There are two tubes running from the tank. 1 goes to the carb. on the engine. The other goes to the muffler. Before you fill up the tank, remove the tube from the muffler. When you fill the tank, the excess fuel will exit through the muffler tube.

Enjoy the Avistar, I flew one for 30 flights this summer until I dumb thumbed it in. Flies great at low speeds. I'm repairing the wing, and building a new fuselage for it. At least your version appears to be covered with monokote, my version had the sticky vinyl covering, only problem with the earlier plane.