That's typically caused by a worn "needle and seat" assembly. You can purchase a needle and seat assembly for a specific carburetor, but it's best to purchase a rebuild kit and do the whole thing.
The float bowl and the venturi tube
One side of a two-barrel carburetor may not be getting gas due to a clogged jet or passage, restricting fuel flow. Another possibility is a malfunctioning float or needle valve, which could prevent fuel from entering that side of the carburetor. Additionally, issues with the linkage or throttle may prevent the second barrel from opening properly. Lastly, vacuum leaks or improper tuning could also affect fuel delivery to one side.
skim heads. does not matter 2 or barrel carb
No
Currently the largest carburetor that Weber sells is the 55DCO SP. It has a retail price in the $450.00 range and is a two barrel side draft compatible carburetor.
The float could be stuck open in the carburetor, causing the leak. The carburetor leak needs to be fixed, this could be all of the problems, it also causes a fire hazard.
I was looking at my Chilton's Auto Repair Manual and for a 1978 Mercury Cougar it only shows a two barrel carburetor for the 351 Windsor V8 engine
in cars, most of the time the carbs are two barrel which means there are two main terminals, if you want more power, you'd go with a four barrel carb witch has four main terminals.
In older cars 4v = a four barrel carburetor. 2v would be a two barrel and a 1v would be one barrel
I would say a 500 CFM but no bigger than a 600 CFM. I don't know if you are looking for an answer on flow (CFM) or just a generic answer like "two barrel" or "four barrel". As I recall, in 1963 the 283 was only offered with a two barrel carburetor. I believe the rated horsepower was 195. The carburetor was either a Rochester or a Carter. Flow was likely not more than 400CFM.
The 1968 Ford Mustang offered a variety of engine options, and the carburetor used depended on the specific engine. The standard engine was a 200 cubic inch (3.3L) inline-six, which typically came with a one-barrel carburetor. The more powerful 289 cubic inch (4.7L) V8 featured a two-barrel carburetor, while the optional 302 cubic inch (4.9L) V8 could be equipped with a four-barrel carburetor for enhanced performance.
In automotive terms, a 4 barrel carburetor that has two accelerator pumps, one for the primaries and another for the secondaries.