Gravity Feed Gas Tank

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Cubman
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Gravity Feed Gas Tank

Postby Cubman » Sun Jan 18, 2015 11:40 am

If I want to gravity feed my motor do I need a gas tank with a pickup on the bottom of the tank, or can I use a regular gas tank and a primer bulb to start a siphon?

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skinnypockets
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Re: Gravity Feed Gas Tank

Postby skinnypockets » Sun Jan 18, 2015 12:50 pm

Either one should work as long as the bottom of the tank is a few inches above the carburetor. It would be difficult to give you a definite answer without more information about your application. Hope this helps.

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glades cat
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Re: Gravity Feed Gas Tank

Postby glades cat » Sun Jan 18, 2015 3:13 pm

A gravity feed tank with a fitting on the bottom will supply fuel until the tank is empty.
Without a pump, a tank with the pick-up on tops will draw fuel up hill due to a siphon effect as long as there is no air in the line. It uses the energy of the falling fuel to draw fuel up the pick-up tube, against gravity. Once enough air enters the line, the siphon effect will cease.
I would put the fitting on the bottom.
Hope this helps you make a decision.

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Cubman
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Re: Gravity Feed Gas Tank

Postby Cubman » Sun Jan 18, 2015 4:04 pm

Thanks for the replies. So a boat gas tank and primer bulb might work, but if the gas starts sloshing too much it will break the siphon.

I have a 90hp Franklin with no mechanical fuel pump. I am trying to decide whether to put a gas tank up in the rigging and gravity feed, or put the gas tank on the bottom of the boat and use an electric pump. The motor has a starter so I will have a battery in the boat anyway, but I do like the simplicity of gravity feed.

My boat is only 6' wide, would putting the gas tank up in the rigging make it feel top heavy? I am thinking of using a 12 gallon tank.

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Re: Gravity Feed Gas Tank

Postby Gonzo » Mon Jan 19, 2015 1:28 am

I run an A-65 on a 10x6 with a 8.5 gallon in bottom with a Carter electric up to a 1/2 gallon header. Fuel level in header is controlled by a Holley Carb bowl mounted on side. 3/8 feed, 3/8 return to tank, 3/8 to carb and is mounted under seat giving 18" of head pressure to gravity feed carb. My NAS will blow over at about 2.5 PSI trying to hit it with an automotive fuel pump @ 4 1/2 psi. will not work. Ticker pumps I wanted nothing to do with. The way I have my set up is all AN braided, fuel pump craps I pop out my little jumper hose with outboard ball and pump my behind back to the hill. Needle and seat issue it turns into live circuit and we get back to hill. Six years/200 hrs. still going strong.

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Re: Gravity Feed Gas Tank

Postby Gary S » Mon Jan 19, 2015 5:54 am

My first boat was a 5x10 plywood boat with a 65 cont. I had a 5 gal top tank and a larger bottom tank. It had a hand transfer pump on the side of seat stand and when the motor started missing you had to pump like crazy. If the motor died it was a bear to restart while hot. I like the idea of the float with a elec. fuel pump.

Cubman
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Re: Gravity Feed Gas Tank

Postby Cubman » Sun Jan 25, 2015 6:13 pm

I got a 12 gallon fuel cell with a bottom sump. I would like to plumb a fuel shutoff right at the tank. How do adapt the AN fitting to normal pipe threads? Can I use a regular ball valve from the hardware store for the shutoff?

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skinnypockets
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Re: Gravity Feed Gas Tank

Postby skinnypockets » Mon Jan 26, 2015 10:43 am

Cubman wrote:I got a 12 gallon fuel cell with a bottom sump. I would like to plumb a fuel shutoff right at the tank. How do adapt the AN fitting to normal pipe threads? Can I use a regular ball valve from the hardware store for the shutoff?

If you're set on using gravity feed, go to a NAPA auto parts store and get a valve made for gas. They will also have all the adapters and fuel lines you will need. Don't try to use items not made for gasoline, if they fail it will cost you a lot more than the correct fittings. Imagine what kind of fire you would have with gas leaking on the engine and you would not be able to get to the shut off valve. With an electric fuel pump you can shut off the flow by turning the switch off. Just something to think about!

Gravity fed fuel tanks have been used on airboats before but I seem to remember something about certified fuel tanks for boats are required to have the fuel line coming out of the top in order to prevent the boat from filling up with gas if the line fails. Maybe someone with more knowledge will chime in to verify this.

What ever you decide to do, think safety first. There's a reason 99% of airboats have the tank in the bottom of the boat! :salute:

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I'VE DONE SO MUCH WITH SO LITTLE FOR SO LONG, I CAN DO ANYTHING WITH NOTHING!

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Re: Gravity Feed Gas Tank

Postby Gonzo » Mon Jan 26, 2015 9:25 pm

I would think the reason for most tanks being in the bottom is weight, 100LL AV is about 6.0 lbs and MO GAS is 6.2-6.35 lbs depending.

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Re: Gravity Feed Gas Tank

Postby crazycoonass » Mon Jan 26, 2015 10:58 pm

You might want to check with wildlife and fisheries or the coast guard but I think it's illegal to have a fuel tank on a boat with the suction on the bottom of the tank. That's why you see all factory made tanks with the suction on top and a suction tube going down to the bottom.

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Re: Gravity Feed Gas Tank

Postby Cubman » Sun Nov 12, 2017 8:40 pm

Will it be ok to run my tank this close to the motor? Or will it get too hot?

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Gary S
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Re: Gravity Feed Gas Tank

Postby Gary S » Mon Nov 13, 2017 7:43 am

I would not like the tank there. Don't think it would get hot, but it would obstruct air to the prop.

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BenBad
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Re: Gravity Feed Gas Tank

Postby BenBad » Mon Nov 13, 2017 9:56 am

Just my opinion but I would use an electric pump and put a pressure regulator on it so not to push to much pressure to the carb and have the tank low so not to add top weight to the boat.

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Re: Gravity Feed Gas Tank

Postby Cubman » Tue Nov 14, 2017 1:27 pm

I appreciate the input gentlemen. I would like to use a gravity feed tank to keep everything simple, so I can run it without a battery if I need to. Airflow to the prop and the weight up high are things I hadn't considered. My drivers seat will be directly in front of the gas tank, so I imagine I will be blocking just as much air as the tank. Being top heavy may be a problem, though. This boat is 5' on the bottom and 6' at the top. I set the motor height so I can run a 66" prop 6" below the transom. I think the prop in the pictures is 60".

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Re: Gravity Feed Gas Tank

Postby Gary S » Wed Nov 15, 2017 6:45 am

You can go with a smaller gravity tank, with a larger bottom tank. Simple transfer pump to raise the fuel. Either hand or electric.

Cubman
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Re: Gravity Feed Gas Tank

Postby Cubman » Sat Nov 18, 2017 8:53 pm

How about this generator fuel tank? I can mount it under my seat, and it already has a fuel shutoff.

http://www.surpluscenter.com/Engines/En ... 1836-R.axd

Gary S
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Re: Gravity Feed Gas Tank

Postby Gary S » Sun Nov 19, 2017 7:08 am

Not sure that valve would supply enough fuel.

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Re: Gravity Feed Gas Tank

Postby Lwells » Sun Nov 19, 2017 10:52 am

Won't that engine burn around 5 to 6 gallon per hour under high throttle settings?


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