Vintage Dirt Bike Q & A

I have a TT250 that I "rescued" from benign neglect. Carb was gunked up - now cleaned and reset to factory specs. The factory airbox is gone and it has a "straight through" air cleaner hanging off the back of the carb (little resistance).

When I start the bike it is running WAY too lean (reacts funny, plug is white) and I am stuck as to the next path to take... would like your thoughts. A few ideas I have:

a) Idle mixture adjustment. When I cleaned out the carb I reset this mixture to one turn out. I can make it richer (screw out?) although not sure that would help. Frankly, not sure that the one turn out is the correct setting either.

b) Buy a replacement stock airbox. This should provide more intake resistance which should help the mixture

c) re-jet the carb. I am not sure about this one - unless you guys have advice. I have limited experience with this and have found it pretty much a time consuming "hit or miss" situation.

Let me know your thoughts on the best path forward. Thanks