The Mechanics of an Outboard Fuel Pump

[ERock82]

I'm trying to understand the mechanics of an outboard fuel system. Here are some questions I have:

When you pump the primer bulb and it gets hard, does the fuel stop at the fuel pump and that's why it gets hard? or?..does it go through the fuel pump and to the carb bowl and become hard? Why wouldn't it just keep pumping into the carb once the float pin drops?

This is assuming the motor is off. Would the pump need power from the motor to pump fuel to the carb?

Also, what if you had a completely drained motor with no gas whatsoever (empty carb bowls)?..

If the fuel stops at the pump, how would you start it since it would need to get to the carb to even start?

If the bulb gets hard, can you keep pumping fuel? If you are able to flood the carb, how would you if it gets firm first?

 

[flyingscott]

OK thats a lot of questions #1 the fuel pump has check valves in it so when you pump the primer fuel goes to the carb will not let fuel go backward.. The float in the carb raises and closes the needle valve that stops the fuel and the primer gets hard. The motor powers the fuel pump with either crankcase pulses for mechanical pumps. Electric fuel pumps are different they need the battery and the motor. When the bulb gets hard you can keep pumping it you just overpower the float and needle and the carb starts leaking. The primer bulb also has a check valve in it. If you empty the fuel system just pump the primer till it gets hard.

 

[gm280]

Okay let me see if I can confuse you even more...I mean explain it... :facepalm:

When you squeeze the primer bulb a few things take place. First you collapse the bulb and when released, it creates a vacuum and since there are two one-way check valves in there, it keeps from pulling any fuel from the fuel lines going to the engine fuel pump. Instead if pulls the fuel from the fuel tank. And further squeezes then pushes fuel into the fuel pump on your engine as you collapse the primer bulb again and again. And that fuel goes to the fuel pump. But the fuel pump also has two one-way check valves and therefore the fuel goes through to the carb. The needle and seat in the carb is basically another one-way valve as well and since an empty fuel bowl allows fuel in because the float arm is opening the needle letting in fuel. As the fuel builds up in the carb fuel bowl, the float raises and eventually closes off anymore fuel from entering via the needle and seat (the one-way valve). That is when the primer bulb gets firm. If you force the primer bulb, yes you can over power the needle and seat and force fuel to flood over in the carb...not a good thing. But that is how it all works. So you can see how any one of those one-way valves can really upset how it all works... I hope this helps you a little. JMHO!

 

[ERock82]

Ok thanks! I understand this now. Now two more questions:

1.) What exactly cause fuel to be shot out the jets from the bowl when you start the motor?

2.) If you had a fuel tank venting issue, how would that relate to fuel not coming out of the carb to start motor?

 

[flyingscott]

GM280 I generally don't comment on other peoples opinions I see you don't have that problem. I always thought that if people don't understand one opinion they get another which is what made this forum good. I see you feel yours is the best so i will leave you to it.

 

[ERock82]

The reason I'm asking this question is to help me further understand an issue I'm having with my fuel system.

I believe 1 of 2 things (or both) are a problem:

1.) I have a kinked fuel line going from the permanent tank. It was cut too short by me and I just noticed there is a sharp bend where it goes into the fitting at the splash well.

2.) I have a problem with my tank vent. My motor wouldn't fire and when I opened my gas cap it fired right up.

Fuel is getting to the carb bowls.

I think somehow a vacuum was created on the fuel supply going to the pump which restricted the flow to the carb and the motor would either not start or only run a few seconds.

I'm wondering whether it's more likey that it is the fuel hose rather than the vent hose? The vent hose looks fine but the fuel hose has that bend.

Would a kinked fuel hose cause a venting problem where opening the gas cap would fix it????

The bulb dips down in the splash well and only seems to get hard when I hold it out straight.

 

[Scott Danforth]

fix your kink, fix your vent

also, since you sucked the air out of the tank, you may have stretched the fuel pump diaphragm, so you may want to fix that while you are at it.

no, the kinked fuel line wouldnt cause a venting problem, it will however cause an inlet restriction which over time will ruin your fuel pump.

again, fix your kink, fix your vent

 

[GA_Boater]

GM280 I generally don't comment on other peoples opinions I see you don't have that problem. I always thought that if people don't understand one opinion they get another which is what made this forum good. I see you feel yours is the best so i will leave you to it.

You both replied within a minute of each other. So what's the problem? In different words, you both agreed. I'm confused!

 

[jbcurt00]

I think the facepalm at the end of gm's post makes it appear that his explain it/confuse you more comment was directed at flying scott's post....

Not sure why gm asked to have his initial post deleted. As I read them as in agreement. Perhaps the wrong post was deleted.

Misundestandings, remotely via posts on the interweb, who'd have thunk it :watermelon:

 

[tomhath]

Ok thanks! I understand this now. Now two more questions:

1.) What exactly cause fuel to be shot out the jets from the bowl when you start the motor?

Fuel isn't shot out of the jets, it's sucked out by low air pressure inside the carburetor throat.

 

[gm280]

I think the facepalm at the end of gm's post makes it appear that his explain it/confuse you more comment was directed at flying scott's post....

Not sure why gm asked to have his initial post deleted. As I read them as in agreement. Perhaps the wrong post was deleted.

Misundestandings, remotely via posts on the interweb, who'd have thunk it :watermelon:

NO I hadn't even seen OR read flyingscott's post at my initial post time (actually less then a minute difference). My face palm was because of the statement I made about confusing the OP's question and not about flyingscott's comment what so ever. Sorry if my post caused such troubles and even more sorry that flyingscott took it as a direct attack on his post. I didn't even know he even made his post when I hit "post" on my final verbiage... All I can say is sorry we both posted near about at the same time not knowing the other was as well. But if it caused troubles, delete mine. It certainly won't hurt my feelings. There are too many others to try and help yet...

 

[jbcurt00]

Don't worry about it GM It happens.

And now back to our story about fuel pumps, carb floats and primer bulbs