backfiring, once in a while Options

[Frank Marino]

Hey, this is my first post, but I've been reading the forum for a while. dmilzoff is the other driver of the car, you may have seen him post before, he's fairly new also. We just had new headers put on the car. The carbs (weber-48's i believe) have been cleaned and tuned. Its an 82 911 engine by the way.

It used to backfire a lot, before we had the headers replaced. Turned out there were a few episodes of rust on the old headers which caused holes and I know that can cause backfiring. But I'm sure there are no holes now because of the new pipes. (IMG:style_emoticons/default/biggrin.gif)

The car still backfires once in a while, so I assume there is some sort of dirt getting into the carbs still. A friend suggested that the gas tank might be rusty inside and that the fuel filter isn't grabbing all the rust. What do you think (IMG:style_emoticons/default/confused24.gif) ? The filter was installed last year, but very low miles have been put on the car since then. Think changing the filter will solve the few backfires here and there? Or is it more involved?

Frank

[need4speed]

causes for backfiring (as I understand them) (in order they should be eliminated):

1) Exhaust leaks, fresh air mixing with superheated exhaust which also contains some unburnt fuel = boom! usually on deceleration. Check for obvious holes first, try covering the tailpipe(s) while engine is running, if it keeps running, you definately have an exhaust leak. Next, check your mating surfaces, flanges, gaskets for trueness, bolts for tightness, etc.
1a) exhaust obstruction
If one of your headers somehow got clogged with a shop rag or a bananna or something. . .

2) Lean condition caused by:
2a) restriction in fuel supply - rust in tank, clogged filter, kinked line, bad fuel pump, bad float valve, vapor-lock, clogged carb jets or passages, bad accelerator pump, clogged injectors - the list goes on and on.
2b) intake leak - cracked manifold, bad gasket, loose carb mount, untrue carb mount (crooked/gapped mating surfaces), cracked vacuum line, bad airflow sensor, bad choke, misaligned throttle plate, etc. etc. etc.
2c) cyliner leak (YIKES!) caused by bad rings, cracked cylinder, cracked head, loose case-stud, cross-threaded spark plug, etc.
2d) POOR QUALITY GASOLINE! (the usual symptom is excessive knock, but you could probably also get detonation prior to the intake valve closing too, I suppose).

3) bad ignition timing (these aren't super likely) caused by:
3a) wonky distributor - causing it to spark prior to the closure of the intake valve. (check each plug wire with a timing light, make sure the timing mark is pretty stable, make damn certain the right plug wires go to the right cylinders, because a lot of these engines will actually run with two mixed up, but it'll run like crap obviously).
3b) mixed-up plug wires (both wrong order, or wrong length)
3c) bad pertronix unit
3d) crud in the points
3f) bad condenser

4) bad valve timing caused by (I'm really reaching here):
4a) broken valve spring/guide
4b) bent pushrod
4c) valve lash out of spec

Most likely 1 or 2. Now you know why us car nuts are all loony tunes.