[Healeys] Fuel pumps (AGAIN!)

Michael Salter michael.salter at gmail.com
Thu Nov 29 10:53:07 MST 2018

Hi Larry,
Although I can appreciate that decreasing the delivery pressure has
resolved issues with excessive fuel in your float chambers I'm not sure
that you are entirely correct with respect to your interpretation of the
specs for these pumps.
As far as I can determine, despite the fact that this is a fuel pump,
pressures are always expressed in inches of WATER pressure. That would mean
that the delivery head (48") would be the equivalent of 1.7 p.s.i. not a
big difference.
However when I check the specs for the LCS pump (BJ7/BJ8 Workshop Manual)
the delivery head is quoted as 4 feet BUT they also quote the CUT-OFF
pressure as 3.8 p.s.i.(105.18 inches of water which is around 150" of
fuel!!! ).

Now of course the $38,000 question is: what is the difference between
"Delivery head" and "Cut-off pressure"?

Well despite considerable research I have never been able to find a
satisfactory answer however, what I have been able to confirm, is that the
standard metal needle valve in SU carburettors should not leak until the
delivery pressure exceeds 5 p.s.i. so decreasing your delivery pressure to
1.5 p.s.i. from 2.5 p.s.i. should really have made no difference, unless of
course both the cars that you tested has some leakage at one or both needle

Any ideas?


On Wed, Nov 28, 2018 at 9:15 PM Larry Varley <varley at cosmos.net.au> wrote:

> Hi Group. Something worth considering when dealing with SU carburetors and
> fuel pumps. I have been doing some investigation and testing the effects of
> fuel pressure on SU carburetors. The image attached is the correct
> specification for SU fuel pumps for the brass float version carbs. The SU
> HP pump is quoted as 48 inch( petrol ) lift which is about 1.5 PSI. This
> document is from a 1950’s workshop manual. I have tested one of my cars and
> the SU pump sold by all of the usual suppliers has a petrol lift of over 10
> feet. I estimate at least 2.5 PSI. I had my brother test his Healey 100
> also and the same result. The test was conducted attaching a clear plastic
> line from the fuel supply line up in the air to show the lift. We have both
> fitted pressure regulators and using the clear tube method set the pressure
> to 48 inches ( 1.5 PSI ). The result in both cars that have wide band
> oxygen sensors fitted is an immediate improvement in air fuel ratio at
> idle, moving away from overly rich mixture. Both cars maintain a far better
> and regular idle as they are no longer running rich at idle. I believe that
> the carbs are barely able to maintain a constant fuel level in the float
> bowls when the pressure is 2.5 PSI and that pressure pushes the fuel level
> up in the bowls when the engine is not consuming much fuel. At all other
> speeds both engines fine with good air fuel ratio at 1.5 PSI fuel pressure.
> Regards
> Larry Varley
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