For fuel to get that high above the float bowls, the floats are not pushing the needle into the seat to disrupt the fuel flow. Also, the fuel is probably entering the throat of the carburetor and draining into the motor. I would imagine that the float and float needle is bound up or the float is no longer floating.

@8053 Thanks. I'll check.

To check for trash that may be fouling the float valve do this....

1) Turn OFF the fuel.

2) Pop the float bowls off one at a time.

3) Pour the contents of each float bowl through a common white kitchen paper towel. The gasoline will easily pass through the towel, but any trash will remain AND BECOME VERY EASY TO SEE against the white background. Any water present will also "bead up" on the paper towel. Any trash is cause for concern. 

4) Now carefully pull the float pin and drop the float and float needle from both carb bodies. 

5) Place a while kitchen towel under the carb and turn the fuel back on for about 2 seconds. This will wash any trash out of the float seat, and again onto the paper towel where it can be easily seen. 

It is common for older Airhead fuel tanks to 1) lose their anti-rust coating and/or start to rust. Bright Orange-Red is flakes of the anti-rust paint coating, dark Brown particles are rust from inside the tank. Black specs will be pieces of "rubber" from very old fuel line or the seals inside the petcocks. 

What you are looking for is not big pieces, but tiny dark specs about 1/3 the size of a grain of beach sand. It doesn't take much to upset the delicate float needles. 

> If you haven't done so already, then order the "new" updated ethanol-proof floats, float needles and float bowl gaskets from Bing. 

> ANY new fuel line you fit must be marked "SAE J30" or don't buy it. This line will resist rotting and breaking off caused by exposure to ethanol fuels. 

> It's difficult and expensive to have your fuel tank coated, so the better way may be to add fuel filters above each carb. Owing to the limited space on an Airhead, fitting even the smallest filter can be difficult. If you are installing all new fuel lines, then routing the lines in a non-standard method may give you the extra room you need. Consider a layout that has the cross-over run around the rear of the engine (as opposed to under the air box)...

(If your petcocks have the fuel line come straight down, rather than out the rear. Consider replacement petcocks.)

Hope this helps.

@wobbly Thanks for all this input.  I'm having a similar carb leak issue on my recently acquired R90/6.   You suggest pulling the float pins/floats/needles... is that possible while the carbs are still on the bike?  

THX...

-craig

Wobbly might have a different answer but, I would believe that dealing with the float hinge pins would be difficult or impossible with the carbs on the bike. I just installed new floats, pins, and needle valves in my 40mm Bings. I took them off the bike. The pin can only go out and in one direction. It has one serrated end, that anchors it in the cast post that is part of the carb. You have to look very closely.  That serrated end is like a head on the pin. The pin needs to come out head first, and be reinstalled as it was. I gently tap it out with a very small pin punch. Being careful is key - you don’t want to break the cast post on the carb. 

To remove the carbs - I disconnect the choke by undoing the 11mm nut and removing the little arm, along with undoing the choke cable from the boss on the carb top. Unhook the end of the throttle cable from the throttle arm.  Then I remove the carb top and leave the throttle cable attached to it. That way I don’t lose my carb synchronization. Of course then I remove the carb piston and diaphragm. This way, also, the carb will sit flat on the bench upside down - easier to work on.