The why. Where is the leak? Unless the float setting is way off or the needle stuck, the leak is from the emulsion tube. Oh, the seat on the tube must have a poor machined surface - good to check and polish .if needed. Maybe use a $0.20 tephlon washer to improve the seal. Problem fixed? Nope. The problem is the float level.
I rebuilt a couple of carbs to OEM specs and got the same slow drip. I did everything the web mentioned to resolve it. They still had the drip. I verified that the lower carb bowl did not leak with the emulsion tube installed with its jet needle - the tube's machined surface was not the leak!
Note: imagine you ran a compression test on the B&S with the tool screwed too far into the head. Then imagine the intake valve hitting the tester's fitting real hard. Like it made the screw on fitting slightly tilt and strip the spark plug hole. If this wild situation happens, the intake valve may bend and the back draft out the carb will also make the carb leak ' but only during cranking.
The answer: the factory float level is too high. Setting the float parallel to the cover when it is held upside down results in the bowl fuel level being higher the a tiny hole in the emulsion tube(jet) located in the carb throat. The fuel that dribbles out is the famous flo-jet leak.
Fix? Slightly decrease the float level to drop the fuel in bowl level just below the tiny lower hole in the emulsion tube.
IMHO: The factory float setting is too high. I resist not following OEM spec. But in this instance, the spec is bad - assuming your carb body and float are good and you properly rebuilt carb ruled out other possibilities.t.
Edited by dblover, July 27, 2015 - 07:07 PM.