One of the causes of float bowl overflow, especially if the car has set for a long enough period that all of the gas has evaporated out of the float bowl, is what is called excess float drop in today’s lexicon. What this means is that the float or, in case of the H2 carburetors found in our cars, the float lever drops so far down that it traps the needle from the needle valve against the straight section of the fork and jams everything together so that the fork cannot raise to push the needle against it’s seat to shut off gas flow.
(Note: This only happens when needle valves are used to shut off gas flow, regardless of make of the valve. It will not happen when Gross Jets are used in place of a needle valve).
The permanent fix for this situation is to cut slots on either side of the straight section of the fork, next to the mounting holes (see accompanying pictures). These slots (some forks come with the slots already cut) form a tab that can be bent down to limit the amount the fork will drop to the point where the needle valve will open but the needle will not become jammed and cause the float bowl to overflow.