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April 21, 2021, 12:23:37 AM

Fuel Pump, Regulator, Fuel pressure

This is a discussion for the topic Fuel Pump, Regulator, Fuel pressure on the board Beach Buggy Electrical Help.

Author Topic: Fuel Pump, Regulator, Fuel pressure  (Read 768 times)

this user is offline LUDICRUZ

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on: September 01, 2020, 08:26:53 AM
Hi folks,

I have spent several hours today reading up on fuel pumps, regulators and wiring on this forum, however there are some unanswered questions rattling in my head, that i cannot see anyone asking.

The reason could purely be that my questions are totally silly and there there are simple answers to them, however i'm still going to throw them out hoping someone can shed some light please.

i have a new 1641cc engine with Weber twin 40 IDF's.

i have a new Facet Red top cylinder fuel pump Model 480532E which runs at 6.0 - 8.0PSI. I also have a new Malpassi Filter King glass 85mm regulator.

I understand that for my use the current Pump i have may well be overkill for my needs as i should have a setup that produces around 2.5PSI based on my readings.

So plans will be to opt for the Facet Silver top cylinder pump Model 477060E which produces 2.75 - 4.0PSI.

The following are my questions please...

If the new pump i purchase pushes out between 2.75 and 4.0PSI of fuel to the regulator, i then set the regulator to feed only 2.5PSI to the Carbs. What happens to the additional fuel that the pump is still pumping because it can go up to 4.0PSI?

Will the pump start to over heat because its pushing more fuel than the regulator is accepting or does it automatically know to slow the rate down to what the regulator is taking in and how does it do this?

If the car is at idle and the regulator chamber fills up, what will happen to the excess fuel that the pump is still pushing, does this require a return feed line back into the petrol tank for overflow petrol or will the regulator chamber never fill up?

Has anyone fitted a return fuel line?

If the carbs fill up with fuel and the float reaches the top point and and stops any more fuel entering in, can the fuel pressure from the regulator over power the floats to open up again and flood the carbs?

Apologies if my questions sound daft but i am just trying to understand the flow of the fuel.
THE HARDER I WORK, THE LUCKIER I GET - HENRY FORD


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Reply #1 on: September 01, 2020, 08:45:58 AM
the pump will near stall and just cycle, the old SU pumps cut off at pressure then restarted when it dropped my current Facet pumps fast with a loud tick then at pressure slows down and the ticking sound is deeper as the pump sort of stalls the fuel cannot exit so no new is pulled in.
I had a return line on mine years ago as the Fiat mechanical pump had no cut out bypass it just kept pumping until something broke so had a 6mm line into the carb and a 4mm return to the tank filler neck.


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Reply #2 on: September 01, 2020, 12:17:08 PM
High, The Malpassi pressure regulator is a simple single diaphragm pressure regulator, it will TRY and keep the outlet fuel pressure at what ever you set it at but if fuel consumption is low ( ie idle/tickover or low throttle) it will cut of the fuel flow altogether or allow only very minimal flow until the pressure in the outlet side of the regulator drops. Facet say there pumps won't damage under no flow situations BUT....now read my actual experience of using a Facet silver top pump and malpassi regulator.

I run the silver top pump and Malpassi regulator, it had been on for 2 years (4000 miles) with no problems whatsoever, then on a trip to France whilst sat in a 45 minute queue for the return ferry in 40 degree heat the pump went noisy and then stopped altogether, my fault I should have stop started whilst in the queue. I think the pump got that hot the fuel vapourised inside it. I turned off let it cool down, gave the pump a smack with my hand and it started up again,  First job I did when I got home was to fit a return pipe back to the tank. So as not to reduce the flow to the carb I fitted a brass restrictor  in the return pipe, the restrictor only has a 1.5mm hole through it. 
Have fun. Be lucky


this user is offline LUDICRUZ

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Reply #3 on: September 01, 2020, 01:21:15 PM
the pump will near stall and just cycle, the old SU pumps cut off at pressure then restarted when it dropped my current Facet pumps fast with a loud tick then at pressure slows down and the ticking sound is deeper as the pump sort of stalls the fuel cannot exit so no new is pulled in.
I had a return line on mine years ago as the Fiat mechanical pump had no cut out bypass it just kept pumping until something broke so had a 6mm line into the carb and a 4mm return to the tank filler neck.

