Some of this is conjecture and some is empirical from working on my own peds.
If your carb is like mine, it does not have a tapered needle extending from the bottom of the slide to the "needle jet", the opening in the venturi which allows gas/gas air mix into the air stream. In the absence of a needle, I do not know what to call this jet, so I will call it an atomizer which is what it would be called in a "butterfly" type carb such as in a car.
The purpose of this tapered needle is to regulate the available gas/gas air for mixing with the incoming air during intermediate throttle openings.
By the way, I use "gas/gas air" because many carbs have an emulsifying tube which mixes a small amount of air with the gas as it travels from the main jet to the atomizer. This assists in atomizing the fuel into the intake air stream.
If you are not thoroughly confused yet, consider the operation of the carb at full throttle. The venturi size is fixed by the diameter of the passage through the carb. The main jet and air jet to the emulsifier, if any, are also fixed. At some rpm, the velocity of the air through the venturi will create enough of a pressure drop to draw just enough fuel through the main jet/atomizer to give you a perfect mixture. If the rpm increases, the air will flow faster, creating a greater pressure drop in the venturi, drawing more fuel through the atomizer. This sounds perfect except that if diameter of the carb restricts air flow at all, because it was designed for optimum flow at lower rpm, then the pressure will drop even more at the atomizer. This causes too much gas to be mixed with the fuel, essentially a "choke" effect. The air cleaner/air box can also contribute to this.
The reason you never have the problem goin up hill is because you never attain this "excessive" rpm range.
If you are still with me, the contributing factors to this equation are:
Diameter of the carb venturi.
Main jet size.
Emulsifier air jet size.
Atomizer jet size.
Float bowl fuel level.
Any restrictions to air intake, air box, filter, air turbulence around the air box.
Any restrictions to gas flow, dirty jets, plugged filters, float bowl and gas tank vents.
Any of the components which are sized to regulate flow can be adversely affected by over agressive cleaning with hard wire, drill bits etc.
Any air leak in a position to allow more air into the mixture, carb to manifold, manifold to block, crankshaft seals..
Any fuel leak which allows more fuel into the mixture, leaky float needle, leaky, stuck or misadjusted enrichener (choke), loose main jet.
As you can see, the opportunities for error are many. Some cause the mixture to be more rich, some more lean.
As we know that you have a too rich problem, there are only so many things to look at which we can do something about. Main jet size, float level and free air flow are the simplest, although it is worth verifying that the enrichener is ok and the air passages to the emulsifier are clear. You have allready determined that the air filter/air box make no difference, although this surprises me. This leaves you with the float level and main jet size to check/play with.
It occurs to me that you should also check that the throttle slide is opening all the way when you twist the throttle.
If you are still awake, I hope this helps.