ditch reactors
 

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

Do you have a pop-off valve installed? If not, your air box could be badly blown out. Even if you do have a pop-off valve, the extreme backfire could have blown the valve out. Also, you should check the normal stuff- fuel, spark, etc...

check, pop-off installed by previous owner. It's possible I have an intermittent spark thats causing the backfire, I think it blew the innards out of my muffler and I am now considering new headers and a new stainless steel muffler but do not want to blow the guts out of a new muffler.

Intermittent spark can be caused by a bad wire or connection, but most likely if it happens only after the car is warmed up, the coil or the CDI box. Intermittent problems can be hard to diagnose, as you can only troubleshoot while they are not working.

Do you still have points? Do you still have the stock Bosch CDI? Stock coil? How long has it been since you did a complete tune-up (wires, plugs, cap, rotor, points, etc.....)?

You want to consider eliminating the points and going will an electronic pickup module. I had some odd things happening even with correct point gap. Actually had the car shut down (tach. just dropped) -spent some time with the fluke meter in someone’s driveway and found that I had point closure but no continuity across them :confused: Likely a new set of points may have solved it. Pulled the point set sanded the contacts, re-gapped and got the car home. After that experience went and bought a pointless point set from Hot Spark around $60 shipped. Replaced points and condenser and never adjust points / timing again :D
http://www.hot-spark.com/3BOS4U1.jpg

The title of your post mentions "reactors" but that is not your problem, obviously. From your brief description, it certainly looks to be an ignition issue and I'd suggest you trouble-shoot that first before venturing into CIS problems. 36 year old ignitions can be problematic but are relatively easy to remedy.

Now, back to the title subject, reactors. Lose them!