do a search for carb tuning, if you have the stock NT type carb there is no screw for changing mixture, it only has an idle adjustment screw, if you have the newer CNS carb I have read on here that there is a brass plug you can drill out and get to the air adjuster screw for better fine tuning of the mix.
adjusting the clip on the throttle slide metering needle is the way to fine tune the low to mid range fuel/air mix, changing size of carbs main jet is how you make the final tune for overall best performance with the carbs.
do some searches and lots of reading and you will get a good idea of what you need to do to get you carb set up right.
The stock Skyhawk carbs are CNS brand because they actually have
to pass current EPA regs.
Skyhawk 66 cc and 48 cc have only come with CNS carbs for
quite a while now and now even the fuel line is EPA approved.
Did you remove the brass plug that is covering the mixture screw?