Holley Carb Jetting Help, Lousy Fuel Mileage

Well for one thing determine what the advance curve actually IS if you have not, and then HOOK UP the vacuum advance. Just about any car "tootin down the road" at speed will have in the vicinity of 55* or so total, that's initial + mechanical + vacuum.

I assume this is a square bore (not a spread bore)? In my opinion, not the best for mileage. You want little bitty primaries, which is why TQ and Quadrajets did so well. In fact, I ran a QJ on a 340 for quite some time. "Back in the day" when I had a stock 71 hi compression 340 in my 70 Road Runner, with headers, Dana 3.54, and 4 speed (had been a 440-6) that thing would get 17.5 on the highway. This was with "hang on" A/C and I was never all that carful around town, or on freeway ramps. That car weighed in the neighborhood of 3850 empty and most of the time had another 200 lbs in the trunk.

Frankly if he has any kind of "right foot" that might be all you can expect in town.

What do you have for cam/ exhaust?