They are notorious for the accelerator pump wearing out and then loosing it's shot. They are just leather "cups" that ride/seal along a cast iron bore in the carburetor and the leather wears out and looses it's seal to the bore in the carb. A "jiffy kit" (rebuild kit) will have a new gasket and accelerator pump to replace it.
Trick: smear a light coat of grease around the outside of the leather of the accelerator pump to help lubricate it when you install it...
That's why I like to convert my 318's to 4 bbls. I don't get the accelerator pump wearing issue, and I can convert to electric choke, which runs much better...