Have some questions.

That would be for a mild engine (686) and more so street freindly/Hwy. freindly gears. A 650 would perform fine or the 670 Avenger as the case may be. But to get the max from the engine, a 750 (770 Avenger) would be the choice I would recomend to anyone.

With the smaller carb, you'll be restricting the engine in it;s effort to make the power. Basicly, you would want your vacuum gauge to drop to zero at W.O.T. from start to finish. This won't happen with the smaller carb. The engine now works alittle to get the air and fuel.