The Webra .40 - .50 Speed/GT series engines are designed to run at high rpm. Loading them down with much more than a 10x6 prop is asking for trouble.

Yes, it could be that your fuel tank is foaming the fuel from engine vibration. The only real solution is to balance everything as well as you can and then make sure that the fuel tank is not touching the sides of the airframe or the firewall directly. This includes the brass fuel tank lines too. I realize that tank space is limited in the Something Extra.

These are standard things to check. There are others if you perform these things and still have trouble.

In fact, the very FIRST thing I would do is to check for loose screws on the engine. Good luck.