Thank you for your answers, however i am still a bit confused over the pump.

According to Facet, they indicate on their website that the pump is running constantly, so my assumption is that it is constantly pushing fuel at the higher PSI.

As you mentioned the pump slow down when there is back pressure from the regulator.

If there is this back pressure then is there anything within the pump (mechanical or electronic) that controls the flow to the regulator?
THE HARDER I WORK, THE LUCKIER I GET - HENRY FORD


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Reply #4 on: September 01, 2020, 01:47:35 PM
Nav there must be something in the pump thar limits is to the 8psi or it would just keep pressurising  until something failed. I assume a solid state pump will not overheat like a stalled brush motor in stall conditions.
 I have a facet red top on the buggy with a malpessi regulator the first one did 5 years and 40,000 miles plus before it died but corrosion had made it unrecognisable under the buggy.
you would guess the non return valve on the outlet would stay open at 8 psi so the pump pulls no more fuel in but just pulses what is in the outlet.


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Reply #5 on: September 01, 2020, 03:20:22 PM
Think of a facet pump as a solenoid, a spring loaded iron core slug inside a tube with a coil around the outside of the tube, the coil has solid state control to switch on and off, so transistors,diodes,resisters. under normal operation, coil is energized, iron slug moves up, power to coil is cut, spring pushes iron slug back down, repeat this operation 3 or 4 times a second, the slug then pushes fuel along the pipe, the larger the diameter of the slug and the more times per second it moves will increase flow and pressure. If you block off the flow say with a bowl and float as in a carb, the pump still moves the slug up and down to try and pump fuel, you will then have a small standing pressure in the outlet side. Because the iron slug is a loose fit inside the tube with fuel in between the slug and the sides of the tube it is still quite happy to move up and down and no damage occurs. because the the iron slug is then trying to compress the fuel in the outlet it moves a little bit slower. It cannot infinitely increase the pressure in the outlet though because any excess pressure just passes backwards through the gap between in between iron slug and the tube.
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Reply #6 on: September 01, 2020, 03:59:46 PM
Think of a facet pump as a solenoid, a spring loaded iron core slug inside a tube with a coil around the outside of the tube, the coil has solid state control to switch on and off, so transistors,diodes,resisters. under normal operation, coil is energized, iron slug moves up, power to coil is cut, spring pushes iron slug back down, repeat this operation 3 or 4 times a second, the slug then pushes fuel along the pipe, the larger the diameter of the slug and the more times per second it moves will increase flow and pressure. If you block off the flow say with a bowl and float as in a carb, the pump still moves the slug up and down to try and pump fuel, you will then have a small standing pressure in the outlet side. Because the iron slug is a loose fit inside the tube with fuel in between the slug and the sides of the tube it is still quite happy to move up and down and no damage occurs. because the the iron slug is then trying to compress the fuel in the outlet it moves a little bit slower. It cannot infinitely increase the pressure in the outlet though because any excess pressure just passes backwards through the gap between in between iron slug and the tube.

Thank you...now i understand...so there is no large amount of pressure built up between the pump and the regulator that would cause the pump to overheat and tolerances allow all to work well, however there are instances where some people have utilised a return fuel line as a precautionary means.
THE HARDER I WORK, THE LUCKIER I GET - HENRY FORD


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Reply #7 on: September 01, 2020, 04:24:15 PM


Thank you...now i understand...so there is no large amount of pressure built up between the pump and the regulator that would cause the pump to overheat and tolerances allow all to work well, however there are instances where some people have utilised a return fuel line as a precautionary means.

Correct, the maximum no flow standing pressure you can achieve is the the max pressure the pump is rated at  So for the silver top you have seen that would be 4.0 psi, they don't have a check valve in them either so when turned off pressure returns to 0. Another thing to note is that they are designed to push fuel, so ideally should be fitted at the front underneath the tank allowing gravity to fill the pump with fuel, although I have seen plenty fitted at the rear of a buggy. As pumps go they are also quite noisy. The return pipe I have is connected in between pump  and regulator and then back to the tank.
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