You cant just unplug the govenor hose from the back of the carb onto the bottom of the block. Its air pressure vs spring, that governs the high rpm. The air sucks on the diaphragm to pull the poppet in lowering the RPM.
So you can hook a long hose on there and suck and see if that alters RPM.
Air leak.
O-rings on the manifold.
O-ring under the starter.
Theres also an O-ring in the centre of the pull starter.
Bottom crank bearing.
Faulty decomp. Wont shut under suction or diaphragm split letting it suck air through.
Leaking head gasket/warped head.
Rubbed through hose.
You said you swapped the carb so it shouldnt be these.
Crack the the black cover on the carb.
Split in the diaphragm in the carb.
Diaphragm not seated propperly. If they have been like this for ages they can be stretched back out.
To see if the bottom seal/bearing is bad, grab the blade plate and see if its got any movement/slop in it.
If the seal has been leaking look in the side of the block to the top of the flywheel, it will be dirty greasy.
