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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
valves.
Any ideas?
M
On Wed, Nov 28, 2018 at 9:15 PM Larry Varley <varley@cosmos.net.au> wrote:
> Hi Group. Something worth considering when dealing with SU carburetors an=
d
> 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=E2=80=99s workshop manual. I have tested one of m=
y 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 plasti=
c
> line from the fuel supply line up in the air to show the lift. We have bo=
th
> fitted pressure regulators and using the clear tube method set the pressu=
re
> 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 bett=
er
> and regular idle as they are no longer running rich at idle. I believe th=
at
> 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 leve=
l
> 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 pressur=
e.
>
> Regards
>
> Larry Varley
> _______________________________________________
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<div dir=3D"ltr"><div class=3D"gmail_default" style=3D"font-family:comic sa=
ns ms,sans-serif;font-size:small">Hi Larry,</div><div class=3D"gmail_defaul=
t" style=3D"font-family:comic sans ms,sans-serif;font-size:small">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 en=
tirely correct with respect to your interpretation of the specs for these p=
umps.</div><div class=3D"gmail_default" style=3D"font-family:comic sans ms,=
sans-serif;font-size:small">As far as I can determine, despite the fact tha=
t this is a fuel pump, pressures are always expressed in inches of WATER pr=
essure. That would mean that the delivery head (48") would be the equi=
valent of 1.7 p.s.i. not a big difference.</div><div class=3D"gmail_default=
" style=3D"font-family:comic sans ms,sans-serif;font-size:small">However wh=
en I check the specs for the LCS pump (BJ7/BJ8 Workshop Manual) the deliver=
y 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!!! ).<br></=
div><div class=3D"gmail_default" style=3D"font-family:comic sans ms,sans-se=
rif;font-size:small"> <br></div><div class=3D"gmail_default" style=3D"font-=
family:comic sans ms,sans-serif;font-size:small">Now of course the $38,000 =
question is: what is the difference between "Delivery head" and &=
quot;Cut-off pressure"?</div><div class=3D"gmail_default" style=3D"fon=
t-family:comic sans ms,sans-serif;font-size:small"><br></div><div class=3D"=
gmail_default" style=3D"font-family:comic sans ms,sans-serif;font-size:smal=
l">Well despite considerable research I have never been able to find a sati=
sfactory answer however, what I have been able to confirm, is that the stan=
dard metal needle valve in SU carburettors should not leak until the delive=
ry 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 cours=
e both the cars that you tested has some leakage at one or both needle valv=
es.<br></div><div class=3D"gmail_default" style=3D"font-family:comic sans m=
s,sans-serif;font-size:small"><br></div><div class=3D"gmail_default" style=
=3D"font-family:comic sans ms,sans-serif;font-size:small">Any ideas?<br></d=
iv><div class=3D"gmail_default" style=3D"font-family:comic sans ms,sans-ser=
if;font-size:small"><br></div><div class=3D"gmail_default" style=3D"font-fa=
mily:comic sans ms,sans-serif;font-size:small"><br></div><div class=3D"gmai=
l_default" style=3D"font-family:comic sans ms,sans-serif;font-size:small">M=
<br></div><div class=3D"gmail_default" style=3D"font-family:comic sans ms,s=
ans-serif;font-size:small"><br></div></div><br><div class=3D"gmail_quote"><=
div dir=3D"ltr">On Wed, Nov 28, 2018 at 9:15 PM Larry Varley <<a href=3D=
"mailto:varley@cosmos.net.au";>varley@cosmos.net.au</a>> wrote:<br></div>=
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex"><div lang=3D"EN-US" link=3D"blue" vlink=3D"p=
urple"><div class=3D"m_6882488397386520852WordSection1"><p class=3D"MsoNorm=
al"><span style=3D"font-size:11.0pt;font-family:"Calibri","s=
ans-serif";color:#1f497d">Hi Group. Something worth considering when d=
ealing with SU carburetors and fuel pumps. I have been doing some investiga=
tion 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=E2=80=99s workshop manual. I h=
ave tested one of my cars and the SU pump sold by all of the usual supplier=
s 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 conducte=
d 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 a=
t idle. I believe that the carbs are barely able to maintain a constant fue=
l level in the float bowls when the pressure is 2.5 PSI and that pressure p=
ushes 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.<u></u><u></u></span></p><p class=3D"MsoNormal"><span st=
yle=3D"font-size:11.0pt;font-family:"Calibri","sans-serif&qu=
ot;;color:#1f497d">Regards<u></u><u></u></span></p><p class=3D"MsoNormal"><=
span style=3D"font-size:11.0pt;font-family:"Calibri","sans-s=
erif";color:#1f497d">Larry Varley<u></u><u></u></span></p></div></div>=
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