Possible symptoms, roughly in ascending order of severity:
Starting easily from cold without enrichment, poor running at idle and/or small throttle openings, needing to close the fuel screw beyond a "normal" setting in order to achieve the correct idle mixture, ability to close the fuel screw completely and not stall the engine, closing the fuel screw all the way helping but still not providing a proper idle, needing to turn in the idle speed screw in order to keep the engine running, the engine then alternating between a hanging idle and stalling, all of the aforementioned becoming worse as the engine warms, spark plug(s) black, (note: some of the previous symptoms may also be attributable to other causes or combinations of causes), spark plugs fuel fouling/wet with fuel, fuel found exiting the overflow (if the carburetor has one), fuel found exiting the airbox, air cleaner, or main air bleed jet, fuel level higher than 1.5 mm when the float height is set to 14.6 mm for BST33 and 14.7 mm for BST36, BST38 & BST40 carburetors.
Consequences, roughly in ascending order of severity:
Rich mixture, with the associated poor fuel mileage and carbon buildup, washing the oil off of the cylinder wall, filling the crankcase with fuel, and/or hydraulic locking the engine. Running the engine with gasoline thinned oil destroys engine internals, as can hydraulic locking. There is also the risk of explosion or fire from ignition of leaked fuel.
Causes, in descending order of prevalence:
Damaged float seat o-ring (look for a loose fit in the bore, hardening, deformation, shrinkage, tearing, cracking or other damage), dirt/debris lodged between the needle & seat, bad float needle (look for a witness line where it has been contacting the seat - use a bright light and magnification, sacked out plunger spring, and/or stuck plunger), bad float seat (pits in the seating area so tiny that they are hard to make out even under magnification can be enough to cause a leak - applies to brass seats, but not plastic), fuel logged float (make sure it weighs 6.1 g or fewer separated from the cage), worn float pivots (look for ovaled bores, lozenge shaped pins, and/or general looseness), wrong parts (incorrect float needle or seat), damaged body (check for corrosion in the bore the float seat pushes into).
Possible misattributions of cause:
Incorrect float height cannot alone cause an overflowing problem. On a carburetor with a float system that is otherwise in proper functioning order, the float height can be decreased as far the design will mechanically allow (to ~13.3 mm), and the fuel level will not as a result be high enough to overflow. A faulty vacuum operated petcock or a petcock that is left on cannot alone cause an overflowing problem (if it could, then the carburetor would overflow any time the fuel were turned on). The source of the fuel exiting an overflow can have its source in a cracked or pin-holed standpipe.
Fuel that gathers at/drips off of the bottom of the carburetor or end of the drain/overflow may not have its source in an overflowing problem. It could have its source in a bad float bowl gasket and/or fuel feed pipe/tee seal (if applicable) instead.
The overflow (if present) uses the same outlet as the drain, so sometimes people attribute fuel exiting here to a loose drain screw. It is therefore not uncommon to find drain screws that have been significantly over-tightened, resulting in stripped screwdriver slots and sometimes even a cracked float bowl. Mikuni specifies a tightening torque of 2.0 Nm (17.7 in-lbs), and under normal circumstances nothing is to be gained by tightening beyond this.