I get 17-19 with mine, averaging about 50 mph, even with a full load. My truck will missfire if the timing gets off too much, but the fuel milage will not get that bad. Advancing the timing a little gets me about 1-3 mpg increase.
A lot of Rochester B carbs have stuck power valves. You can fix it without pulling the whole carb (It's in the top casting), but check to make sure that it moves freely. Also make sure that you have the carb spacer at the base, between the carb and manifold, there is a vacuum port hole in the flange that has to be kept uncovered so the power valve can see vacuum. The spacer is about 1/2" thick and has notches on the inner diameter. If you don't have one that is your problem. The power valve should get pulled upward when vacuum goes up. It is a piston in the air horn casting that has a needle that pokes open a ball valve next to the main jet. The spring in it keeps the ball valve open, allowing more fuel for a rich mixture with low vacuum. If the piston is hanging up or not getting a vacuum signal it will run too rich. To check the mixture: Put new (or cleaned) spark plugs in the truck and drive for a while at about 50 mph, push in the clutch and shut the engine off, and coast to the side. Pull the plug and make sure it's kind of a chocolate color. If it's black and carbon fouled then it's too rich.
