Glad you are heading in the right direction. Now throw away that plug and buy a spare for the tool box. I never had to play with one of these but have a couple of Buzacotts waiting to play with and they have a couple of ways to adjust those carbies.
Try to remember that some engines need the choke for a while to warm up especially if they are TVO engine (petrol/kero). As suggested by 'oldgoat' check the air mixture passages. I would take the carby all the way apart and start cleaning up everything so it is squeaky clean. I recently had an issue with a carby off a rare mower that belonged to my Dad and it was not until I got up close and personal with a magnifying glass that I found what I thought was a plug to block a passage was actually a blocked tiny hole that a soaking in paint thinners and a poke with a single strand of 4mm 12v electrical wire finally broke through. So the tiniest amount of crap in the wrong place can give you grief. I also use a pressure pack can of carby cleaner or start ya bastard to blow through the galleries and passageways followed by compressed air from the air blower but at a low pressure of around 30-40psi. I sometimes re-surface the mating surfaces to ensure a good seal and that no air gets sucked in where it doesn't belong. Also check the throttle shaft it seals.