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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 02-24-2013, 09:47 AM   #7
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A vent is a must have. We have diesel tow vehicles at work with no vents on the tanks. If your not careful when filling them will take a gallon or so then burp half a pint all over you. I would think you could run a solid line into the main tank carefully aimed so it doesn't vent to close to the filler hole.
interested in seeing what you come up with. I love bikes with killer range.
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Old 02-24-2013, 03:33 PM   #8
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Quote:
Originally Posted by wos View Post
A vent is a must have. We have diesel tow vehicles at work with no vents on the tanks. If your not careful when filling them will take a gallon or so then burp half a pint all over you. I would think you could run a solid line into the main tank carefully aimed so it doesn't vent to close to the filler hole.
interested in seeing what you come up with. I love bikes with killer range.
you might like my 1150gs with killer range, i had a custom aluminum 12 gallon main tank and a 2 gallon aux tank in the back.
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Old 07-06-2013, 03:44 PM   #9
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seeking advice with hat in hand.

I have the bike mostly done and have been riding it. The other day I was riding it was quite warm (90ish) and I smelled fuel. 20 seconds later fuel was coursing down the tank, obviously coming from the tank "overflow" that I had repurposed for the subtank vent.

I have the bike plumbed like this. The fuel line from the main tank to subtank is gravity feed with a 3/8" ID line. The vent from the subtank uses the "water" drain around the fuel filler neck that is supposed to drain water from rain etc and filling whoopsies down to the ground. I hadn't quite anticipated the rain water issue as I thought that area was sealed. Oversight I guess.





Here's where the vent comes out of the tank. This is what I have connected to the subtank vent.



So I think I need to rethink this. Problem is that the tanks are now sealed and I don't want to have to do anything internally. I guess I need to put the overflow vent back to its original purpose. But how to make the subtank vent? It needs to be higher than the fill level of the main tank.

The way I see it, I have 2 options.

Vent up underneath the headlight using either a fuel vent or making an upside down "U" and having that line drain down under the bike.

I see a couple issues with this. One, using just a vent, I could have the same issue I have now, spraying fuel all over the front of the bike and as speed, that could be unfun to say the least.

The other issue is the "U" could potentially drain the fuel out of the bike if there wasnt a break, also unfun.

I thought about just putting in a small valve to vent the subtank when filling and then closing it during normal operation as I don't think it's really needed other than to evacuate the air while filling.

Any other ideas?
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Old 07-06-2013, 03:50 PM   #10
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One more thing, the fuel stopped coming out of the overflow the second I opened the cap. I suspect that maybe the cap wasn't venting, prevent the subtank "overflow" from returning back up the preferred gravity feed line and going up the much higher vent line. So the vent line is the path of least resistance for the fuel at the moment.
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Old 07-06-2013, 04:01 PM   #11
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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, 04:05 PM   #12
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Quote:
Originally Posted by sailah View Post
One more thing, the fuel stopped coming out of the overflow the second I opened the cap. I suspect that maybe the cap wasn't venting, prevent the subtank "overflow" from returning back up the preferred gravity feed line and going up the much higher vent line. So the vent line is the path of least resistance for the fuel at the moment.
Gas expands when heated or shaken. Your open vent was the path of least resistance. Under normal circumstances a gas cap only lets air in , not out.
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Old 07-06-2013, 04:08 PM   #13
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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   #14
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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, 04:11 PM   #15
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Gas expands when heated or shaken. Your open vent was the path of least resistance. Under normal circumstances a gas cap only lets air in , not out.
I think that's the problem, the positive pressure in the main tank from the warming fuel is forcing the fuel down into the subtank and the only vent at this point is up the overflow tube.

I really shouldn't have the overflow tube as the vent anyways because rainwater can now get into the subtank.
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