HI, I'm just getting running a 1985 K100 RS that's been sitting for a few years.  The gas tank was amazingly clean on the inside and the bike runs surprisingly well.  I did notice that while the bike is running, there is quite a strong turbulence (no bubbles, just flow) on the left hand side of the inside of the gas tank, which I found odd considering that the fuel system is meant to be closed.  Any thoughts on this?   The gas tank is more than half full and there's no sort of problem with the idle nor acceleration, so I don't think the fuel filter is cracked.  I really don't now how to describe it other than a flowing movement of gasoline in the tank.

That is quite normal. The fuel pump circulates a lot of fuel and what the injectors don't use returns to the tank.