Honda NC750S

~Martin~

RBLR1000 Finisher
IBA Member
#1
I'm planning a gravity fed aux tank on my NC750s which has the main tank under the pillion seat. I’ve read a lot on here about not having the aux tank larger than the main tank which is only 14lt 3.8gallons and the danger of the main tank flooding, but what if I just create one large tank?

Aux tank mounted directly over the main fuel tank.
AN8 weld on bulkhead fitting in left and right side of main tank. Yes, twin fuel lines as I want a quicker fill from both tanks being empty .

I plan to weld a blanking plate over the existing gas cap hole.

The breather tube just behind the gas cap I will reroute using fuel line to the top of the aux tank but I’m concerned that a) its a tiny bore b) it probably has a roll over bearing in it which may stick and cause a vapor lock and due to its location be inaccessible, so perhaps blank this off and fit a third bulkhead fitting to run a larger bore pipe up to the aux tank with an inline roll over valve.

Aux tank breather run down and connected to old breather tube run.

It sounds to simple to me so that makes me think I’ve missed something important! Any advice?

IMG_0804(1).jpg
 

EricV

Premier Member
IBR Finisher
#2
You would be attempting to seal the OEM tank, that was never intended to be sealed. It should have both a vent and over flow line plumbed to the OEM tank, and blocking both of those off, and the main cap are really a lot more work than needed and potentially fraught with issues.

Add to the fact that unless you're running 2" lines to the OEM tank from the aux tank, your fill up times will be slow at gas stops. Long ago people did something similar with the Honda ST1100 bikes. The Aux tank filled via the main, commonly thru 5/8 hose. It was painfully slow to fill up. I doubt very much that two lines would have improved things much.

I installed a 4.5 gal aux tank on the pillion seat area of my wife's BMW F650GS twin for the 2011 IBR. The stock tank sits directly below the rider seat as it does on the NC750S and is 4.23 Gal. It was the poster child for that year's IBR on how to install a fuel cell for the IBR.

What you're not seeing yet is flow rate. As in a lower flow rate is better than a higher flow rate sometimes. The fuel lines I used were 5/16" I.D. The process was to run the main tank down nearly to reserve, then open the aux tank valve and continue riding. Fuel flowed at such a rate that you could not use fuel faster than it was transferring, but it didn't transfer so fast that it would end up over flowing the main tank. It would slowly come back up to full, but by the time the last of the aux transferred into the OEM main tank, there was no over flow.

The exception was if you forgot to close the valve, filled both tanks and took a break... Then you would come back to a big puddle of gas on the ground. Or you'd be trailing gas spewing out as you rode. Not advisable. My wife only made that mistake a few times in 100k miles of use. Once on the IBR. Someone hit the pump shut offs and freaked out. She told them she was moving her bike as they told her Hazmat was coming. Then she got the heck out of there!

Build a good rack to mount your aux tank. Secure the aux tank well to the rack. Plumb a vent/overflow line from the high point of the Aux tank with a roll over valve in the tank and run the line down past the rear tire so it's not in danger of puking fuel on the tire or worse, in front of it. Consider a dry break fitting on the vent line so you can easily remove the tank if needed. Install a ground wire/strap on the Aux tank to the bike's frame. The line from the aux tank to the main tank needs a dry break fitting and an inline fuel filter. Use a clear plastic one and buy two so you have a spare on the bike. Do NOT use a glass or "serviceable" type. It's a lot more prone to leaking with the vibration of a motorcycle. Do not use plastic dry break fittings either.

What I did on the F650GS was to remove the charcoal canister and the plate with the vent line, fabricate a new plate from aluminum and drill and tap it for a brass hose barb for the line from the Aux tank (edit) AND a fitting for the main tank vent that ran a line into the Aux tank's vent line up high. That eliminated the check valve and problematic CA emissions charcoal canister that could potentially fill with fuel in a bike drop and clog, causing a big problem for fuel flow w/o a decent venting situation.

I don't know the NC's system. If the check valve is not in the tank itself, you might be able to use the vent line as your incoming fuel line from the Aux tank. But I'd try to avoid that and instead install a new fitting for the line from the Aux tank. This way the main tank retains it's stock vent line and the aux tank has it's own vent line. This will eliminate any chance of pressurizing the main tank or getting a vacuum there, both of which will result in fuel transfer issues in a gravity feed design.

