1960 Rochester 4gc idle bypass big screw specs

[paulkersey]

Hello,

My Rochester 4GC  Instruction Sheet (on left pic) shows Idle by-pass screw: 2 turns out.
My rebuilt carb will not idle, just dies. mixture screws 1-1/4 out.

1959 Shop manual 10-69 shows Idle by-pass screw: 3-4 turns out.

next try will be at : Idle by-pass screw 3 out and starting with idle screws 1-1/2 out.

Just checking to see what  4gc owners are setting  big screw since i have different spec. sources

[dplotkin]

The big screw adjusts to get the highest possible manifold vacuum with the smoothest possible idle, you use a vacuum gauge while adjusting, it is not necessarily a prescribed number of turns.

Dan

[1959Fleetwood]

Hello,

I've rebuild many carburetors including the Rochester 4GC. When adjusting i always go by the shopmanual specs because the sheet that comes with a rebuild kit is not specific enough for Cadillac.
Turn the by-pass screw 3 to 4 turns out and the mixture screws 2,5 turns out to start with. When the engine is running you can use a vacuum gauge or a smog tester to adjust further after reaching normal operating temperature and choke off. Most of the times it's just listening to the sound of the engine when adjusting ( rough or smooth idle ). Keep in mind that these engines need something to drink to operate well so don't go to lean while adjusting.
Matern Harmsel
CLC 15331

[paulkersey]

Thanks all.
I will get a vacuum gauge and go forward.
I saw the shop manual air by-pass screw specs differed than the 4GC manufacturer specs a little too late and broke in the car big screw 2 turns out. Maybe that is why it would not idle.

[INTMD8]

Timing is set?

[paulkersey]

Dwell steady around 30. timing very near 5.
I will check it again once I can get it to idle down OK.
If not it will be another two-person job.