Effect of Fuel Pressure

[Brian B.]

Hello.  I searched previous posts for clarification on this but I could not find what I am looking for.

1. Could someone please advise how fuel pressure can affect how rich or lean a Quadrajet operate?
2. Can higher pressures impact the fuel curve like a larger jet? If so, how is this physically possible? 

Thank you.

[Brian B.]

Sorry. I did not phrase my first question correctly.  Can fuel pressure impact how rich or lean a Quadrajet operates?

[77cruiser]

Higher pressure can raise the fuel level slightly.

[Kenth]

Increased fuel pressure rises the fuel level in bowl.
Increased fuel level starts fuel delivery earlier, less pressure difference between atmosphere and intake manifold vacuum needed to pull over fuel from bowl to venturi.
Different Qjets have different length and sizes of the primary brass nozzles and due to that different float settings. Short nozzles generally has higher float settings.
Recalibration always starts with factory specs on fuel pressure and float setting, then go from there with idle tubes, idle channel restrictions, idle needle holes, idle bypass air, jets and rods.

[Brian B.]

Thanks to you both!

[Cliff Ruggles]

Something very important but not mentioned is that you MUST use the correct fuel inlet seat diameter as it also has an impact on fuel level in the bowl at any given fuel pressure.

Most if not all rebuild "kits" that you but do NOT have the correct fuel inlet seat in them so you aren't getting the fuel level where it should be right to start with. 

Unless I'm asked to supply something else I put the .135" fuel inlet seats in my rebuild kits.  Every single carb I get in here that was just rebuilt or restored has a small inlet seat in it, and every kit I see being sold these days also has the smaller ones in them.......FWIW.....Cliff

[Brian B.]

Thank you Cliff.

[old cars]

just to clarify. .135" seat would not be the original size seat necessarily

[Cliff Ruggles]

I'd also add that you MUST use the original type OEM float. 

NEVER use a brass float in a Q-jet.  Not only does this new fuel eat the seams out of them and they will sink to the bottom of the bowl they are too heavy and NOT the same buoyancy as the original nitrophyl units.  This effects fuel level and carburetor metering.

The new closed cell nitrophyl floats we sell are 100 percent dead solid reliable in all fuel blends.  Have had zero failures with them to date and they are far superior in fuel control than the brass floats folks are selling for these carburetors........Cliff