I had my 'b' carb set up with a 62 main jet and a power valve restriction that was too big. I had scavenged the carb off a 250. After running way too rich for a while and only getting about 10 mpg I re- jetted it and put the right power valve assembly in it from a scrap carb. Now I get about 18 mpg in the city (mostly 35-45mph roads) with a #57 main jet. My truck is a 1/2 ton with a '62 261, single exhaust with glasspack, 4 speed, 4.11 gears, and 27" rolling diameter radial tires. I was getting about 20 mpg with the 216, but I was babying it because the bearings were so loose. I like to romp on the 261. Also check that your air cleaner isn't restricted. I had a 'hot rod' air cleaner on mine, and it was so restrictive I stacked another element on top of it and extended the center bolt to get enough air flow. That really helped.
You might try a step leaner on the jetting and a lower restriction exhaust, and make sure your carb heat riser flap is working correctly. Also, check your valve lash settings. I like to run mine slightly on the loose side. It keeps the seats sealing well and helps compression. I let my BMW valves get too tight and was getting a check engine light, and after I adjusted them the cel went away and I got about 3 more mpg.
