Actually I'm a lot more familiar with the aluminum-headed OM60x engines, but in this case, I think I know what the white dial is.
That's the primer pump. You unscrew the knob, and then it will pump up & down to pump fuel up to the main filter. The pump is notorious for leaking once you touch it after years of non-use, so don't mess with it unless necessary. There's a new style primer pump now that cures the leak problem, so if yours starts dripping, a new one will cure that.
