You need to take apiece of copper wireto all 4 jets, make sure the floats arent sticking and run spray cleaner through all the orifices making sure cleaner comes out all of them. You plug off someopeningshere and there until you are able to get it to flow out of each small opening. Also clean the pin holes on the sides of the carb throats. Theres some idlerelated partsyou can clean but since it seems to idle fine dont bother it. But if you want to make sure remove the idle mixture screws (you have to remove the epa caps)and clean the orifices and make sure you put them back together correctly. Turn them in all the way then back out 2 1/2 turns.

Youll also want to make sure your diaphrams arent damaged, dried outor stuck, Ive had the plastic slides get sticky. They move up and down by vacuum so its important they are moving well.

Are your vacuum lines routed correctly?

When they dont run right its usually the carbs, its likely you missed something. Old gas turns into shellac so getting it out of the orifices can be time consuming. Also make sure you have fresh gas in the tank not something thats been sitting for the lawnmower for a couple of weeks.