TR3A Fuel starvation

[MarcoD]

Hi. Im working on a 1959 TR3A. Its recently been fitted with an electric fuel pump, I dont have the spec on it yet. It also has an inline fuel filter just before the carbs.

The inline filter never gets full, you can see fuel almost dripping into it.

It starts, eventually, runs and drives well at speed, revs fine.

But if its left idling for more than a minute, you can see the fuel that is in the filter drain out and the car stalls.

Ive disconnected the fuel pipe at the carbs, turned on the ignition and got a good fuel flow into a clear bottle.

The float chamber breathers are clear, the needle valves are not sticking.

So my conclusion is poor fuel pump pressure.

Does anyone have any other thoughts?

Thanks

 

[RobH]

That sounds very odd. The pump can supply enough fuel for the car to be driven at speed, but can't supply enough to keep it idling?

I wonder whether the pump is very voltage-sensitive and only gets enough power to run properly when the engine is revving?  

 

[MarcoD]

Hi Rob. Yes its driving me, and the owner a bit nuts! 

Good point about the voltage, although the car has also been fitted with an alternator, so it should have enough power.

It seems like its getting back pressure from the carbs and shutting off too early. It pumps really well into an open container when idling.

Seems like im going to have to do a pressure check on it.

The mechanical pump has been left in place, but gutted, not by me!

 

[RobH]

For an SU the pump only needs to supply 1.5 -3.5 psi, and that is more to do with keeping the float chamber full at speed rather than delivery at idle. If the float drops far enough to open the valve there is no back pressure at all to the pump. The float chamber is vented by the overflow pipe so there can't be any pressure in there. 

You say you can see the filter drain - so the fuel must be going into the carbs under gravity.  It sounds like the pump just can't replenish the filter.

Have you tried it without the filter in circuit?  Perhaps that is the problem. 

[MarcoD]

Hi Rob

Thanks, Ill try bypassing the filter.

Ill post what happens!

If we are also getting a pressure drop through the old pump that may be part of the issue.

The glass bowl next to the old pump is full.

[harlequin]

I had similar problems last summer when the weather got hot. I finally tracked it down to the glass inline filter just before the carburetors, I believe that the nylon filter element was closing up in the heat, once I removed the element I had no more issues even in the recent very hot weather.

I should add that where the filter was just before the carbs also put it directly above the exhaust manifold which probably compounded the problem 

George 

[Hamish]

Try the filter before the pump it’s how I have mine set up on a sucker Huco pump that’s under the bonnet.

i had fuelling issues but that was high revs tho.Turned out to be a 3” rubber joining section of pipe near the rear wheel of my TR3a  that had turned to jelly.

[MarcoD]

Thanks for the info.

The glass bowl appears not to have any filter in it.

Ill try the filter bypass this week.

The fact that shes ok at high engine speed, and when its warm, is the mystery!

Its not helped by the pump being installed just next to the fuel tank, its very difficult to get to without a car lift and Ive had petrol pour over me before from a faulty pump so Im trying to be careful!

 

Edited August 17, 2020 by MarcoD
Fuel pump info added

[RobH]

Ah - thats new info. So the pump is a pusher type mounted low at the tank and has to shift fuel through the old pump body to get it to the carb. I wonder how the old mechanical pump was modified to allow that?

 

[MarcoD]

Yes my thoughts exactly!  I may have to take the mechanical pump off to find out whats going on.

I take it its more common to fit an electric pump under the bonnet?

Ill also see if I can temporally bypass the mechanical pump to check if thats the issue. 

I think the pressure drop through the old pump is causing the issue, the flow to an open end pipe is good but there is no oomph behind it! 

Id rather reinstate the mechanical pump, but I think they the binned what ever parts they removed from it!  

[RobH]

You can do it either way of course but the under-bonnet pumps work more like the original setup and don't need modification to the tank outlet pipework. Common 'pusher' pumps often have too high an output pressure for SUs and need a pressure regulator fitted in the engine bay. The unusual bit is the use of the old pump body. 

[MarcoD]

Thanks, I think they wanted to keep the under bonnet looking as original as possible, negated by some of the other things they have done though!

  

[mikej]

I always thought that the electric Lucas pump needs to be gravity flooded so mounting it above the (current) tank fuel level will cause it to starve.  Ethanol -loaded fuel is more volatile and should ideally be pushed rather than sucked (well, that's ideally true of all petrol) . Either way, there should be absolutely no impedance to flow before the pump.

M

[MarcoD]

Thanks Mike, The pump is right next to the bottom left side of the tank, so it should be getting a good enough flow. 

[Dic Doretti]

Hi Marco

                Both MGBs and Austin Healeys manage with pusher pumps next to the back axle and SU carbs, I suspect the problem lies with the bodged mechanical pump in the circuit.

                Cheers Richard

[MarcoD]

Thanks Richard I think you are right!

 

[Hamish]

I wouldn’t want to force fuel via an electric pump though a decommissioned mechanical one-

i considered doing it but then thought that if the seals or mechanical pump failed in any way you just be pumping fuel into the crank case, wouldn’t you ?

[RobH]

Yes Hamish which is why I wondered what they had done. I suppose it would be relatively easy to just remove the valves and blank it off with a plate where the diaphragm usually sits.  If they did leave any of the innards it might explain the odd symptoms .....

 

 

[MarcoD]

Yes those were my thoughts, but I dont know how they did, they may have removed the lever from the pump and fitted a blanking plate. 

I can see a plate between the pump and block.

Ill bypass it and see what happens!

[MarcoD]

I now have the pump info and it answers a few questions!

Link is here, if its ok to post links? https://weber-carbs.co.uk/shop/product/huco-low-pressure-12v-fuel-pump-133010/

Its a 2,1 psi pump, its a suction type, so it should be mounted in the engine bay not next to the fuel tank!

So the issue is with the new pump!

Ill try the bypass hose and post what happens!

[MarcoD]

Update and Conclusion

I bypassed the gutted mechanical pump and hey presto it now has enough pressure to keep the inline filter at least 1/2 full. so she now starts and idles ok.

Ill fit the correct pump (4 psi version) later but shes now running well enough for me to do the final ignition and carb tune.

Issues with the ignition switch, which is faulty but Ill start a new thread on that!

Thanks for everyone's input!

Mark