The solution might be as simple as changing brands of fuel and finding a better blend.
In Canada I have found that Shell "V Power" works best in my 67 E-Type. (no ethanol)
I have done some mileage comparisons with gas from the usual suspects in my SUV and discovered that Shell 87 Octane (Bronze). delivers a minimum of 10% better mileage. (contains ethanol).
Good luck and happy motoring.