Besides the internals in the carb malfunctioning look at the basics such as gasket deterioration under the carb, and intake to cylinder heads gaskets. Vacuum hoses are also a culprit many times causing lean conditions. I'd be looking for a vacuum leak first, before condemning the carb.
You also have the distributor vacuum advance canister and depending upon what distributor you're using the wiring inside of the distributor on HEIs that tend to fray on the pole pieces. Also on HEI's make certain the mechanical advance weights and springs under the rotor aren't binding, or frozen.
