A few addendum to previous post -
Contrary to reputation, four barrel carburetors have the potential for better fuel economy than two barrel carburetors. This is because the venturis in two barrel carburetors have to be large enough for the highest anticipated engine load. The primary venturis on four barrel carburetors can be relatively smaller because the secondaries can take up the 'slack' in response the occasional heavy engine load. (i.e., acceleration or climbing hills.)
The four barrel carburetors have a reputation for greater fuel consumption because many owners make excessive use of the secondaries.
Both my 318 engines had two barrel Carter BBDs. I anticipate a 2-4 MPG increase when I switch the '69/70 318 to a four barrel Carter AFB. (Switching also requires a different intake manifold ... a four-to-two barrel adapter is a less effective -- and unacceptable -- alternative.)
People sometime change carburetor jets in a usually misguided attempt to increase engine power. Unless you have lots of knowledge and experience, doing so very rarely succeeds. Usually, they just cross the too-large line and 'fall off the cliff'. (Larger jets usually just cause excess fuel being dumped into the engine without significantly affecting engine power.)
I discovered an ignorant previous owner had drilled out the jets when I rebuilt my '80 Bronco. (Drilled out because the jets were larger than any standard jets for that carburetor.) I couldn't find the right size jets. So, I had to get the next smaller size and carefully drill them out a 64th at a time. (I had to make a special drill-press jig just for that task.)
People rarely go to smaller jets because it reduces engine performance. (In this case, falling off the cliff means the engine stalls under load.) However, small jets are appropriate if you're sure the carburetor is dumping excessive fuel into the engine ... but only if you're sure.
