Basic first sanity checks: hand brake releasing fully; all brakes free; air filter blocked. If all these OK, is fuel quality impaired by standing? What do the spark plugs look like? Try running with the air filter removed, see if that makes an obvious change in power.
I don't have any specific figure to hand but I'd have thought you should get at least around 40mpg. Don't forget the plug gap - if too wide your spark may be too weak and not fully burning the fuel available.
I'm assuming the obvious has been checked - correct carb size, jets and needles all set correctly etc. Also timing. Tyre pressures hard enough? Bag of cement in the boot?
Is all the fuel going into the filler reaching the engine - no leaks along the way?