It has always, and I do mean always, been my experience that a 4bbl will get better gas mileage than a 2bbl; PROVIDED, you keep your big foot out of the back two barrels. I've ran both Rochester and Holley 2bbls and have always gotten poorer gas mileage than their 4bbl counterparts.

Rochester Quadrajets in particular get excellent gas mileage for 4bbl carbs. Holleys will also, especially the vacuum secondary 4160 models. As for Carters, the only one I've had experience with to any degree was a WCFB on my '57 Bel Air, and it always got horrible gas mileage no matter how I drove it. I don't think all Carters are like that, particularly AFBs and AVSs, but I don't have any experience to back it up.

I have personally never heard of a plate that blocks off two barrels of a 4bbl carb, although there is an adapter that will adapt a standard flange 4bbl to a standard flange 2bbl, but I can't imagine what good it could possibly serve. The best advice I could give you concerning improvement in your gas mileage would be to go very lightly on the gas pedal, and avoid opening the back two barrels. I will admit, it's kinda hard to do, knowing that you've got some extra punch under your foot if you want it. And, it's really hard when one of those noisy Honda Civics with a big wing and some stupid decals starts buzzing around you like a pissed-off bee. Man, they are just asking for all four barrels when they do that!

Randy