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The PO installed a new (rebuilt) carb. I figure I need to remove and see what size jet it has. Does anybody know what it should be? Then I figure I need set the idle mixture screw. I presume the shop manual has something about this. Any other ideas for me?


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Engines require different air-fuel ratios depending on speed and load. At idle, the mixture should be slightly rich (generally in the 13.5 to 1 range). At cruise, it should be close to stoichiometric (14.7 to 1). at wide open throttle, generally around 12.5 range.

Carburetors of this era run an idle circuit (throttle plates barely open) where the mixture is controlled by a screw for each barrel. After the plates open the orifice in the jet (or jets) controls. At WOT the power valve adds a little more fuel. You also have an accelerator pump that gives a shot of gas into the venturi if you quickly open the throttle to prevent bogging and stalling.

Always tune the carburetor last. Perform a good ignition tune first, including new spark plugs properly gapped, as well as cleaning the PVC system. Idle mixture should be set by gently bottoming out the mixture screw than opening up 1.5 turns. Then start the engine and let it warm up. Tighten the screw by 1/4 turn increments to where the idle speed drops or turns rough, then back out 1/8 turn. Do each barrel separately. If your idle speed needs to be readjusted you may have to tweak the mixture too.

Verify accelerator pump operation and proper placement of the levers. A rebuild kit for the carb will have instructions.

Aside from using a wide band O2 sensor with appropriate electronics to determine AF ration during cruise operation, the old school method is to simply drive the car for several hundred miles then inspect the spark plugs. Line 'em up in order on your bench and compare them to color charts that are widely available on line. Generally a dark color (black) indicates too rich, white too lean, and a light tan spot-on. Swap out jets in small increments, clean the plugs and drive the car again...
 
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