Don't know how familiar you are with that carb? They're the single most 'adjustable' carb ever built. Within that you can swap both the mains and secondaries jets. They screw into the floor of the float bowl. The metering rods, actuated by vacuum signals, lift or descend in the jets. Only 2 metering rods, secondaries are direct fed based on WOT vacuum.

On top of carb there are two little metal covers held by single screws. These cover the step-up pistons and springs and metering rod attachment. The springs can be changed for different pressures, allowing a tailored step-up function. The step-up pistons are directly connected to the metering rods. You can change jets & metering rods & step-up springs to tailor the carb for user preference.

Possible that a previous owner got a mismatch of parts going there, throwing off the balance. Might want to remove step-up piston covers and take a look see at those parts. Check carefully how step-up piston spring captures metering rod, and watch that spring that it doesn't fly off if removing rods! Metering rod size is stamped into side of metering rod, and jet size into top of jet body. You only have to remove carb top cover to see or change the jets or accelerator pump.

Vacuum circuits are cast/machined into carb body so unlikely there's a leak there. 3 separate vacuum circuits, idle, cruise and WOT. On front of carb there's a ported vacuum nipple and a full vacuum nipple for distributor vacuum advance signal. The unused one should be tightly capped. Seems to me there's another nipple on the side of carb near throttle shaft, cap if unused.

Check throttle blades to make sure they're identical left/right, and check the enrichment cluster [shooters] at top of carb bores, there's supposed to be a thin paper gasket under the aluminum body there. Different orifice size shooters are available too. The carburetor top cover gasket might merit close inspection.

Best carburetor ever built! Designed by Carter, re-engineered for ultra-performance by Webber, manufactured by Edlebrock.