I have a newly rebuilt(less than 100 miles) 292 with a Rochester Monojet carburetor. When I shut the engine off sometimes it will keep running(sometimes backwards)and sometimes it will backfire. An old book I have says there are three possible reasons for this:
incorrect timing, incorrect grade of gasoline, or incorrect carburetor adjustment. The book really doesn't go into anymore detail than that. The timing is set pretty well, and I've been tinkering with the carb trying to tune out some other issues. Any ideas how to tune this out? It sure can be embarrassing!

sounds like a timing issue to me. can you go thru your timing for me. explain how you set it up. and did you check the mechanical vs vacum timing? tom

Set your idle speed to spec. you may experiencing "dieseling" due to a high idle setting and higher cyl temps with rebuilt engine. Some GM engines used an electric soleniod to control the idle so that when you turned off the ignition it retracted closing the throttle plate completely to prevent dieseling. After reading your other post you may have other problems which is causing you to have to run too much throttle plate opening at idle just to keep it running. You might want to clean and rebuild your carb completely. Also check for vac. leaks and pluged or misrouted hoses. It is probably something so simple you are just over looking it. Good Luck