Float/needle/seat is usually the culprit, however a plugged tank vent can also cause that problem, especially with a full tank.
Yeah...the 'full tank' thing is what alarmed me at first. The bike was registering a half tank when I got it and I haven't got to ride it much. I was keeping a real close watch on it for leaks. I know it hadn't leaked in almost 3 weeks, ever since I got it. When I saw it leaking right after I filled it up I first thought it had a high tank leak but as I traced it down, it was coming from the carb.
I'll hafta get my service manual out and pull the needle float out of it and check it out.
I can't get to it today. Got about a foot of snow on the ground and I can't get to my shop where I keep the bike.
Exactly, the difference in "head pressure" between a full tank or less is minimal. When the tank is full there is less air space to compress, so it forces the needle open. Park the bike with the cap loose and see if it still does it.
I would think the auto shut off on the fuel tank valve would keep the pressure from getting to the carb. You could try shutting off the fuel valve at the tank, and see if that stops it. I am guessing, but it probably has a little crud under the needle as said above. It is pretty easy to clean. I can take the bowl off of mine without removing the carb.