Pumps R Us.

[Staghound]

The new SU fuel pump I have is the AUB 662/ AZX 1307...

Is this pump suitable for flow and return or is it only intended for the original single ended Stag system?

Here's hoping one of you fine fellows can help me.

Thanks, Andrew.

[thebadleys]

Hi, I'm a new visiting stag owner who has come over from the dark side (MGB ownership, which I still have )

The SU site ( http://sucarb.co.uk/technical-electrical-fuel-pumps ) give a description of pumps and specification which may help you.

Regards

Nigel

[V Mad]

The volume of return fuel should be very small, and therefore negligible relative to the main demand, so you should OK providing the return goes via a small jet sized orifice.

[Staghound]

Chris,

Why would the return need to be small? I was intending to fit supply and return pipes of the same diameter, am I asking for problems if I do this, if so what?

Andrew.

[V Mad]

The return pipes dont need to be small, but somewhere in the return path there should be a restrictor or jet fitted. Just a 0.5 to 1mm jet would be sufficient to allow any excess pressure to be relieved when the engine is switched off. For example, the Weber 38 already has such a jet fitted into the return outlet at the carburettor.

If you do not fit a restrictor then the pump will have to work hard all the time, which might reduce its life, and may cause it to overheat since you will have vastly increased its normal duty cycle. But more important than that, it will rob the carburettor of some fuel, leading to poor performance at full power.

Think of it like this. A pump is designed to deliver the maximum demand of an engine, say X litres per minute. If you return x litres a minute to the fuel tank, there will be none left to feed the engine. I hope that makes sense.

[KOY 23]

Chris.

I believe that its an accepted fact that whatever carb is fitted, many people are experiencing flooding and difficult hot starting, due more to change of fuel rather than than carb quality issues, which I guess is why Weber fitted a return stub, they wouldn't have done if unnecessary .

Not having a pop but out of interest ask why don't you use it on your kits? as its a simple job to run a line back into the return stub on the tank. It was on my list of things to do and I believe would have cured the hot starting issues I had.

[V Mad]

I disagree there John. Weber fitted the return path long ago, when fuel was much more user friendly. All the hot starting issues reported on this forum seem to be related to Burlen needle valves, as now fitted to the Strombergs, not closing and allowing the carburettor to flood. I dont believe that this applies to the Weber or any other Stag carb conversions.

May I suggest we do not start another 'debate' on this subject as it is unlikely to be in the interest of the forum.

[KOY 23]

Shame that a simple genuine question causes a problem.

I had hot starting problems caused by flooding on both alternatives carbs I had---fact, neither had a return.

Anyway if you don't want to discuss, that's fine by me.

[Staggard]

I found this problem ( in my case anyway) not related to needle valves but to pressure being held in the fuel line by the pump after turning the engine off causing fuel to syphon in to the manifold ( verified by whipping the carbs to find fuel pool in the manifold) after fitting a fuel return to the tank thus relieving line pressure, the problem of pooling and hot starting was solved. IMO a needle valve is a needle valve irrespective of what carb it's fitted to and will only hold pressure back for so long until it weeps past raising the float chamber level.

[Bob Heritage]

I had a Stag once with a RS3.1 Capri engine in complete with the same Weber carb. That flooded in the entrance gates at Brands and held up the queue until they pushed me to one side. Got all the comments from the know alls saying 'they do that you know' and I was shouting back 'It's a bloody Ford'. For the record it didn't go any faster than a TV8.
Also had a 3ltr Granada with Weber and that flooded on the M23 They all do it sometime.


Bob

[Stagsongas]

I found this problem ( in my case anyway) not related to needle valves but to pressure being held in the fuel line by the pump after turning the engine off causing fuel to syphon in to the manifold ( verified by whipping the carbs to find fuel pool in the manifold) after fitting a fuel return to the tank thus relieving line pressure, the problem of pooling and hot starting was solved. IMO a needle valve is a needle valve irrespective of what carb it's fitted to and will only hold pressure back for so long until it weeps past raising the float chamber level.

If we can keep things civil and professional I do think this issue needs finishing up, that's why I'm posting.

I agree wholeheartedly with what Graham has said above, and a lot of what Chris has to say. Let's just strip it to the bare bones for clarity:

The Problem

1. Engine stoppage results in an increase in the temp. of the fuel held in the pipes (due to rising heat from exhaust manifolds etc.)

2. The result of that is an increase in pressure within the fuel supply pipe.

3. Increased pressure results in the float valves (of any make and type) being forced off their seats.

4. Excess fuel is then forced into the manifold, making hot starting a problem due to the resulting, overly rich mixture.

All that is happening here is something we all accept in another part of the car - it is very similar to what happens when engine coolant is heated and expands (I'm deliberately leaving out evaporation) but to cater for that we have an expansion tank. The fuel system does not have any expansion measures, the pump has a non-return valve, so the expanded fuel cannot go back into the tank, it pushes past the float valve(s) instead.

Graham had his own way of solving this problem (as far as I recall he used a solenoid valve which opened a fuel return to the tank on engine shutdown). That is a clever solution, but there are various other ways of relieving the pressure increase. Another way is to provide a constant return of fuel to the tank, the line pressure cannot then rise, but;

If you send too much fuel back, all of the things Chris describes can result - too weak a fuel mix for the engine and an overworked fuel pump etc. That is where the return restrictor comes into play. I have special knowledge here, as during LPG conversion of some vehicles, fuel must be returned whilst the engine is running, but there is no fuel return on the car as the pump is of the 'Stepper Motor' type. For 15 years or more I've been using a 0.6mm MIG tip fitted into an installed fuel return hose which makes a great restrictor, and making holes that small yourself is quite difficult anyway. It works perfectly, and MIG tips are easy to obtain.

For the Stag, I did a good deal of thinking about a fuel return and realised I'd been using the solution in other ways for a very long time on other cars (Jaguars etc). Running a fuel return all the way back down the car's length is uneccesary and quite ridiculous when all we need to do is release excess pressure in the fuel line, and that fuel line begins at the fuel pump.

So, we have the choice of either fitting a solenoid controlled and/ or restricted fuel return very close to the pump, right at the rear of the car, to release excess fuel back to the tank right next door. There's even a connector for this purpose on the standard tank. Both measures have merits and drawbacks, but both of them will solve the problem described above. I was poised to write about this some time ago, but other things got in the way, and Mike (Lingen) had come to a very similar conclusion, so Ieft it.

One final thing, and in the hope that things don't 'kick off' all over again, is that if all of the posts above are read carefully, it is clear that everyone has got major parts of the story absolutely correct (IMHO) and it must be also clear that the problem is not specific to any type or make of Carburretor - if it has float valves they can be pushed off their seats by excess pressure whoever made them.

Regards

Steve

[Staghound]

So...Chris, the Weber 38 I have has a restriction in the carb outlet, have I got that right? Do I need to restrict the return pipe at all?

Thanks Andrew.

[V Mad]

Andrew, I dont know anyone that has used it, but yes, as I said the Weber already has a restrictor in the outlet pipe, which we usually blank off. So no need to put any a restrictor in a return pipe.

[V Mad]

3. Increased pressure results in the float valves (of any make and type) being forced off their seats................

................and it must be also clear that the problem is not specific to any type or make of Carburettor - if it has float valves they can be pushed off their seats by excess pressure whoever made them.

I can only go by what I know from direct experience. I have had no reports of anyone having this problem with the Weber 38.

I am not saying its impossible, but it is not a known issue with this carb.

[Staghound]

Gentlemen,Thank you all for your help in this matter, I can now proceed...

Andrew.