On some engines I've adjusted the mixture screw in the same position as this Honda and it doesn't seem to make much difference to the RPM, the manual just says to fully seat the screw in then turn the screw out to 2 and an 1/8 of a turn , then if it needs a slight adjustment from that position you can turn the screw a little more or less.
I've never had the motor over rev from this screw adjustment so sounds like a governor problem or (linkage / spring problem etc )
