It does sound like a carby that is running too rich or too lean remery, you haven't mentioned if the main jet and emulsion tube were removed for cleaning ,sometimes I drain the fuel from the tank and run the motor by spraying fuel into the carby or use starter fluid and if the motor revs out fine and runs fine you know it's a carby problem ,l would also check there isn't a vacuum leak ,like a loose intake manifold or an old damaged gasket on the intake side, l usually tighten the head bolts on old motors as they can come loose when they haven't been checked for decades.
Cheers
Max.
