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Old 02-18-2013, 07:59 AM   #1
sailah OP
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Fuel tank (and subtank) plumbing questions

Little background, this is for my Ninja 650 project bike. I needed to relocate the stock in tank fuel pump from the Ninja tank somewhere else due to the shock tower and decided to build a subtank to house the pump in the subframe and add some more volume while I'm at it.

I am currently building the subtank and have some questions regarding plumbing and venting of the tank so that fill ups are hassle free

Here's the CAD drawing



And the real thing starting to get formed



Stock Keihin fuel pump in custom flange that will be welded to bottom of subtank.



Hopefully that gives enough info.

Plumbing questions:

The main tank I am going to seal off and weld in a bung. I have purchased some -6AN fuel fittings from Summit along with hose ends and hose. My plan is to run a feeder line from the main tank to the subtank.



Now if I never hit the subtank on "reserve", it's fill as usual. But say I run the bike completely out of fuel and start to fill it through the tank filler on the main tank. Obviously the fuel is going to fill the main tank faster than the feeder line can fill the subtank so in order to press the tanks, I'll need to be patient at the pump.

My question is this. Say I run the feeder line as -6AN which seemed a reasonable size. The fuel draining into the subtank is going to displace the air in the tank and it needs somewhere to go. If I vent the subtank to ambient air, it's going to need to be higher than the top tank level.

If I vent it back to the main tank, I'm going to need to run an internal line inside the tank and up to it's highest point?

Below is a crude schematic but I think shows what I am trying to accomplish.



I could go bigger on the fuel line with the hope that the larger diameter will fill and vent effectively in the same line, but I suspect that it will get annoying and create a bottleneck which will dramatically slow down the filling process.

Thoughts?
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Old 02-20-2013, 06:25 PM   #2
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Could you run a larger than -6 AN fule line to the main tank and put a smaller line inside it as a vent but run it to the filler hole area of the main tank?



Edit: forgot to add the - (in -6).

Hastelloy-X screwed with this post 02-20-2013 at 06:28 PM Reason: Forgot something...
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Old 02-21-2013, 05:38 PM   #3
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i ran a bunch of aux tanks on some of my bikes and one way i did it was build a spare tank with filler bung and vent and a out line on the bottom then ran the bottom line to a t fitting on the main tank line that went to the carbs.then put a on-off valve on the line going from the t fitting to the aux tank. if it was higher than the carb it was ok to turn on when i ran out but on some tanks it didnt have enough gravity feed so i put a small fuel pump between the aux tank and the t fitting so when my main tank was empty i would flip the fuelpump and on off valve so it would feed the carb and the extra would go back to the main tank. i plumbed it another way too but will have to try to remember how i set it up lol!
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Old 02-22-2013, 05:02 AM   #4
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I got a bunch of feedback in my build thread on this and the consensus was to def run a vent and I think the best place to have the vent terminate is through a straw high up in the main tank so it can vent properly.

Thanks!!
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Old 02-22-2013, 10:40 AM   #5
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i would think that the existing vent line to the main, would be fine without any additions to the venting side of things. afterall with the one fuel line, all you really got is one bigger tank now...no valves inserted,between the tanks, is there? one small question..is the fuel line between the two big enough to allow you to refill at a gas pump? this without waiting for the fuel to slowly trickle down to fill the lower first? with a second vent in place it might speed up gas stops.

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Old 02-22-2013, 12:05 PM   #6
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There is no existing vent from the subtank as I built it. The connector line isn't going to be large enough to allow filling and venting, most likely it will burp and vapor lock.

The idea of the vent is to allow the subtank to displace the air that is being displaced by fuel during fill up. If I never reach a level where the fuel drops into subtank alone, yes it's basically one big tank. But if I run it dry, the subtank is going to get filled first being the lowest point and thats where I am going to need to vent it so I'm not standing at the pump for hours waiting for it to fill
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Old 07-06-2013, 04:01 PM   #7
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The subtank vent only needs to be higher than the other line in , if you run it into the main tank. Is the bottom of the main tank higher than the top of the subtank? If so , you could run both lines through your new blockoff flange. One at the bottom and extend the one for the vent towards the filler. It can still be lower than the filler. The weight of the fuel will push the air up.
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Old 07-06-2013, 04:08 PM   #8
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Quote:
Originally Posted by joexr View Post
The subtank vent only needs to be higher than the other line in , if you run it into the main tank. Is the bottom of the main tank higher than the top of the subtank? If so , you could run both lines through your new blockoff flange. One at the bottom and extend the one for the vent towards the filler. It can still be lower than the filler. The weight of the fuel will push the air up.
The subtank vent is lower than bottom of the main tank, yes. I don't really have a lot of real estate in the bottom of the main tank. Since it's lined I can't weld on it.

I'm not clear on your suggestion or exactly how to implement it
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Old 07-06-2013, 04:09 PM   #9
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Didn't your flanged fuel pump come from the bottom of the tank?
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Old 07-06-2013, 04:12 PM   #10
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Quote:
Originally Posted by joexr View Post
Didn't your flanged fuel pump come from the bottom of the tank?

Yes, but I didn't leave the original flange I needed to remove it because of the shock tower so it's welded shut.
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Old 07-06-2013, 04:29 PM   #11
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That sounds like the only option , short of welding on the tank again.
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Old 07-06-2013, 04:55 PM   #12
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I took apart the fuel cap and you are correct it only vents one way. Why?

I disabled the one way venting system so now it vents both ways and it pulls air around the fuel filler "moat" Hopefully that will allow the main tank to vent enough so that it's not forcing it up subtank vent. Now the question is how to make the subtank vent so that water can't get in from rain.

I think I can form a wall with some JB weld and shove some gas foam down there to keep most of the water at bay.
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Old 07-06-2013, 07:06 PM   #13
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post9-are you quite sure that's a vent? forgive me for asking, but to my limited experence, it looks like a drain for sloppy fueling/rain water. i have wrestled with aug tank problems too, in my case a st1100 with a sampson aux. fender type tank-got it used with NO Installation instructions. the problem sounds like what i had. the cause in my case was that the fuel pump raised the pressure in the tank, and forced fuel out the aux fuel tank vent line.(fist go round it was ventedto the air right above the r. tire. and for a 1/8 inch line, it delevered an alarming amount of fuel to my rear tire) the fix for me was to reroute the sub's vent line into the main, and at the highest point on the main.

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Old 07-06-2013, 09:19 PM   #14
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Quote:
Originally Posted by sailah View Post
I took apart the fuel cap and you are correct it only vents one way. Why?
Letting fuel fumes out is probably prohibited by emissions regs, except via a charcoal canister. That's just a guess, though.

Quote:
I disabled the one way venting system so now it vents both ways and it pulls air around the fuel filler "moat" Hopefully that will allow the main tank to vent enough so that it's not forcing it up subtank vent. Now the question is how to make the subtank vent so that water can't get in from rain.

I think I can form a wall with some JB weld and shove some gas foam down there to keep most of the water at bay.
Another option would be to run the vent line into the main tank through the fuel filler cap. Any spew would just go into the tank. Routing the vent line to the front of the tank would prevent length problems when the filler cap hinged forward to open.

Or, you could [edit:never mind, you'd just dump fuel with this idea.]
There might be problems with both suggestions, but they might also be helpful.
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Old 07-07-2013, 10:52 AM   #15
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I'd try the bulkhead fitting. Maybe on top in front of the cap.
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