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Gravity Flow Fuel
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drs1023
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PostPosted: Tue Mar 27, 2012 5:35 am    Post subject: Gravity Flow Fuel Reply with quote

My buggy is 1641 cc DP, Engle 110 cam, Solex 34 Pict and 009 w/ electronic ignition. Before I get hollered at about this distributor/carb matchup, remember this is an off road vehicle which usually runs at higher rpm's in lower gears, so the infamous "flat spot" is not a factor. I searched the topics, but didn't find the one specifically dedicated to this question, so here goes:

Does anyone have experience in gravity feeding fuel to the carb rather than using a fuel pump? There's a lot of reasons that this will work OK, but somewhere in the back of my mind (from MANY years ago) that says this shouldn't be done, but I can't recall the reason. I've never tried this, but the tank on my woods buggy is off the rear roll bar and well above the carb inlet, and I have a petcock w/RESERVE at the bottom of the tank as a shutoff. I have an electric pump right now.

Yea or nay? and thanks in advance for your help.
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markhuebbe
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PostPosted: Tue Mar 27, 2012 6:31 am    Post subject: Reply with quote

I do, but my VW is in an experimental airplane. Gravity feed slide carb works just fine. 80hp 2276 @3200rpm constant. I don't know if it would work in your case. Laughing
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hotratz
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PostPosted: Tue Mar 27, 2012 11:48 am    Post subject: Reply with quote

There are a lot of early vehicles and tractors that run gravity feed. It could be just a matter of having enough delivery volume for your carb size. It would be easy enough to try and switch back if it doesn't work for your application.
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joescoolcustoms Premium Member
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PostPosted: Tue Mar 27, 2012 1:18 pm    Post subject: Reply with quote

As the fuel gets lower in the tank, less pressure will be fed to the carb. Less pressure will usually result in a lean condition. A fuel pump allows for a consistant fuel pressure so you can tune your air fuel mixture and the engine run decent in all conditions.
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drs1023
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PostPosted: Tue Mar 27, 2012 2:10 pm    Post subject: Reply with quote

Good thinking Joe, but I don't quite follow you. The fuel for the fuel/air ratio is sucked through the carb via vacuum, not forced through by the pump. It is only forced internally some times by the accelerator pump.

But, I won't discount your scenario since I can't remember any other reason not to do it. I do think I'll give it a try. I realize that off road vehicles have a tendency to slosh fuel away from the exit point, but my tank is fabricated from an old BBQ grill tank with the drain at the bottom where the valve used to be.

If I get stuck in the woods, I'll report it back so no one else will try it. Thanx
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PostPosted: Tue Mar 27, 2012 2:27 pm    Post subject: Reply with quote

It takes a certian amount of force to overcome the float valve in the carb fuel bowl. The less the pressure applied to the float valve, the less fuel in the fuel bowl.

The float valve does two things, first it stops the fuel from free flowing by shutting the fuel off. Second, it keeps an exact amount in the fuel bowl. This exact amount keps a steady pressure over the main jet in the carb. As the level decreases in height over the main jet, less pressure is applied to the main jet and more vacuum is required to suck it out of the main jet well and through the venturi.

Think of it like this, the lower the soft drink is in your glass, the harder you have to suck on the straw to pull the liquid up into your mouth. A carburetor cannot suck harder just to get the same amount of air fuel mix all the time.
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drs1023
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PostPosted: Tue Mar 27, 2012 3:19 pm    Post subject: Reply with quote

Well, that makes a little more sense. I should leave the electric pump hooked up. Thanks guys
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GA_Boy
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PostPosted: Tue Mar 27, 2012 4:10 pm    Post subject: Reply with quote

Gravity is my backup plan in case of tank trash or fuel pump failure.
This can with hose attached strapped to the luggage rack. Never used it but there should be enough pressure to work.
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PostPosted: Tue Mar 27, 2012 4:20 pm    Post subject: Reply with quote

I have brought rail buggies out of the woods on gravity when mis-haps occur. I have driven home on the street by gravity.

But just did it temp. to get back for a better repair.
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adams77
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PostPosted: Tue Mar 27, 2012 7:49 pm    Post subject: Reply with quote

You guys have never owned a motorcycle? All of my bikes have been gravity fed.
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PostPosted: Tue Mar 27, 2012 9:00 pm    Post subject: Reply with quote

adams77 wrote:
You guys have never owned a motorcycle? All of my bikes have been gravity fed.


Yes. And generators, tillers, mini bikes, lawn mowers and more.

Their carbs and float valves were engineered from the start to work with gravity flow. The VW carb was engineered to work with pump pressure.
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PostPosted: Tue Apr 17, 2012 8:41 pm    Post subject: Reply with quote

If you wanted to do that, you could always pressureize your fuel tank, that would put force on the fuel in side the tank and lines making it want to flow out. And it wouldnt need alot of pressue just a few lbs.
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PostPosted: Tue Apr 17, 2012 8:47 pm    Post subject: Reply with quote

the_hub wrote:
If you wanted to do that, you could always pressureize your fuel tank, that would put force on the fuel in side the tank and lines making it want to flow out. And it wouldnt need alot of pressue just a few lbs.


Welcome to the samba.

But, describing how to make a bomb with your first post is not usually a good idea.

Something inert like Nitrogen would need to be used to make the fuel tank safe. Seems that procurirng the compressed N2, regulators and low pressure gauges would be more of a pain than just putting a stock fuel pump on the unit.
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