Is my 340 Overcarbed? Avg 5-6 MPG

As you go down in jet sizes, you have to open up powervalve restrictions. You need to
determine best size jet for cruise, then open pvcr(power valve channel restriction) for wot. Your motor wasn't responding to more timing because your wot fuel ratio is too lean. Also, secondary jet size a couple sizes bigger. Timing 36-38° total.