[pat in Santa Cruz]

sounds rich to me. You should be getting 18-20 mpg on the highway. The first thing I would check is fuel pump pressure. If its higher than 1.5 psi, the float is being over ridden by the pump. Do not rely on the regulator dial, use an accurate pressure gauge. I cannot see why you need a pump on an updraft carburetor unless you are not using the cowl fuel tank.

Also, The float in the carb might be set too high or could be sinking, the main jet could be worn too large, there could be a leak from one of the jets, or your GAV could be too far open or have a leaking seat. If you're not the carburetor tech you would like to be, Berts sells some really well done rebuilt Zeniths with flow tested jets.