Saito100/GK
That is the crank case breather. Oil from the fuel that bklows by the piston ends up in the crank case to lube the parts there. Excess oil will blow out this nipple. I just hook a fuel line up to it and route it out of the cowl on my plane. The Saito 100 is a great engine. Follow the break in procedure to the T. It is very important for ringed 4-strokes to be broken in properly. Should be a sweet engine on your stang. Good Luck!!