Fuel Hose Diameter - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 18 (permalink) Old 08-12-2009, 09:31 AM Thread Starter
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Fuel Hose Diameter

Had some trouble with not getting the same flow to the rear carb than the front, rebuilt petcock and a good clean of the carbs and some seafoam seemed to do the trick. But now just wondering if the size of the hose matters on how much fuel is getting to the carbs. Have 1/4" now, what I was sold, but was really hard getting on. Would going up to 3/8 or even 1/2 in. make a difference?

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post #2 of 18 (permalink) Old 08-12-2009, 10:00 AM
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I have 3/8" on. Under the assumption that you can never have too much fuel available. I don't remember seeing 1/2" at my local Advance Auto. I think 3/8" is the biggest they had.

There would be 2 problems w/ 1/2". #1, getting it to clamp around the fittings. Would there be too much "slack"? #2, finding room for it. I have enough trouble getting the 3/8" in there and not having any kinks, etc. It has to be pretty short to avoid kinks and bends and that doesn't leave you very much slack when attaching and unattaching it.

So myself, I would probably not try 1/2". But if you do, let us know how it goes!


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post #3 of 18 (permalink) Old 08-12-2009, 10:04 AM
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I think it is 3/8 stock, but not sure. Most important not to have kinks! Makes it really difficult if you put filters on the hoses!

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post #4 of 18 (permalink) Old 08-12-2009, 05:34 PM
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Stock fuel line size is 5/16. There is no point in using a larger diameter hose because the nipple on the carb and the petcock will still be the same size so they will always flow fuel at the same rate.


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post #5 of 18 (permalink) Old 08-12-2009, 06:17 PM
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Quote:
Originally Posted by Knifemaker View Post
Stock fuel line size is 5/16. There is no point in using a larger diameter hose because the nipple on the carb and the petcock will still be the same size so they will always flow fuel at the same rate.


KM
True that, and the little passage in the petcock it has all has to come through isn't very big, either!

But when the engine is running and the petcock is feeding (via gravity) wouldn't a larger diameter hose hold more in it POST tank and petcock, thus having more available when the carbs call for it? Or is the difference insignificant?

The petcock passes more gas than the bike can burn under most conditions, that's why the floats are there to shut it off. Seems like bigger hose would have more available for quick delivery. But maybe I'm all wet.


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post #6 of 18 (permalink) Old 08-12-2009, 09:16 PM Thread Starter
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Quote:
Originally Posted by krebsne View Post
True that, and the little passage in the petcock it has all has to come through isn't very big, either!

But when the engine is running and the petcock is feeding (via gravity) wouldn't a larger diameter hose hold more in it POST tank and petcock, thus having more available when the carbs call for it? Or is the difference insignificant?

The petcock passes more gas than the bike can burn under most conditions, that's why the floats are there to shut it off. Seems like bigger hose would have more available for quick delivery. But maybe I'm all wet.
See, thats what I was thinking. There would be a some fuel in the hose right after the PC, a void and then just a little right at the carb. Any good revs and would all but dissapper. Bigger hose, more in the line avail. But while I'm thinking about it, what would keep the flow of fuel less in one line and not the other. The front carb always is full(er). vaccuum is good on both carbs also.

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post #7 of 18 (permalink) Old 08-12-2009, 09:41 PM
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Quote:
Originally Posted by eberghold View Post
See, thats what I was thinking. There would be a some fuel in the hose right after the PC, a void and then just a little right at the carb. Any good revs and would all but dissapper. Bigger hose, more in the line avail. But while I'm thinking about it, what would keep the flow of fuel less in one line and not the other. The front carb always is full(er). vaccuum is good on both carbs also.
A bigger longer hose will not change anything, if one carb has more fuel then your floats may need adjusting or you could have a problem with the petcock.


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post #8 of 18 (permalink) Old 08-12-2009, 09:51 PM
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Quote:
Originally Posted by krebsne View Post
The petcock passes more gas than the bike can burn under most conditions, that's why the floats are there to shut it off. Seems like bigger hose would have more available for quick delivery. But maybe I'm all wet.
Well, you are pretty damp. And you kind of answered your own question... Because fuel flow is gravity fed, in theory, the extra "weight" of more fuel in a fatter line would increase the pressure on the "drain" (that being the hole leading to the float bowl)

Think of a tall rain barrel full of water. You take a pistol and shoot a hole in the barrel right at the bottom, and also right near the top. The holes are the same size, but the water comming out of the bottom holes stream comes out with more force, the stream hitting the ground is farther away from the barrel than the impact point of the stream coming out of the upper hole.

You can see this with just punching two tiny holes in a 2 liter bottle , the bottom stream shoots out farther than the top one....so your thinking here does have merit.

But as you surmised, the difference between the the added weight of fuel in a 1/2 inch tube compared to a 5/16 tube would be fairly insignificant...but would be a moot issue as the valve in the float bowl meters the amount of fuel in the bowl and by design it never really "runs out" so the nano second you saved filling the bowl would never be felt. Not to mention...

That it is the weight of the gas in the tank that makes the differnce here, not the weight of the fuel in the lines. So keeping your tank topped off would do more to keep fuel flowing quickly to the carbs.

The last thing that blows using that theory is the fuel in the lines can not flow any faster than the rate coming out of the petcock to replace it. Even if you had 6 inch diameter fuel line, the tiny hole in the petcock would would only let fuel flow as fast as it can come out of that hole...

Got it?


It also brings up the issue that if you could increase the "pressure" of the fuel going to the carb enough , you could over-ride the float valve and flood the bike.


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Last edited by Knifemaker; 08-12-2009 at 09:54 PM.
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post #9 of 18 (permalink) Old 08-14-2009, 09:28 AM Thread Starter
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Soooo what your saying is, no flow, check the floats/valves. Got it. But will still put on 3/8 hose instead of the 1/4.

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New! Dunlop 110/90-19 front
150/90-15 rear
Aftermarket pipes
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"STUPIDITY SHOULD BE PAINFUL"
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post #10 of 18 (permalink) Old 08-14-2009, 11:04 AM
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