The bigger jet would have been intended for a smaller-displacement engine, maybe a 216. Less air velocity through the venturi for any specific RPM equals less vacuum, requiring a bigger jet to make an acceptable air:fuel ratio. A Quadrajet for a 305 will run VERY rich on a 350 for the same reason. Float level makes a big difference in overall mixture, also. Higher float=richer mixture.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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