Re: carb tuning
There is no screw for the main jet. You adjust the clip by removing the slide and taking the needle out of the slide, moving the clip up or down, and re-installing the needle/slide top of the carb.
You can also fine tune these engines by other methods, such as tapering the jet needle (you can get a slightly richer setting at any needle setting this way), opening up the air filter cover with some holes (makes it leaner, but you have to make a lot of holes) drilling or cutting part of the baffle inside the muffler (makes it leaner) and adjusting your gas/oil mixture. (more oil will make the overall mix leaner, less, richer- but don't even think about this if you are not really adept with small 2 cycle engines)
Best to stick with the needle clip adjustments for the newb.