the throttle plate was opened with a spring and the governor would over ride the spring to close it.
Steve, you got that 100% correct.
To the O.P.... note the line to the distributor.... manifold vacuum is supplied through a small oriface (jet) in the carb to a governor valve in the carb, and onward though that line to the distributor.
When the engine over revs, a governor weight in the distributor blocks the flow of air into that line, causing vacuum to build up and act on a diaphragm that pulls the primary throttle shaft closed, and the little valve also "dumps" vacuum to the diaphragm that opens the secondaries so they close as well (IF open at the time).
Try this... disconnect the line from the diaphragm area on the RH side of the carb to the base of the distributor AT THE CARB and take it for a drive and see what happens. (To be clear, we are talking about the vacuum operated GOVERNOR line to the base of the distributor, NOT the vacuum advance line to the distributor.)
If it now works without the unnecessary governor action, it generally would indicate the governor area in the distributor base has gotten dirty and free air flow has been restricted, causing the unneeded governor action.
If it DOES NOT work any better, the problem is in the carburetor side of the vacuum operated governor system. We can address that, if needed, post back after the trial run with the line disconnected.
Do NOT continue to operate the machine with the governor line disconnected, as dust will be sucked into the carb governor mechanism, and on into the engine,
As a workaround, if it works with the line disconnected and you can get along without the overspeed governor or don't want to take the distributor apart for a good cleaning right now, install a hose barb in the carb, at the governor port and connect it to a clean, new in-line gas filter, leaving the other hose barb on the gas filter "open".
The gas filter will act as a mini air filter, allowing filtered air to pass freely into the governor area of the carb so the overspeed governor isn't activated.
As goofy as that all sounds, I've done exactly that on a number of old, dirty farm truck engines that were doing exactly what yours is doing.
Post back and let us know how the test run went.