Hello D.,
Remove the air cleaner and take a couple of pictures of the carburetor linkage for us to review. (My mind's eye, needs a little help to focus)
If I remember correctly, this is a larger flathead engine that had an internal mechanical governor driven off of the cam gear.
It is rare but not impossible for the governor fly weights to get sticky. I have only seen this happen with these older engines after they have been left sitting for years with very old oil in them.... Then someone tries to bring them back to life with that old sludge inside them. The old oil that has been sitting in the sump has basically turned to thick goo.
Option 1 I suggest you drain and refill the sump with a newer 5W-20 oil. I do not recommend any kind of engine "flush" treatment on one of these engines. Run it up to full temperature and keep manually working the throttle, but don't over rev the engine. This will likely correct the governor hanging issue after a few heat cycles.
If that does the trick, drain the oil and move up to a conventional 10W-30 engine oil. (Full synthetic would be a waste of money)
Option 2 would be to remove the engine and open up the crankcase sump to clean and inspect the parts. (make sure of parts availability before you attempt this)
