Re: My GZ 's top speed seems low
You said a number of posts above that you turned "that screw thing" and increased the idle because your biker stalled. You probably adjusted it too high, and that wasn't your initial problem anyway. That idle adjustment screw is only to be used when the engine has fully warmed up, and then set to about 1300 rpms, then you don't touch it again. Ride your bike, warm it up, and turn down that white knob until the idle sounds right, then leave it alone.
And make sure you are also not riding around with the choke on.
You may need the choke to start the bike after it sits a while and this is normal and may be why your bike would only run for a second and then stall, being up in cold Pennsylvania. As you ride and the engine warms up, slowly turn it down 'till it is off after a few minutes or about 1 mile of riding.
If you haven't yet, read the owners manual and it will answer and solve a lot of problems before you start making new ones. It can be downloaded off here if you don't have one.