fuel tank connection problem

[davehump]

Hi Folks.

I have a very strange problem that I cannot get my head around. I am part-way through an amateur restoration and have overhauled the SU pump, replaced it,filled the tank and fired the pump. No fuel was delivered. Working back down the fuel line I have located the fault to be at the union connection to the tank. When the union is tight, no fuel is drawn from the tank. When the hex union nut is loosened off, fuel is drawn from the tank but there is fuel escape. Tighten it up again and the fuel is stopped.

I have drained the tank, looked in the tank connection and checked for blockages. I have blown comprerssed air into the tasnk no problem. I have also blown compressed air down the fuel line and it is clear. When the fuel pipe is reconnected and tightened, blowing with compressed air fromn the engine bay causes air blowback, indicating a full pipe closure at the tank union.

I can't work this out. Has anyone else experienced something like this?

Thanks,




.....................Dave

[Neil MG]

Definitely sounds like a blockage at the tank/pipe union, but you might think that should be easy enough to find.

If it's not that then, well just thinking aloud really...

Is the carb already connected? Could it just be that the system is full and the pump is only operating when you allow fuel to flow back from the pump by releasing the joint at the tank.

Or is the pump not drawing enough pressure (vacuum - sucking) unless you introduce an air leak. As it is the pump that was reconditioned is the throw over correctly set? This is covered in detail in section B5 of the workshop manual.

[davehump]

Hi Neil.

The carb is not connected. The fuel pump discharge is open to a container via a braided hose.

The pump draws fuel up the line when the end of the fuel line is disconnected from the tank and bent down into a container of fuel therefore I think the pump is okay as it does draw. I have pressurised the tank with the pump running to give it some assistance, but without success.

At the union, compressed air can be readily blown into the tank but when the fuel line is connected tightly, compressed air cannot be blown into the tank from the pump end (with the pump out of position of course).

In summary, everything is clear and unobstructed until the fuel pipe union is tightened down. My conclusion is that the pressure exerted by the back-nut on the union faces between the fitting in the tank and the end of the pipe is somehow causing full closure of the pipe. I'll be blowed if I can see how though. My brainstorm solution is to fit a 3/8" BSP nipple to the tank outlet and get/fabricate a connector to pick up the fuel line, thereby bypassing the union interface but this seems like a bit of a bodge really.

ANy thoughts will be much appreciated.

Dave.

[Neil MG]

Is there any flex hose along the pipe?

Some pictures might help if you can...

[davehump]

Hi Neil.

No, there is no flex on the suction side of the pump. It's the 5/16" copper line from tank to pump.

I will try and get some pics tonight

Thanks.

[bmcecosse]

I think you need to take the tank out and clean it out thoroughly - and have a good look inside through the gauge sender mount.

[davehump]

Good call. If my adapter plan does not work that will be the next step. I have just received a 1/4" BSP.M x 8mm Cu adapter which I hope the olive will pull down to 5/16". Good job for the 48" stilsons

Dave

[bmcecosse]

[IslipMinor]

Hi Dave,

The connection to the tank is not as simple as it appears. The fitting that secures the fuel line in place also secures the internal pick-up tube. If you use a standard adaptor, the internal pick-up tube is not secured and you can 'run out' of fuel at about 1/4 full, especially going up a hill! I found this out after replacing the complete fuel line, and finding the new tank fitting was distorted, I used a standard 1/4" BSP adaptor - all was fine until we were on the west coast of Ireland, up and down hills, and suddenly had a rapid power decrease, unless I used a very light throttle up to the top of the hill, then was all fine again.

If you look into the tank fitting from the outside, you will see an inverted 'V', like a brake pipe fitting - that is the pick-up tube. Has that been damaged, so that when you tighten the fuel line, it closes off the supply?

The end of the fuel line should have a long nut that bears up against a swaged end to the tube; this swaged end engages with the 'V' of the pick-up tube to seal everything up. The nut must be long enough not to bottom on its threads. Is this swaged end correct? In brake tube speak, the swage is a 'single' one, i.e. not doubled back on itself, and obviously must have a hole more or less the same size as the fuel line bore right through the swaging. Has it?

If you remove the fuel sender unit and blow air into the tank from the pump end, where are the bubbles coming from? The end of the pick-up tube filter, or at the connection at the front of the tank itself? They should come through the pick-up tube filter.

[davehump]

Thanks Richard.

That gives me a good shopping list of things to look at.

With regards to the bubbles, these are def from the end of the pick-up tube as the fuel level is below the outlet.

Many thanks.



.......................Dave