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<p>Now THAT'S a new data point. I was confused as to why SU pumps
came in 'low pressure' and 'high pressure' but it appears 'high
pressure' is relative:</p>
<p><a class="moz-txt-link-freetext" href="http://sucarb.co.uk/technical-electrical-fuel-pumps">http://sucarb.co.uk/technical-electrical-fuel-pumps</a></p>
<p>So, the HP pumps are meant to be mounted near the tank. Usually,
these are quoted to be 3psi, so your estimate is pretty close.
Still doesn't explain why some of us 'claim' to have had issues
with them.</p>
<p>Bob<br>
</p>
<div class="moz-cite-prefix">On 11/28/2018 2:05 PM, Larry Varley
wrote:<br>
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<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Regards<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Larry
Varley<o:p></o:p></span></p>
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