All California models will have the gas tank vented into a charcoal canister, which is connected to the intake somewhere. The idea is to collect the fuel vapors that escape into the atmosphere, store them in the charcoal canister, and have them sucked into the engine and burned when started. To the best of my knowledge this system has never worked properly on any bike. Even on a new bike, if you fill the tank more than about 3/4 full, liquid gas will run out the vent and into the canister, causing a super rich mixture, resulting in hard starting and poor running. The best thing to do with this system is to remove everything, vent the tank to the atmosphere, and plug any holes or fittings that might allow air to get into the intake system.
All four cylinder Goldwings also have an air injection system which injects air into the exhaust ports. The idea is that more air will help burn some of the unburned fuel in the exhaust from incomplete combustion. This is also a bad system, as it causes the exhaust system to overheat, and also causes backfiring due to an overly lean condition in the exhaust. I completely removed it and plugged everything from my 1200.
Some of these systems are really complicated. My Kawasaki Vulcan 750 was a California model, and had both the evap system and air injection system. I removed them both, and had a full 5 gallon bucket of junk, which I happily tossed into the dumpster where it belonged. In my case, on the Vulcan, I also had to rejet the carbs one size richer on both the main jets and pilot jets, and turn the idle air screws out another full turn to really get it to run right.
But if you have an evap system, getting rid of it will likely solve your problem. The fuel is probably either coming from the tank vent or the charcoal canister.