Spooky Tooth has a cool video on hooking the kill switch and maintenance and coaster brake over haul. You must register to view.
Spooky Tooth Cycles - Motorized Bicycles: Gas Powered and Electric Bikes - VIDEO: Kill Switch Wiring
Sounds like to me you have to much slack in the throttle cable. You can adjust the throttle slack on top of the carb.
ThatsDax has some pretty good instructions on installing the throttle cable