Has an electric fuel pump been added? If so, maybe a regulator that failed???

And just curious, why does an "older restoration" 1951 GMC have a Rochester B? The first GMC to use a Rochester was in 1973, and then only on the motorhome with a Chevrolet engine.

GMC used Carters, Strombergs, and Zeniths, with an occasional Holley.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop