If you can't control the engine speed by pulling the governor rod that runs from the governor to the carburetor down, then its either:
-sticking in its pivots,-That rod has to move freely, it closes the throttle pretty much by gravity alone.
-rod is too long. (I haven't seen this on a ZB, but have seen it on a U and the governors are similar.-probably due to wear inside the governor.)
-the idle speed screw on the throttle shaft-NOT the mixture screw- is screwed too far in and keeping the carburetor from idling.
-its getting extra air from somewhere else besides the intake, like a worn throttle shaft.
-something worn, or sticking in the governor itself. I have seen a couple that had so much end play in the governor shaft that they could not control the carburetor properly. Also, the pins in the flyweights and the block they push against can wear out.