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MG Midget and Sprite Technical - Primary pump - gizmo to run it 50% of time?

Hi

I finally got round to plumbing in the old SU pump to prime a swirl-pot so I can run injector pump with tank below 1/3 (on road) and below 1/2 on circuits. I know... should have done this a long time ago.

So the SU pumps away energetically to keep the one-litre swirl pot full (and overflowing back to the tank).

Does anyone have an idea for a simple gizmo that will run the pump for (say) 50% of the time? Can be electronic, electro-mechanical... might be best if there was a unit that did something similar already... otherwise will break out the 555 timer and pin-board...

Thx in advance.

Anthony
Anthony Cutler

something to measure flow on the return pipe and switch the pump off/on accordingly.
Mick struggling with the wiring

Why would you only want to run the pump 50% of the time?
Robert (Bob) Midget Turbo

We have a simple gizmo, its called a needle valve.
We set an SU float chamber lid into the top of our swirlpot. The SU pump feeds into that. The secondary pump then does its bit cycling from the swirl pot to the engine as usual. When the swirlpot is full, the needle valve shuts off in the normal manner.
We have connected up the overflow to vent to the ground just in case it's needed, but this has happened only once.

A word of caution: You definitely need something to save the pump running all the time. For some reason which I can't recall, the primary pump once ran continuously. It may have been an air leak on the inlet due to a damaged hose I think. Whatever the reason, it wasn't noticed by the son who was driving it, and the end result was that the pump ran for quite a long time and overheated, causing the points pedistal to sag & crack.

Paul Walbran

The SU pump filled the 1-litre swirl pot in around a minute - faster than I expected. This means it's pumping in the order of 12 gals / hr. That's around 500-miles worth of fuel every hour. So (if my arith is correct), the SU pump only needs to run for between 1/10 and 1/5 of the time; using the higher figure means the pump might last 5 times longer than otherwise - that's the motivaton.

Will Munns and I discussed using an SU-float-lid some while back ...what's kept me away from this to date is that I'd need to cut a round hole in the brand-new swirl-pot top... not easily reversible, without a TIG; hence I've been looking for alternates.

A
Anthony Cutler

Using Paul's solution does that simply stop the pump operation because of pressure build up (needle valve closed?)

If so do you believe that is better for the pump than simply running to a tank 24/7 ?
Bob Turbo Midget England

Hi Bob

That's how the SU pump is designed to work... the solenoid pushes against the spring (creates a vac to let fuel in), and releases the spring to pump the fuel out. So the phase of pumping the fuel out doesn't require any electric contact/power; the pump is stalled in a benign manner (as if it were switched off).

[OTOH if the pump is stalled when solenoid engaged / sucking the fuel in, the points are closed and the solenoid is powered / heating up... the pump life would be short as the points burn and the coil melts... took the pump apart to clean it and find out how it works.]

So I'd say stalling the pump as it was designed to do, or cutting power amount to ~same thing.

If I were to work out how many pulses the original pump did in 80K miles, I guess it would be a considerable number... still, it seems unnecessary to have it running all the time when it could be stalled for 4/5 of time.

A
Anthony Cutler

It was a simple thing to do for us as we started from scratch making our own swirl pot. If we hadn't, the possibility of mucking up one which had cost money might have given us the same cause to pause & think.

The SU pump is energised only briefly each cycle as the solenoid retracts the diaphragm quite rapidly. Once retracted, the points open, turning off the current to the solenoid and allowing the spring to create the pressure as Anthony says. This can clearly be heard in the standard application - each click of the pump is the solenoid retracting the diaprhagm. Once the bowl is full, the pause of several seconds between each click is the time when the power is interrupted and the spring is doing its thing ... ie most of the time.

If the pump is cycling continuosly on what is effectively open circuit in the fuel line, (for example, when the carb bowl has been drained and is refilling) then the proporion of time with the coil energised is much increased.

Having said that, the proportion of time spent in the power-on part of the cycle will still be much less than if the pump is sucking air, when the cycle is very much faster again.
Paul Walbran

the original pump stalled once the floats were full IIRC, so if there was some kind of restriction on the return from swirl pot to tank then might that result in the pressure in the pot being big enough to stop the pump? as the return from the high pressure rail will also be pressurising the pot? just a thought..

I'll be making one soon as well, as rapid progress is halted by intermittent fuel if I've got less than half a tank...
Rob Armstrong

This thread was discussed between 11/07/2011 and 12/07/2011

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