What's the purpose of each of the two mixture screws on the Rochester 4GC 4-Jet carburetor on my '65 Vista Cruiser?

 

They serve the same purpose as a 2GC or Q-Jet mixture screws. It sets the idle mixture. Best adjusted using a vacuum gauge or by ear if you're good enough. They are for idle only and are non operational once throttle is applied and the primary throttle plates open.

 

X2 - they are the idle air/fuel mixture screws. As stated you can adjust by ear, with a vacuum gauge, or tach. Needs to be adjusted last after dwell (if applicable) and timing.

 

So the left screw adjusts the mixture for the left primary, and the right screw the right primary? Meaning you should adjust them equally, which is what I did to lean the mixture.

 

Yes, adjusting the passenger side screw is for the passenger side bank of cylinders, adjusting the drivers is for the drivers side bank.

 

Nope.

A dual-plane intake manifold will have one set of runners feeding the outside cylinders of one bank, and the inboard cylinders of the other bank. The other plane will be reversed.

The effective firing order is that every cylinder firing alternates between upper and lower plane of the manifold.

The idle mixture screws adjust the idle mix for the two planes of the intake manifold, not for the two banks of cylinders.

 

Major brain fart in typing. Sorry