

RRitt

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Re: questions about fuel pump rebuild

I'm looking at your picture.<br />I replaced item #1 and #16 only.<br />did not even open or check #2-6<br />the big piece (#20) stayed on engine along with gaskets 19 and 23.
 

eurolarva

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Jun 24, 2003
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4,182
Re: questions about fuel pump rebuild

The diaphram and the gasket should go on without any problems. If there is a small hole in the diaphram it might give you grief. NAPA and Iboats sells the rebuild kit part number 18-7806 for about 10 bucks a kit. If you take it apart again check the fuel valves. There should be three. One has a bolt holding it in place and the other two are press fit into the fuel pump. Remove the one that is held in place by the screw and verify the gasket is intact and that the spring on the valve works freely. The gaskets that come with the kit may not fit. Those valves cost about 5 bucks a piece and the correct gaskets are 50 cents each. The valve is part number 21-F177330 and the gasket is 27-F84792. Your nasty merc dealer should be able to get you these parts.
 

roscoe

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Re: questions about fuel pump rebuild

I can't imagine the compression dropped that much and so evenly across both engines.<br />Perhaps your gauge is sticking, or you didn't have the throttle opened up.<br /><br />???
 

RRitt

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Re: questions about fuel pump rebuild

oops. you're right (again). didn't have the throttle opened at all. <br /><br />thanks eurolava. if I understand you then you are saying i should open it back up and check spring on valve. maybe I should just get valve and gasket in advance and plan on replacing them? what about #19. do I need to replace it?
 

eurolarva

Rear Admiral
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Jun 24, 2003
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4,182
Re: questions about fuel pump rebuild

Part 19 is just a spacer plate which should be okay. If the pumps are working now I would not take them apart. If you feel you are still getting a restricted fuel flow it would not hurt to replace the fuel valves. The press fit ones are a bear to get out. One is impossible without damaging the valve so if you are going to remove them have a few on hand.
 
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