All K bike gas tanks have two vent pipes protruding from underneath the right rear of the tank. Oftentimes these are connected to two hoses. The forward hose, from the factory, went to a vent on the top of the engine block above the middle of the timing cover. (Rumor has it that this was so that gas/fumes would vent back into the engine to meet California emissions requirements.) The rear hose was an overflow hose that drained to the pavement behind/under the right side peg plate.
Many times the vent on the front of the engine has been capped and the hosed plugged since it really doesn't do much.
These hoses can be a pain to keep connected and also keep them unkinked when re-installing the gas tank. An alternative is to install "The Cup" (Part number 16132307467 and called an "Air Accumulator" in the BMW parts catalog - cost is about $3) under those two vent pipes. Then attach a hose to the pipe at the base of the cup to drain behind/under the right side peg plate.
Here's what the cup looks like in the parts diagram for the gas tank:
Here's what one looks like when installed:
The upside of doing this is that it makes tank removal and installation much easier and you don't have to worry about those vent hoses getting kinked. The downside of doing this is that you'll occasionally get a whiff of gas when you're riding. Big deal.
