Inliners International Forums General Discussion Engines After run and backfire?

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!