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Old 09-21-2018, 05:14 AM   #1
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How does an outboard fuel pump work?

I didn't know where else to put this but, I started watching this video and started wondering how exactly they work.

I understand that its a diaphragm pump, so the in and out motion of the spring causes the pumping actions, but what makes the spring move? There's nothing electrical on these small pumps, which means that its got to be activated by some other mechanical means. I'm going to assume that this is somehow tied into the vacuum created by the cylinders, is that correct? Cylinder comes down, creates pressure, compresses the spring moving the diaphragm?

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Old 09-21-2018, 07:01 AM   #2
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The pump you show is like most outboards. They utilize the pumping pressure of one of the pistons to to work the pump. The spring is there just so the diaphram doesnt get stuck against the pumps port.


So when the piston goes up in the cylinder it creates a vacuum in the crankcase and that vacuum pulls the pumps diaphram in which in turn sucks fuel from the fuel line into the pump chamber.

Then as the piston goes back down it pressurizes the crankcase which pushes the diaphram back out (same direction the spring pushes). When the diaphram is pushed out by the crankcase pressure the fuel is then pushed out a fuel line and up to the carb(s) float bowl.


simple and no real moving parts.
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Old 09-21-2018, 10:03 AM   #3
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Quote:
Originally Posted by what_barnacles View Post
The pump you show is like most outboards. They utilize the pumping pressure of one of the pistons to to work the pump. The spring is there just so the diaphram doesnt get stuck against the pumps port.


So when the piston goes up in the cylinder it creates a vacuum in the crankcase and that vacuum pulls the pumps diaphram in which in turn sucks fuel from the fuel line into the pump chamber.

Then as the piston goes back down it pressurizes the crankcase which pushes the diaphram back out (same direction the spring pushes). When the diaphram is pushed out by the crankcase pressure the fuel is then pushed out a fuel line and up to the carb(s) float bowl.


simple and no real moving parts.
Thanks!

So I was thinking correctly but reversed...

Gees, when you think about how much that little
Diaphragm moves over the course of its life, it’s kind of a mechanical marvel.
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Old 09-21-2018, 10:34 AM   #4
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That is why you have to squeeze the primer ball on the gas line first. There is no fuel moving until the motor starts.
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Old 09-22-2018, 03:01 PM   #5
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The above is true for a two stroke. The four strokes work off a cam
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Old 09-22-2018, 03:33 PM   #6
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I remember it well!!


A paper thin flat metal "Reed Valve" fuel flow suction apparatus that worked off piston movement pumped gasoline into the single cylinder of my first outboard motor. It was a gold colored 1.7 hp. "Neptune - Mighty Mite" tiny two blade prop engine that I purchased from my own earned money in 1961 at 9 yrs. old. Cost was $77 earned from community area snow shoveling in NY.


The reed valve was pressed behind the large-thimble sized "tickle bowl" that held gasoline. Said bowl had a [pencil lead thin] metal spring loaded "tickle post" on its top that you "tickled" up and down to prime the cylinder from the two quart gas tank that set atop motor. Tickle valve often wore out from thousands of flexes and needed replacement... which took minutes by removing two small screws.


Then you pulled the motor through with rope starter cord you hand wound on top of motor. If no start - repeat till it does. Always keeping slide bar accelerator bar in mid front of motor in mid position.


This was placed on our 6' fiberglass "Pixie" dink.


I had great fun!
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