[NeighborsComplaint]

Check your float level first. With the engine shut off, remove the sight plug and bounce the front fender. Fuel should trickle out when you bounce it. Sitting static, the fuel level should be just barely below or even with the bottom of the sight plug opening. Also try rotating your choke slightly leaner. It may be staying on a touch too long and loading up on warmup.

That carb appears to be a Holley Spreadbore 650 with an electric choke it added. An older design, they are not equipped with the power valve protection check valve. The power valve diaphragm can be damaged by a backfire. You can
field" check the power valve without disassembling the carburetor. Warm up the car to clear the exhaust of any condensation. Have someone rev it (let it drop to idle, rev it and repeat) while observing the tailpipe tip. If you see sooty exhaust or liquid fuel drip from the tail pipe, then a leaking (blown) or out of spec for the engine's manifold vacuum is likely the cause. If the car smells gassy while driving and the tailpipes are always sooty, this is the probable cause.

That carb came from the factory with an 8.5 primary power valve which which can cause a partially opened (enrichened) condition if your engine idle vacuum is less than 15"-17". Pick up a vacuum gauge and check your idle vacuum through a port below the throttle blades (manifold vacuum) and see what you read. Divide that number by 2 and round up to the nearest available power valve size (8.5, 6.5, 5.5, 4.5, 3.5 are standard power valve sizes).

Regarding secondary operation, Holleys are different than Q jets. A Holley carb has only a single set of throttle blades, they do not howl on secondary opening as you may be accustomed to with a Quadrajet (air taking a more torturous path around the secondary air flap and secondary throttle blades and causing the secondary air flap to oscillate).

Properly adjusted vacuum secondaries on a Holley do not induce a snap when opening (which is actually a stumble) but are simply drawn open with a turbine like smoothness so as to be unnoticeable (other than the tire spin lol) as you roll into the throttle. You can check for proper function by holding a mirror at angle over the carb while operating the throttle and observing the secondaries or just watch the secondary throttle shaft rotate as viewed from the side of the carburetor while going to full throttle.

The rate and percentage of secondary opening will vary based on the engine's vacuum characteristics. On some cars, the secondaries may not fully open even at full throttle. Being based on the engine's requirements, full opening may not be required even at full throttle as the engine has already reached its vacuum limit and maximum air flow requirement. This is particularly true when a large carburetor is installed on a small CID engine. On a 400 like yours, the 650 should be pretty optimal for a stock or mild state of tune engine.

If you favor a more rapid secondary opening rate, Holley sells lighter springs that install inside the vacuum secondary actuator (flat-round canister just behind the choke). You have to remove the carburetor to remove the cover plate as the screws are threaded in from underneath "to keep the kids out". It is a very simple modification and some engines can benefit from more rapid secondary opening.

Companies like QuickFuel incorporate adjustable vacuum secondaries through an external screw which adjusts the speed of secondary opening by varying the internal spring's height (tension). Holley also sells a QuickChange cover for the vacuum actuator that allows the screws to be installed from above allowing the springs to be changed without taking the carburetor off.