I did a spark plug test and it's definitely running lean. Which way do I turn the mixture screw to remedy this?

 

uh...you will probably need to go bigger on the main?

 

Probably should rejet first

 

Bigger main, raise the needle, maybe both??
The screw is just for idle, it has less effect on the spark plug test than the midrange or full throttle jetting.

 

I have no recent knowledge regarding Keihin carbs. but I believe they have two adjustments. Mixture screw is not a correct term. The pilot screw is nearest the engine intake & the other is an air screw located at the rear of the carb nearest the air filter. Neither one has much influence on air/fuel mix. They merely fine tune the jetting after installing the proper jet. Your problem is a lean main jet. Buy a couple of main jets, one a size larger than what you now run, and the other two sizes larger. Using a new plug, run a test/plug read again for color with the larger of the two new jets.If the plug indicates rich, drop down to the smaller jet.
The nominal adjustment on you pilot screw is about two turns out from seated. Jet needle position is three.