I have found the Zama's tend to have the check valves on the fuel tank return line fail (common on mccullochs for some reason) so they wont primer properly.
AVB, the best way I have found to do the barrel carbs is unscrew the low jet that is down the middle of the barrel (which I suppose acts like a variable jet) until the threads don't contact, then screw back in 13 turns (from memory). Screw the idle screw in all the way, and then unscrew 3 full turns.
Start the machine as normal and fiddle with the choke to keep it running until warmed, and then screw the mixture screw in until it smooths out. Unscrew 1/4 turn to richen a bit more, and unscrew the idle screw until a moderate idle is reached. Now you can adjust the mixture for best pick up off idle.
Thats the way I did 2 that were a real pain
