A local magazine surveyed dentists, asking "who, besides yourself, is the best dentist in our city?" By not allowing dentists to vote for themselves, the survey produced a much truer guide to where to get quality dental care. Similarly, a constitutional amendment that bars congresscritters from seeking to have money spent in their own districts would boost the overall effectiveness of government. Lockheed would finally be pressured to source its F-35 components from the most efficient suppliers, rather than from the most pork-ified network of suppliers.
I have my doubts that such an amendment would help. Over time, Congress critters would simply enter into quid pro quo relationships with each other. Critter A proposes to spend $$$ in Critter's B district while B proposes to spend $$$ in A's. The net effect would be the same.