I just drove my 68 d100 up to Canada this week. It has a problem where after exiting the freeway(or freeway speeds) and stopping, shutting the engine off, it has a tendency to run on and then backfire. Even when just shutting it down after driving around town it wants to run on. It has a supersix with the holley two barrel. What could be causing this? I can post pics of the carb tomorrow.

_________________
Excuse me, does this smell like chloroform to you?

What are your timing and idle speed set at? There is a tendency to speed the idle up with the speed screw to compensate for the timing being set late. The wider opening of the throttle plates leads to the run-on.

_________________
Official Cookie and Mater Tormentor.