Make sure your tank line (clunk) is not too long and the clunk is not hitting the back of the tank. Also, I use a larger size fuel line for the intake side of the tank, all the way from the clunk to the carb. with zip ties securing all of the fittings. Have never needed a pump on eother of mine and I have the tank mounted over the CG on both of my 1.20 powered planes. The Saito's draw fuel very well so a pump isn't really needed. However, the fuel line that comes with a lot of ARF's fuel tanks is to small for this engine to draw sufficient fuel under load and sometimes will stretch on uplines, allowing the clunk to hit the back of the tank, cutting off the fuel supply.

Hope I helped.