I've had this happen over the years on everything from a lawn mower to an outboard.
The common denominator is that you really need to remove the shaft, clean it well, and lube it.
Penetrating lubricants might give a temporary fix, but not for long.
Thanks all. I had to remove everything. It was seized where shaft went through the choke tube but there was another thing. The locating clip at the bottom which locates the shaft vertically would cause the throttles to hang also, apparently due to throttle plates jamming. I had to tweak them. All this is bad because as the throttle is only returned to closed by those weak springs it can jam open in use.