12,000 rpm idle? Really?
The only thing that would make the engine rev that high is the carb.
You have a stuck Wide Open Throttle, which usually is the result of a stuck cables or open butterflies inside the carb. The butterflies are spring loaded to close without cables. Even with the idle adjustment knob turned up all the way and choke stuck, the engine does not idle that high.
Recheck your cables, and try to manually close the carbs where the cables attach. You might find your problem.