I've see it done two ways. One is that you use muffler pressure with a check valve and the second method is to put a tee in the crankcase vent line. One outlet goes to a check valve and onto the tank and the other outlet just goes to the atmosphere through a nominal length of tube.

These two set ups only work when you use a Cline or Iron Bay type fuel regulator. Otherwise the pressure in the tank builds up too much to just run fuel straight to the needle valve. You will never get your mixture settings to settle down.


Enjoy,

Jim