1972 Chevy Truck Backfires through the carbureator

This engine is in a boat. In a 1972 firberform cabin cruiser. I don't know what vehicle might fit this engine. I assume all these engines could have this problem. The engine is a Chevy 350 225 hp. The problelm is that it backfires through the carburetor. But only when we throw the throttle quickly forward. This engine has a mechanical advance. Cintrifical. I am thinking that the mechanical advance may be faulty. However, When I move it manually by hand, the spring tensions seems OK but does not return the mechanism all the way back when released. It does not, however, feel sticky. Also, The manual says that the timing should be set at two different degrees. One place it says 10 Degrees, and another says 8 degrees. I am inclined to believe the 8 degrees. Could this be the problem also.