Hello…two questions on fuel pump pressure. What should be the fuel pump pressure for a 74 Etype V12 with Stromberg carbs? My car currently has an aftermarket pump, and not sure if the pressure is right and is causing some of the plugs to foul up. So the 2nd question is at what pressure will the plugs start fouling up? Sandy
I’m going from memory here – and can’t find the spec in the repair manual – by believe fuel pressure for naturally aspirated engines is 2-4 PSI.
I can’t even begin to answer question 2
Thanks Craig. This is helpful…i am also trying to understand at above what PSI does the Carbs run rich? Also is 2-4 a wide range? If my carbs are running rich what PSI should i be at? Sandy
The v12 has a fuel return, so any excess fuel returns to the tank. The purpose of the fuel pump is only to refill the float bowls in the carburettors. Once these are full, the floats rise and cut off the feed from the fuel pump. Accordingly, the fuel pump can only have an effect if the pressure is so high that it is easier to push the floats down than it is to return to the tank - that means the petrol will have to be overflowing into the carburettor throats.
The carburettor needle height dictates how rich/lean the carburettor will run, subject to the airflow being set correctly (throttle butterfly, linkages, vacuum leaks from peripherals, choke,etc) .
Doesn’t sound like pressure is the problem, sounds like fuel levels in the float chambers?
Flooding due to stuck valve and or float problems is where my head was at Marek.
Thanks…two follow up questions…Marek on your note you say…that a fuel pump can have an effect is too high it can push carb floats down versus return to tank. Any feel for what that pressure level is? At this point i do not know what the pressure of this after market pump is in my car. I will find out this week.
If the issue is fuel levels in float chambers…what is the right way to diagnose this?
The fuel has to go somewhere - as Les says, if it doesn’t return to the tank, then it’ll flood out of the carburettor throat if the float doesn’t rise and shut off the supply.
I have no idea what pump you are dealing with so can’t advise, but there are only two flavours of fuel pump:- low pressure for carburettors; high pressure for fuel injection. Google the part number or connect a gauge.
Questions I’d ask myself are:-
Are all four carburettors behaving the same way?
Does fuel come out of the needles/jets if I peer into the throats?
Is the choke disc so badly corroded that fuel leaks through?
Is the fuel return working?
It seems unlikely that all four floats are set so that the level is so high it’d always run rich unless someone has set it up incorrectly.
I just had both of the original floats in my Strombergs sink: it ran, but ran very rich. Be prepared for $50 each, to replace them, If that’s what it needs.
I have a pressure gauge on mine. I usually run it at about 4 to 5 PSI. Indeed excess fuel is being returned to the tank. Too much pressure should not affect mixture but could cause the grose jets to be overwhelmed and the carb to overflow.
The fuel return line has a valve just being the left side rear carb. It s just a spring with a SS ball inside. It can cause issues. If it doesn’t open, pressure may be too high. If the spring is too weak it will dump too much fuel back to the tank and cause fuel starvation on the left bank.
Although not in the ROM, fuel pressure was quoted as 1.5 psi in the original S3 Press Release.
The same figure was quoted in the 1971 and 72 US Brochures.
Thank you….this is so helpful. So if the recommended pressure is 1.5PSI, and it is a full recirculating system….is the only issue of a much higher pressure above 4.5 some leakage/overflow or there other issues on how the car will run? Are there driving performance symptoms for a fuel pump that is putting much more PSI than recommended?