I had a similar problem with a walbro carb. Turns out when I did some work on the throttle butterfly, I put it back on backwards. Had it opening the wrong way. Engine would not idle at all. When the butterfly opens the wrong way, the low speed ports are blocked off. Dont know it the same holds true on a Zama but you might want to check it. On the walbro carb there are three holes in the side. The two lower ones are for the low speed and the last one is for the high speed. The butterfly should close against the side with the holes. When closed the lower one will be below the butterfly and the other two will be above.

Just a suggestion.

Jim