Pepper,
All of the Cox backplates that I have looked at have a relief cut into the backplate which will admit air into the venturi opening (which has the mesh cover to keep out pieces of grass and dirt). This even works when you mount the engine flush with a solid piece of wood.
I would venture that your poor running problems are probably coming from the reed--it may be sticking or not sealing to the tank, and cleaning this may be the best bet. However I warn you to take things apart over a white piece of paper so that you don't lose parts--especially the tiny rubber gasket that fits between the internal stack of the venturi and the inside of the backplate--next to the spraybar area. This is very tiny and can easily be lost. If it is left out, you will also experience poor running due to air leaks (and fuel, depending on how low the fuel level is) directly into the downstream side of the venturi. It is easy to replace a lost gasket with a very fine slice of 1/2 A fuel tubing, but the best is not to lose the gasket in the first place!