The effect of a carburetor larger than needed (more rated CFM than the calculation)?
1. if the primaries are about the right size, the idle, response, mileage, and part throttle will be the same as a smaller carburetor with the same size primaries.
2. if the primaries are too large, response will be soft, some delay on tip-in, lower mileage, etc.
3. if only the secondaries are too large, a vacuum secondary (or secondary air valve, such as AFB, AVS, QuadraJet, ThermoQuad) will simply not open fully, so the CFM delivery will be less than the rating.
4. if mechanical secondaries, the WOT vacuum will fall below the factory calibration rating at 1.5" Hg, and the A:F delivery curve (BFSC, brake specific fuel consumption) will not be a good match for your engine, and may deliver less power than the correct size.
Biggest reason not to use a QJ? Not many good manifolds for the weird base flange pattern and entrance shape.
I've worked on them for 50 years on Buick 350, 455, and V6 turbo, Cadillac 425, always good results with minor adjustments.
