The valve lash should be set at 0.03-0.1mm (0.002 - 0.004").

As stated above the nipple on the backplate is essentially the crankcase breather. Ambient air needs to be pulled in and pushed out of the crankcase in order the for the piston to move up and down freely. A buy product of this is that the oil in the crankcase also will weep through this nipple and so a length of fuel tubing so it exits the cowl helps to keep everything nice and clean.

Pete