I have a 70 Chevelle, 350/350 2 barrel. When cold, I start the vehicle and it continues to rev at a high idle even when warm. I noticed a copper color disc on the side of the carb where the accelerator cable attaches and turned it clockwise and restarted the car. Idle was back to normal. Is this the "butterfly" valve? What does it do? Why is it sticking? Is there an easy fix? Thanks for all responses concerning this subject!!!