I can't see how the governor can be implicated Paul, since you found that it wouldn't run properly when you removed the governor's link to the throttle, and operated the throttle by hand. The result you got by doing that, was exactly the same as when the throttle was operated by the governor. That is the way I read the previous posts, but please confirm whether that is correct.
Because the problem, whatever it was, was consistently overcome by partly closing the choke, it had to be mixture-related. That almost certainly means it was caused by a carburetor fault or an intake air leak. An intake air leak is unlikely to have been the problem because it would idle: it only went lean above idle. Therefore, a carburetor fault, or fuel supply to the carburetor. I think I recall that you checked the fuel flow to the carburetor at the beginning of this process, but please confirm that.
What can cause intermittent lean mixture when the carburetor has been cleaned out? Perhaps some loose crud or loose component was still in there after cleaning, and caused the problem, so a moving mass could move from being a problem to not being a problem without human intervention. If so, it is still in there now, just not causing a problem. My favourite explanation is that your float arm is binding on its pivot sometimes, or the float needle is sticking sometimes. That might result in passing enough fuel all the time for idle, but not enough to rev the engine. Now the needle or float pivot has unstuck itself, so all is well until the next time.
It is a Tecumseh carburetor, and those have an evil reputation. Whether this problem would not have happened on a better carburetor is difficult to say.