You will simply have to learn the process on that bike for when you can safely open the valve on the Aux tank. It may end up being necessary to use a smaller fuel line, 1/4", to slow fuel transfer to avoid over flowing the main tank, but I suspect not. An alternate method would be to turn the aux tank valve on at Reserve, then off again at 3/4, then back on at 1/2 to accept the remaining fuel in the aux tank. Too much hassle for most riders in the heat of a rally, but muscle memory is an amazing thing.

I hope that helps give you some ideas and points you in a better, less complicated, and less problem prone direction for your aux fuel. KISS and don't reinvent the wheel when it's already been proven to work fine.

Please feel free to ask any questions you have. And please remember that this advice is worth exactly what you paid for it, so no harm in ignoring it if that's your choice. It's your bike and your choice. I'm just sharing what worked for another bike with that same configuration and set of problems.
 
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~Martin~

RBLR1000 Finisher
IBA Member
#3
Thanks for that Eric, and the penny has dropped now you've explained fuel flow, and you are right, it makes it a lot simpler. I also take your point about the fuel line size and agree it will be trial and error and only one required, I will just have to fill both tanks separately.

I will get it all fabricated and fitted and then finalize the fuel line run, the tank is steel and lots of space to weld on a bulkhead fitting. There is an ideal spot to put the fitting where I've removed the pillion grab rails, but it does make for a short run to the aux tank so not a lot of room for shut off valve, dry break and fuel filter. I may forego the filter as the bike is a Honda commuter and probably designed with cruddy fuel in mind and use the shut off as the break point if i need to remove the aux tank.

I'm planning on using push fit hose and connectors, they specify an operating pressure of 200psi and the NC fuel system works at 50psi max so should be good.

Have you got any photos of your set up?
 

EricV

Premier Member
IBR Finisher
#4
Please share a link to the push fit hose and connectors you are planning on using. Pressure is one thing, fuel rated is another.

What I did on the shut off valve and filter is to have the main tank inlet on one side and the aux tank outlet on the other side, so my hose could run across the bottom of the aux tank with the inline filter, then down under the seat to the main tank. The valve was right off the tank, which made it nice and solid, but also within reach of the left hand. Some pics:


The custom plate with main tank vent line outboard and fuel inlet line inboard.

Showing the marine drybreak fitting as the hose comes from the aux, thru the inline filter and drybreak, then back and curves around down to the main tank. You can also see the custom plate on top of the main tank that covers the area where the original charcoal canister and vent line were that now has a new vent line and aux fuel inlet. The brass T in the forground is where the vent line from the oem tank meets the vent line running down from the top of the aux tank. Note the black wire, that is the ground wire for the aux tank as it comes down to the bike frame.

This is the top of the aux tank which shows the ground wire attached to the cap area and the roll over valve/vent and line that runs down to the T, then continues to the rear of the bike.

Another shot of the vent line running down from the aux tank. Clearly shown T, ground wire with bullet connector that allows easy removal of the aux tank w/o removal of the wire from tank or bike and the drybreak fittings on the vent line, (for ease of removal on the road, and act as a built in spare set for the fuel line drybreak fittings, which are identical.)

All the efforts to allow an easy removal of the aux tank have more to do with field repair/access than any regular removal of the tank. Consider if you have to get to something under the aux tank? Tail light bulbs, for example. Not something you want to spend an hour doing by the side of the road.

Don't skip the inline filter. It provides you with a visual clue if you get bad gas. And it's a hell of a lot easier to change than the in tank tea bag filter on the fuel pump. When the bike starts running like crap, you're going to think of several potential reasons, being able to quickly look at the filter and see clear or clogged is a huge jump on the diagnostic process. Here is another install where I had minimal room and used a different size/shape of inline filter. Filters come in many sizes/shapes and line sizes. The ones shown are 5/16 I.D. since I find that size fuel line works best for my applications. In your case, I would definitly start with 5/16 I.D. fuel line, 3/8 will flow too fast. 1/4 is easier to route, but flows quite a bit slower. It may be necessary for your application, but hopefully not. If you went with 1/4, you would have a longer transfer time and likely need to open the aux fuel valve closer to 1/2 full on the main to ensure you didn't out ride the fuel flow and run out when you still had gas in the aux tank.
Disk style inline filter:

Another shot showing the valve and filter. This was on a '12 Yamaha Super Tenere.

