All YS engines are closed circuit pumped fluid systems including the two stroke types. It cannot vent to atmosphere. As indicated by RCKen The check valve must be in the return line to the tank (The port that is normally considered the vent). This valve has an small embossed arrow on the body and this arrow must be pointed toward the tank. If not sure about the arrow just blow into both ends and it will be obvious. The pressure must be toward the tank.

Caution these systems can retain fluid pressure in the tank for long periods of time after the engine has stopped. Always unplug the check valve or a capped tee line in the main return line first before fueling through normally in the other line which is the tanks clunk to engine line. Unpluging this line first can result in a ten foot stream of fuel shot randomly long after the engine is stopped.

The best tanks for YS are either Hayes or the clunk type Tetra's. The simple precaution of wraping the tanks in a few wraps of filiment strapping tape will go a long way in trouble free operations. Most cheap no name Arf fuel tanks and copies of Hayes tanks will soon present problems weeping from the Bung plug or the exit tubes, even ruptured tank seams.

YS engines are unmatched performers as well as user friendly but they are plumbed differently and it is a big mistake to use cheap tanks.

If your engine is new suggest you not touch that big pressure regulator screw on the bottom of the crankshaft only the needle valve and possibly a slight leaning of the mid range screw after running in a bit.

John

The pictured tank is not a Hayes (far preferrable) but it shows the strapping technique.