Similiar 428 in a heavier Bonneville. I get 12+ around town. I suggest a tuning problem.
Vacuum advance on the distributer? on mine it makes a huge difference on the street, not only with making the idle circuits work better with a 230 degree cam, but the increased timing gets WAY better gas mileage and healls greatly with heat in the summer. That car has a Qjet, Ive used a late type AFB and it ran well but used more gas though not as much of a gas hog as when I tried a Holley.
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 My Daddy bought me a car but all I got was this old Pontiac.
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