Correct fuel pressure for twin Zenith-Stromburgs

[kpytoi1]

Does anyone know the correct required fuel pressure for a twin Stromburg set up? The OEM fuel pump has been replaced with an aftermarket one, the car runs too rich and throws fuel out the tailpipe, trying to remedy situation one step at a time. That's not to say that the carbs and everything else is perfectly set up, but I'm starting with the fuel pump. Can people weigh in on fuel regulators, or if anyone is using anything other that Malpassi FilterKing and why that has been the go-to, preferred set up. Also, is there a fuel return line on a 1968 2+2, and if not, where does the excess fuel go or when does the fuel pump stop pumping?
Thank-you.

Peter

[christopher storey]

I cannot speak for Stromberg equipped cars specifically, but as far as I know there is no fuel return on any of the 6 cylinder cars : the SU delivery pump is pressure sensitive and only pumps when the line pressure falls as a result of the needle valves opening. When SUs are used the maximum line pressure desirable is no more than 3 lbs/sq.in ( about 0.2 bar) and preferably nearer 2.5 lbs/sq.in ( about 0.16 bar) , and it is pressures in excess of these which are responsible for much rough idling on SUs. I doubt that the pressure required for Strombergs is different from this - the only real reason for any significant pressure is to counteract the backwards force in the fuel line under hard acceleration

[PeterCrespin]

Correct re specs and absence of returns on carbed sixes (Series 3 injected XJ6 have one). Pressure, however, is more a function of fuel head, i.e. the need to raise fuel from tank to carbs. In theory, the geometry and configuration of differing float systems (pivot vs direct-acting etc) could alter the pressure cut-off point but in practice everything from Webers to SU to Strangleberg float systems operate fine on 2-2.5 psi and it's flow at WOT and high revs that becomes a critical performance parameter.

Even pumpless gravity-fed systems operate at a slight pressure head. Anyone astride an old British twin, who has seen fuel spurting from the carb(s) onto the running magneto directly underneath, will remember the queasy feeling of incipient 'nut roast', immediately followed by a lunge for the kill switch and fuel tap in quick succession...

Pete