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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><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></div></body></html>