Assuming your overflow tube is the carburetor bowl vent, then possible causes (there are probably others):

(1) Leaky float
(2) Float incorrectly adjusted
(3) Too much fuel pressure
(4) Float stuck
(5) Defective fuel valve

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