That isnt the problem, I used to fly rc helis professionally I know the engines are not much different, But i was king at tuning and troubleshooting the heli stuff,
Its not vapor lock,

Now I did some thinking, gravity is a ***** we all can agree. This planes a tail dragger. i make the plane level and the fuel doesnt go back into the tank.
I dont know if this is normal but I didnt even think about gravity being the problem, I tried a different tank and it did the same thing.
What I am doing is priming it, and seeing if the fuel goes into the tank, I tried that on both tanks and same thing. I think the tune of the engines just off, IE the idle mix.
Now this 7m carb I know really nothing about any insight would be nice