Malpassi fuel pressure regulator

[tim wood]

Hi all,
I have one of these fitted and have set the pressure to approx 2.5 psi. The car runs well.
Whilst fiddling today I fitted the gauge and watched whilst the engine was running. Most of the time the gauge hovered around the 2.5 mark but occasionally the reading fell to zero. Is this to be expected or am I about to see fuel pump failure?

Thoughts appreciated

[mgcjag]

Hi Tim....is it a newish pump...i have heard that some are eratic and not as good as the old ones...dont know how true this is.....or even if it may be your problem...Steve

[tim wood]

Hi Steve,
The pump is old but I’ve recently fitted one of the boards made by Marek which reduces load on the points.
New points at the same time.

[mgcjag]

Try running the engine but listen to the pump....should hear it ticking....does it stop when pressure drops

[Series1 Stu]

Surely this is normal operation. The pressure will fluctuate as the engine uses fuel and the pump cuts in to restore the level in the float chambers.

Don't worry about problems that might occur. Just enjoy driving your car.

Regards

[Simonpfhc]

Hi Tim,

I have a gauge fitted to mine and the pressure never fluctuates. The pressure drop you experience has to be caused by the pump but may not necessarily be a fault. I run an in-tank pump from SNG and that runs constantly with the ignition on. Your SU may cut in and out depending on load.

Cheers.

[MarekH]

A regulator is typically only useful if a non-standard higher pressure pump is fitted and works by capping the pressure and returning the excess to the tank. The SU pump already has that cap enabled by design. The pump spec is chosen to shift enough volume of fuel at redline rpm and do so at a pressure sufficient to overcome the spring/jet holding the float bowls full of fuel and no more.

As an experiment, you could up the pressure setting and see how strong the pump actually is.

If you have a failure to close the float bowl at the carburettor, then all that will happen will be that fuel pours out at 2.5psi instead of 3psi (or whatever the SU pump normally delivers).

If you are accelerating hard or running at redline rpm, you probably want those float bowls refilled as fast as the carburettor allows, rather than at a lower cap, to avoid any potential lean condition under heavy load.

kind regards
Marek

[mgcjag]

Hi Tim.....are you only running the car a low revs....if so and your not useing much fuel the bowls could be full and the pump cut off....if however you run at higher revs the bowls should be down and demanding fuel......also are you floats set correctly...Steve

[tim wood]

Steve,
Good thought about the floats.
I’ll check these out


Tim

[christopher storey]

I rather think that Marek is thinking of the V12 system which has a fuel return. The 6 cylinder cars have no return flow . Your pump is clearly a traditional pulse type SU, which will show steady pressure for a while,( whilst float valves are shut and the pump is not working ) then when a float needle valve opens the pressure will drop until the pump beats when it will rise again, stabilising as the float valves shut and the pump stops pumping . I doubt that there is anything wrong with your system . The pressure you have set of 2.5 lbs is probably perfectly adequate although if you felt there was any starvation under hard acceleration you could raise it to 0.2 bar = 2.8 lbs

[MarekH]

No I'm not Christopher.

Perhaps I should have used the phrase "may return fuel to the tank", as a regulator designed for use with a low pressure lift pump such as the SU doesn't need a return, whereas one used with a high pressure, high volume pump for fuel injection, will.

The word "cap" doesn't imply that that pressure is seen 100% of the time and at no point in my post have I remotely implied that the pressure is held constant at that cap. All I have said is that having a regulator almost certainly serves no purpose as the oem hardware has already been chosen accordingly.

By contrast, the regulator may serve a purpose if a non-standard higher pressure pump is fitted. In this case, it'd want to ensure that the line pressure is not so high that the already full float bowls are overfilled which can happen when pump pressure exceeds atmospheric pressure. Pumps used for fuel injection typically run at three times atmospheric pressure.

kind regards
Marek

[ralphr1780]

I have an SU pump and a Malpassi regulator on 2 cars, which I have set at about 2.5. Incidentally last week I controlled the pressure on both with engine running at idle and raising revs to 3000rpm: pressure never drops to 0 even if you let engine on idle for few minutes. Furthermore pressure is retained when engine is shut, but did not test for how long.
I do this test every spring to check for any change in running pressure.

[Heuer]

From the SU publication 'SU Carburetters Tuning Tips & Techniques':

[PeterCrespin]

“..previous types of S.U. high pressure electric fuel pumps..” ???

I’ve never seen any pumps lower than SUs. Even American carb fuel pumps tend to be a pound or so higher.

[Gfhug]

I'm getting more confused . Is fitting a Filter King to a car with the standard SU pump worth doing or not?
Previous posts, including from illustrious people, seem to recommend them, but is that only if you've got a none standard pump and/or none standard air filter and carb needles?

[mgcjag]

Hi Geoff....have a look here...an old thread by Heuer..explains why he fitted one....if you think your haveing problems it may help.... http://forum.etypeuk.com/viewtopic.php?t=164

[Gfhug]

Thanks, Steve, that's the illustrious post I was thinking of. It's just that there now seems to be contradictory information, hence my

[Heuer]

1. The SU pumps run at a pressure of 4 psi or more (I measured 3.8 psi on my car)
2. The SU carbs require a pressure of 2.5 psi to work correctly
3. High pressure can cause the carbs to leak, increase fuel consumption and overly rich running at idle
4. The Filter King allows owners to easily adjust the fuel pressure delivered to the SU's
5. If you are happy to run your car with 50% over pressure then leave things as stock although there is the potential for problems to develop sooner than they might
6. It is just an option but many of us find it worthwhile as it removes a potential problem source when diagnosing issues

[Gfhug]

David, thank you as always. You never know, I may not be the only one who appreciates this clarification.

Geoff

[Richardhealey]

David, thank you as always. You never know, I may not be the only one who appreciates this clarification.

Geoff