Yeah, backfires will do that. It blew off a home-made vacuum plug. That is why the timing is so important. Plug up all the vacuum ports for now as vacuum leaks cause backfires too. All you need to use is the vacuum advance port on the distributor and hook it to the ported vacuum port on the carb. I will explain that later. . Don't run it anymore until you beg, borrow, or steal a timing light. Once the base timing is set then you can tune the engine properly. Again, WELL DONE!!!