The problem on the 750 (and can be applicable to any of the carbs that have the air valve)oh you say whats the air valve? It's that butterfly that sits in the bore above the Secondary butterflys. As the air pressure increases when opening the secondaries the AV limits the amount of air entering the secondaries till the pressure increases flowing past the AV causing the AV to open. The quadrajet and thermoquad also have these but they're controlled by a spring to increase or decrease the amount of air it takes to open them. Now back to the point on the 750 they open to quick and the engines have a tendency to bog or seem sluggish. On my Carters I drilled a hole in each of the airvalve counterweights. This causes them to open easier, and then if it bogs are seems sluggish I push in a bb fishing weight to make them a little harder to open. The primary side is just fine on all these carbs but the secondary side needs to be played with to tune properly.