fuel return line and twin tanks
Turbo Mini Forums

hiya, long time lurker, first time poster.... quick background, I'm in Australia, 1970 MK2 Cooper S, home-made SC12 supercharger install, previously suck through with a carb, now suck through with home-made injection using an old Haltech - very low budget, around the $400 mark to do the blower, and $100 for the injection - http://www.ausmini.com/forums/viewtopic.php?t=66558

Tonight I was driving it around and fiddling with the mixtures, after filling up both tanks, then driving a few miles, the left hand tank started gushing fuel out of the breather tube. I took the fuel cap off and the tank was full to overflowing, the right hand tank was half empty.

I have the fuel return line running into the left hand tank, so obviously what's happening is that I'm drawing fuel from both tanks, but only returning into one of them. The tanks can't equalise fast enough so the left hand tank fills up.

The right hand tank is an original, I'm not really willing to put any holes in it, so splitting the pressure regulator return line and feeding both tanks isn't an option, neither is enlarging the fuel feed lines or breathers. I haven't fitted a swirl pot, but I don't think it'd make a lot of difference. Once the swirl pot is full, it'd start filling the tank again anyway

I can avoid it by keeping both tanks only half full, or putting a tap in the right hand tank's line and opening it when the left is nearly empty, but how have other people with twin tanks dealt with this?

any thoughts?

hello. I'm doing an efi SC12 setup too, but I've cheated and bought efi of the shelf...

Yours looks good.

Have a look at MAdMK1's build in the show us your's section. He ran twin tanks for a while and had it working.

Sounds a bit strange - unless it is really small, the balance/suction line should be able to cope with both balancing the levels and feeding the pump.

I'd check it for blockage/restriction on the left hand side (tank to pump suction Tee off) first in case the pump is actually only sucking from the right hand tank.

Otherwise, the only fix I can think of is to double the size of the left hand suction line (up to the Tee piece) compared to the right hand one to make it preferentially suck from the left hand tank. That will most likely mean modifying the left hand tank to make the outlet bigger.

Otherwise, try the opposite, ie, put a restrictor in the right hand line, same effect, suction preferably from the left, but if you do have restrictions in the left hand connection, it might starve the pump a bit and make it cavitate.

Schrödinger's cat - so which one am I ???

They used to do a twin tank conversion kit for the MPI, wonder what was in that?

If something is worth doing, it's worth doing half of.

Just one other thought.....

How are your two tanks vented ???

If it's 70's presumably to atmosphere or did they tighten up the rules in Oz earlier than over here ???

Given one tank is "original" presumably the other isn't so may be better vented ???

And given your hot climate it's "possible" the right hand tank is pressurising and forcing the balance pipe not to be effective.

Schrödinger's cat - so which one am I ???

thanks guys - good thoughts.

The left hand tank is an original too, but not a rare one like the right hand. I'll restore the car properly in a few years time, so want to keep the original S parts as unmodified as possible. In fitting the blower and injection, I've drilled a total of 3 holes in the body

The breathers on both tanks are identical - atmospheric - 1/8" plastic hose running down through holes in the boot floor. I'll check if the right hand one is blocked

The tanks do balance out (IE put fuel in the left and it'll run into the right). It did drain OK when I emptied the tanks, and when I fill up with fuel I do the left first and always need to top it up a bit after filling the right... but I will check for blockages too.

The line between the tanks is the standard 5/16" line, but the fuel pump is a monster. There is a LOT of fuel coming back from the regulator...

I was planning on putting a tap in the line to the right hand tank - just so I can get around this issue for now, but obviously I can use the tap as a restrictor too - good idea :)

I thought about putting a solenoid in the line to the right hand tank, and opening it up when the level in the left tank drops below a set point... but that'll take longer than bunging in a tap

put a tap in it for now, long term I think I'll put another fitting in the bottom of the left tank, and have the right hand tank feed into that. Then the fuel will just come from the left and the tanks will go back to self levelling

Mine has a large fuel filter ala diesel truck style under the boot - this doubles as a surge tank. The return tees into the filter body prior to the tank. Never had a problem.

Cheers

Dan

First 5-port EFI turbo in the world (built 1997, running in 1998)

Great info, but maybe a little late!

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk