Super Six Jet Size recommendation

[Grasshopper]

I was hoping I could get a recommendation for a jet size that will be suitable for a daily driver. I currently have a rebuilt 1971 - 225/6 bored 30 over with stock cam , a Carter BBD and 2&1/4 exhaust pipe with stock Walker muffler.. The motor is broken in already. It is getting poor gas mileage - approx 14 mpg. I have the stock Holley 2bbl carb for this motor (new) and was looking replace the BBD with it. I was hoping to increase mpg without losing too much in performance. Any input on a jet size is appreciated.
Thanks,
grasshopper

 

[jos51700]

Pull the spark plugs.
Black? Lean it out a size and check again after driving.
White? Rich it up a size and check again after driving.
Goal: Golden brown like McD's fries.

Sometimes too lean costs more fuel because they're so gutless at that point.

 

[Slantsix64]

as said by Joe pull the plugs check if your running rich or lean, Check float level, and timing give it as much as you can before it pings under load check your valve lash also make sure to use 180 degree thermostat.

 

[RustyRatRod]

Where is the initial timing set?

 

[slantsixdan]

The problem is probably not being caused by an incorrect jet size in the carburetor. There's something else wrong -- carburetor is faulty and/or other problems. Where'd this Carter BBD come from? What's its history?