get your engine nice and warmed up, then pull the clutch and come to a stop with your engine "idling". slowly uncrew (lefty loosey

) the idle screw on the left side of the carb (as you look down while sitting on the bike). This should lower the engine speed. If it does not, your throttle cable is not long enough. Fix this problem by screwing in the top throttle cable that sits right on top of your carb. You will notice that it screws in, though you might have to loosen the lock ring in order to do this.
A good way to adjust all this (assuming you have the room on your frame) is to loosen the clamp on the intake tube and slide off the entire carb, leaving everything on the carb alone. Once you have the carb free, look back up inside to view the little gold shuttle, and note its position. adjust the idel screw, and as well the position of the top threaded screw on top. The idea is that if your idle screw is allll the way out, you have just enough throttle cable to let the shuttle sit ALL THE WAY down, blocking the intake. You then adjust your idle screw to open up the intake a bit, and set idle speed accordingly.
On a side note, many times I'll start the engine on a cold day and let it warm up for a minute using half choke while screwing the idle screw in a bit.
If you need any more help I can take and post pictures.