[930cabman]

I own and tune a 1975 911s, overall a great car, but have had an issue with a very loud backfire, similar to the Lincoln in "Uncle Buck". I am not sure if this is a cis or cdi issue. The car generally runs great, but does "loose spark" from time to time. Once as entering a church my girl and I were visiting, I wound up pushing the hulk into the parking lot as several onlookers kinda gawked. After the service I was approched by a few people wondering if I was the one with the car that was not running???
I realize after 36 years things break down, but where to look?

Thanks in advance