Importance of fuel pressure on carburettor (down draft) tuning

Oldskoolrob

Active Member
So I'm slowly improving my electric fuel pump installation. I now have an inline pressure gauge just after my filter and just before the carby. I'm getting a consistent 5psi. My understanding is that holleys like 6/7psi best. Will it make much of a difference only giving it 5? I only have a basic understanding of tuning these carbies but I'm looking to learn.
 
It is not only pressure, you need to consider flow as well. I would disconnect the fuel line at the carb and point into a bucket and time how long it takes to dispense a gallon of fuel.
Then find out how many gallons per hour your motor will need at wide open throttle.
IIRC I worked out my 4.6 needed 16 gph, and my fuel line gave 20 gph.

In making any adjustment to flow / pressure you will need to re set the fuel height in the bowls.

Edit
I found this site which might help you. "What Size Fuel Pump Do I Need?" Your Top Tech Questions Answered! - OnAllCylinders

I ran my stats into their calculator and came up with a fuel requirement of nearly 21 gph. So my fuel delivery is only just sufficient. I will not be able to run at WOT for a sustained period so I am not overly worried about it.
Bear in mind the Holley site uses US galls as a measure.
 
Last edited:
Hello,
You lost me a bit there, but you may be able to help me.
I got my car to start after many years of non use
I had a leak on the fuel bowl, so decided to clean it out.
The filter has a diaphragm control, this was in a mess, having cleaned the whole lot, and not altering the pressure setting, on starting the electric pump, independent to starting the car, I had a deluge of petrol coming from the carb overflow pipes. ( small bore tubes from the top of the carbs)
It seems the gummed up diaphragm must have controlled the flow and I have may be increased the flow some how.
So, I will need to get a new diaphragm and filter, soon, but I think until I get these, I can still run the car if I can reduce the fuel pressure to a correct level to stop the overflow, I don't think the carbs non return valves are the problem
HIF6 carbs
P6 3500 auto
1974
Thank you
Brian
 
I think I ran my HIF6 at something like 2 psi, they hate pressure, 3 psi and mine would flood and they were overhauled.
 
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