1979 Porsche 928 backfiring. Why?

[townsendfm]

I have been trying to figure this problem out for about a week. I bought this car about 3 weeks ago from the second owner, he replaced the timing belt on it himself but i think he got the belt of by a tooth which might be causing the problem or it is a vacuum line need help to make sure that it is the problem or something else. The car is a 5 Speed and does all the backfiring when I shift or rev it up.

HELP!http://forums.pelicanparts.com/ultimate/confused.gif