It is very easy to do a simple experiment to see if it is lean on start up. Add some extra fuel, do not use ether, it knocks. I use either propane or carb cleaner. With the carb cleaner you can stick the hose in the snorkel and spray. If the stumble goes away, you were lean, if it gets worse, you were already rich.
You also can pull off the tubes to the evaporate emissions, this will admit a little air, if you are rich this will lean it a bit. This will tell you which way you are off.
I suspect if you are lean, it is because of some defect like an air leak, low fuel pressure, slight injector imbalance, air in fuel line.
If you are rich check for a dripping injector.