Tank Position
Just to put the final nail into the 4 oz tank coffin....
Most of these model engines can only draw from so large a tank. You can put a bigger tank on there but what happens is that it's so hard to draw the fuel CONSISTENTLY (note emphasis here) that you usually end up getting SHORTER runs from the big tank than from a properley sized tank. If you set the needle for a good peaked run it goes lean way too early on the big tank but on a proper size tank it can draw the fuel properly. Add to that the fuel sloshing around during flight and creating larger pressure pulses with the larger tank and you have a recipe for rotten running.
Hopefully this helps you understand what's going on.
And muffler pressure is highly over rated. It's a very weak pressure that's only slightly better than open air. It's not like other higher pressure systems where you could run larger than normal fuel supplies. In fact some engines actually run better without the muffler pressure.