When i switched to very small tubing such as the Sullivan 1/2A tubing, and put a full circle of tubing between the tank and venturi that launches went better. I suspect that the surface tension in the small tubing causes the fuel to resist moving back on launch. The small circle of fuel tubing rather than a straight run seemed to help as well. Again I suspect the inertia of the fuel in the tubing is causing it to flow backwards on launch. The fine tubing ( and the surface tension being higher in the small tubing ) and the loop or circle resists this rearward motion of the fuel. YMMV.

I also always mount my venturi equipped COX engines at 90degrees so the venturi is lower and it is easier to get the venturi opening closer to the horizontal center line of the tank.