I got an RV a while back & had to replace the holley 4 bbl. got 7-8 mpg at the time. The dealer just unbolted the old, bolted on the new, it started and off I went. I log every tank and got 12 mpg the first, 11.2 the 2nd, 9 (ish) the third and around 8 after that. Had it checked out by an old timey 460/4 bbl mech (semi-retired) and he adjusted on it a while and it started and ran better… I use octane booster and it purrs on the hwy.
Q.= What happened??? Why no more 12mpg???
Air filter? Spark plugs? Carb cleaner? PCV valve dirty?
There is nothing to adjust on the carburetor that is going to cause the mileage to increase or decrease, short of jet changes and this involves tearing into the carburetor.
If the mileage figures are accurate then I would examine the ignition timing closely. Depending on the year of the engine and which system is used you could have a faulty vacuum diaphragm on the distributor, sticking advance weights, problem in the vacuum tubing or thermovalve between the vac. advance and manifold, or a fault in the Dura-Spark if it is equipped with that particular system. There is more than one Dura-Spark ignition system.
I’m also assuming here that you don’t have a transmission slipping as this could cause a mileage loss.