I too have struggled with a Zenith on a 140. Either it would overflow gas and flood, or it would not take gas at all.
I must have disassembled and reset the float level a good 8 or 10 times. Thoroughly frustrated I finally discovered the viton tip on the new needle valve was molded off-center. Also I found small dimple worn into the float arm. This was catching the bottom end of the needle and preventing the float from operating properly.
Solution was to polish out the dimple in the float arm, then stick the old needle and seat back in.
But finding the right float level was still difficult. Took a bunch more trial and error adjustments to get the tractor run without leaking.
Most miserable carburetor I've ever worked on...