As an aside, I have installed four custom fuel cells and also had a main tank fabricated into a larger tank. The expansion of the main tank was more expensive and resulted in less overall fuel on board. All worked very well.
 
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~Martin~

RBLR1000 Finisher
IBA Member
#5
These were recommended to me by a friend that works on race cars Speedflow but I'm still investigating what my options are on fuel line fittings.

Looking at the pictures certainly helps, thanks for all the input, it has certainly given me food for thought. Once things get underway I will update here.
 

EricV

Premier Member
IBR Finisher
#6
Thanks for the Speedflow link. I was afraid that was what you were referring to. Far better than plumbing bits! ;)

I used to use a lot of Aeroquip hose and AN fittings with similar press on fittings for race cars, high performance street and track cars, etc. The quality and function is not in question, if you choose to go that route. However, AN hose is not as flexible and generally larger in diameter for a comparable flow automotive fuel line. There is also the aspect of potential field repairs. If you set it up like an aircraft or race car, you can pretty much forget about fixing anything in the field with commonly available parts/hose. This may not be an issue for you.

Hose flexibility may not be an issue either, considering the relatively open run and short lengths you need. For bikes with traditional fuel tanks, running the hose from the aux tank to the main involves snaking it thru under the seat and it can be a challenge to keep a short, smooth route w/o any possible areas where air pockets can be trapped.

Press on hose/fittings are not something you toss on in the parking lot. Typically a vice and proper hose holding tools are used, and make the process a LOT easier than attempting with bare hands. You will end up with dedicated hose assemblies instead of something you can easily change and sort once you're using it to correct any issues that may come up. If a hose is damaged, you're done until you do a proper repair, as attempting to find a work around with common parts from an auto parts store that will work with your Speedflow fittings and push on hose will be a real challenge.

Go to a hose shop in person and talk with them about what you're doing, examine and handle some of the hose in the AN size you plan on using and consider what I've said above before you make your final choice.

Another issue is I.D. and flow rates. AN -6 is 3/8" I.D. (~10mm) and a AN -4 hose is 1/4" (~6mm). You won't find AN -5 hose, it's not a size used. So you're likely stuck with too small and too large, from my experience. Common 5/16" I.D. fuel line, (8mm), is much more available.

If this were a race application, I wouldn't hesitate to go with Speedflow bits. But race applications go to the track on a trailer and come home on a trailer. You're going to be out in the wild urban and not so urban jungle with your bike. If something happens, you don't want to trailer it home because it can't be fixed at the local shop or hardware/auto parts store.
 

cacomly

Premier Member
IBR Finisher
#7
Another consideration if you are having your aux tank built is to have it split into two compartments with their own outlet. That way you can open the valve when the main tank is low and not worry about overflowing if you stop, and then repeat when it gets low again.

I have not built that tank yet, however I am considering it if and when I replace my now aging rally bike. It certainly won't be before the 2021 IBR as I would not make such a change only a year out
 

EricV

Premier Member
IBR Finisher
#8
Another consideration if you are having your aux tank built is to have it split into two compartments with their own outlet. That way you can open the valve when the main tank is low and not worry about overflowing if you stop, and then repeat when it gets low again.

I have not built that tank yet, however I am considering it if and when I replace my now aging rally bike. It certainly won't be before the 2021 IBR as I would not make such a change only a year out
That's a great idea if you were doing a custom cell. Two valves would be a minor thing to get used to, or a Y valve at the aux tank in addition to the main valve.

"only a year out"? You have a year to sort it. ;) A week out would be tempting fate.
 

Marc11

Premier Member
#9
That's a great idea if you were doing a custom cell. Two valves would be a minor thing to get used to, or a Y valve at the aux tank in addition to the main valve.

"only a year out"? You have a year to sort it. ;) A week out would be tempting fate.
Exactly. I built my rally bike in about 5 weeks in 2019 when my intended bike pooped the bed heading home from the Jax dinner. You can do it!