I have fitted an aux tank and used a low pressure pump (5-6 psi). To get the fuel into the bike main tank I have teed into the fuel return line and used a one way valve at the joint to stop any feed back into the aux tank.
The aux tank is fitted in a sidecar.
All worked as expected in the garage with the engine off.
However, when travelling the system is very unreliable. The pump is on a timer switch and sometimes I get a full quantity of expected fuel (rarely) and others I get varying amount from half the expected amount to nil (more often it is the latter). Always the expected amount is delivered when pumped with engine off.
I am suspecting that the pump needs to deliver higher pressure for the fuel to get into the return line if the system is to work properly.
If I am right does anyone know the what psi is in the return line, all I know is that it is low pressure, hence the low pressure fuel pump. I have used this system before but must have fluked the correct fuel pump because it has been successful. But at $230 per pump it is a bit pricey.
If I am on the right track with my thinking about the pump psi being too low I could try to match the pump to the pressure in the return line OR what would happen if I used a standard high pressure pump, would it put undue pressure at the FPR or would it merely take the line of least resistance and deliver the fuel as required into the tank???
Open to suggestions.

Return line pressure should be 36 PSI. Search Aliexpress for pump just type in motorcycle high pressure fuel pump. For an OEM copy type, in BMW K100 fuel pump.
Regards Martin.

I would expect the return line to be around 10psi but it would be getting lower as it gets towards the outlet. You could use a roller cell type pump like the standard pump but put a restrictor of about half the diameter of the internal size of the tube going into the tank (probably 3mm) after the non return valve.
Restricting the flow to 1/4 of the pumps capacity will cause a much lower pressure but because of the pressure behind the restriction it will always overcome the return line pressure.
You may need to play around with the restrictor size but 3mm is a good starting size.

Thanks Rick, will give it a go, knew I could rely on you to come up with something.

"I would expect the return line to be around 10psi but it would be getting lower as it gets towards the outlet."
Just on that would it be worth putting the "T" up as close to the tank spigot as possible on the off chance ???

Packo wrote:Thanks Rick, will give it a go, knew I could rely on you to come up with something.

"I would expect the return line to be around 10psi but it would be getting lower as it gets towards the outlet."
Just on that would it be worth putting the "T" up as close to the tank spigot as possible on the off chance ???

Yep close as possible

I've had a gauge on the inlet to the fuel rail and the outlet return line and they both showed 36PSI.
Regards Martin.

but after the regulator its quite low martin
I observed return flow in tank whilst replacing my pump recently and no way was it higher than a few psi

DUH.   I'm getting old and dementia is setting in. I'll go take my medication.
Regards Martin.

  its all good ...lol

Hi Packo, can't help you with sorting this out but I was wondering if I could trouble you for more details/pictures even of how you have this auxiliary tank set up.  I have a similar idea for increased fuel capacity on a project that I am starting and would love to get more information on where to start.

Thanks, Woodie

When I was designing my K75GS I found a 9ltr fuel tank for a go kart which would have fitted neatly into the space where the battery is. I will have a search around and see if I can find one.
What I was going to do was put a large (20-25mm) hose between the top tank and the lower one with a breather also and run the pump in the lower tank.

[quote="Woodie"]Hi Packo, can't help you with sorting this out but I was wondering if I could trouble you for more details/pictures even of how you have this auxiliary tank set up.  I have a similar idea for increased fuel capacity on a project that I am starting and would love to get more information on where to start.

No worries. I will take a couple of photos over the next few days if I get the opportunity, but in the meantime a bit of a description is the best I can offer.
I have 2 sidecar outfits and both are set up the same, except one has a 17ltr tank under the sidecar and the other has a 40 ltr tank in the rear section of a cargo box. Both have a breather which is simply a piece of 8mm fuel line coming from an outlet on the top of the tank to a the outside of the sidecar and fitted with a marine type breather outlet. From the outlet at the bottom of the tank is another short run of fuel line to a filter and then to a fuel pump (low pressure pump). From the pump the fuel line the heads up towards fuel return line and is fitted with a non-return valve just before a T piece into the fuel return line. In both setups the fuel pump is switched on manually but controlled by a timer switch which cuts the pump off after a certain period so the bike main tank does not overflow. Just a bit of pumping and timing and measuring volume pumped in the garage to get this to what you want.
For the timer switch on one of the setups I used a REDARC potentiometer which is the same setup pin-wise as a relay, it is excellent but expensive $200+AUD. But that works from one switch , manual ON and auto OFF and resets once the switch is returned to the OFF position.
The other is a much cheaper timer switch from Jaycar approx. $40 AUD. This requires manual ON and is auto OFF with another Master OFF emergency switch. The wiring for both is not difficult
Both have worked very well but I replaced the pump on one of them with a cheaper pump that looked the same as the original at half the price and from my post above you can see I haven't got the pressure right yet.
Will try to get some photos but in the meantime if you require more info just ask, Hope this is what you were after
Some people run a gravity feed from the main bike tank to the aux tank under the sidecar and then pump fuel from there up to the fuel rail and all else remains as is. I think this is the set up that Rick was going to use with the aux tank acting as the main supply and the bike tank as the "aux"- much simpler way to do things I think.

Thanks Packo, greatly appreciated.  I am currently looking at the space occupied by the battery and toolbox/EFI and thinking that a fuel tank there might be more useful.  I have gone back and forth with the merits of setting it up as a transfer tank or plumbing it with the main tank draining into it.

Woodie wrote:Thanks Packo, greatly appreciated.  I am currently looking at the space occupied by the battery and toolbox/EFI and thinking that a fuel tank there might be more useful.  I have gone back and forth with the merits of setting it up as a transfer tank or plumbing it with the main tank draining into it.

I did a lot of research into this one mostly because I do a fairly regular 175 miles each way day trip, almost all on motorway. Its very easy to make up a polystyrene mock up for a fabricator.

The simplest solution I had was to relocate the standard fuel pump to an auxiliary tank in this position made in stainless steel and with a removable top access panel to access the pump without a need to remove anything. Routing of pipework was quite simple, outlet from 'new' tank goes at 60psi to the fuel pressure regulator [FPR] as standard, then the fuel pressure regulator to injector rail and FPR return to tank main as they are. Original tank supply spigot is rerouted from FPR as a gravity feed into the 'new' tank. This also deals with venting issues as it all uses the original tank venting.

The top removable panel in the 'new' tank allows easy access to the pump and filter. just let the fuel level drop enough to remove the access panel. You can leave all of the existing wiring in place with a re route of the fuel pump pos and neg wires, either leaving the original warning and level indicators in place or putting new ones in the 'new' tank.

Not wanting to highjack your thread Packo so I've moved my questions over to "K75 Somethin'" .