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914World.com _ 914World Garage _ backfiring

Posted by: Frank Marino Jun 17 2003, 06:43 PM

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.

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 ? 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

Posted by: Rob Ways Jun 17 2003, 06:47 PM

I had exhaust leaks which caused backfiring under deceleration. When does yours backfire?

Posted by: ChrisReale Jun 17 2003, 06:55 PM

Has the gas tank ever been removed and properly cleaned? When the tank gets low, shine a light in there and see if there is sludge at the bottom

Posted by: Frank Marino Jun 17 2003, 06:55 PM

When it backfires, which isn't very often at all, its on deceleration. I guess a little bit of backfire from the a carburated car is not abnormal. In the case of there being rust in the gas tank. Could that cause backfiring? What kind of steps can I take to check to see if its rusty in there and how can I get a rusty tank cleaned?

Posted by: ChrisReale Jun 17 2003, 07:00 PM

Por-15 sells a tank cleaner and sealer kit, but I took mine to a Radiator shop and they cleaned and sealed it.

Posted by: Rob Ways Jun 17 2003, 07:11 PM

What did they use to clean it???

Posted by: goobu Jun 17 2003, 08:40 PM

My 914 recently developed the same issue...it backfires (almost all the time) when I lift off the gas.

This only started happening after I swaped out my transmission
Time Elasped Video here for anyone who cares
http://www.gtc-motorsports.com/tech/914-trans-swap/trans-swap-in-under-5-mins.mpg

I have heard all the above mentioned here (bad gas, check you fuel tank, fuel filter) although I have not done anything yet.

I was wondering..maybe I forgot to put an exhaust gasket back on...could that do it? Before the great trans swap, I always noticed a little white smoke from the exhaust when coasting (but never backfires). It still emits white smoke now.

Posted by: Frank Marino Jun 17 2003, 09:26 PM

Not putting an exhaust gasket on would be equivalent to having a hole in your exhaust which would cause backfiring to occur. Check that out.

Posted by: goobu Jun 18 2003, 04:07 PM

my 914 is at the shop right now getting tech'ed for my next event (Zone1 - WG in 1.5 weeks) and also m&b a new set of hoosiers. WHen she comes back I will slide under there and check the gaskets.

Thanks

Posted by: need4speed Jun 18 2003, 08:42 PM

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.

Posted by: Rockaria Jun 19 2003, 09:54 AM

Mine used to do the backfire thing on deceleration all the freekin time. Actually it sounds cool driving down the street, though I know it's bad. One day I decided to eliminate the Cold Start Valve since the summer was coming up. All I did was to take the fuel lines off the CSV and put a through pipe where the valve used to be on the lines and right away the Backfiring went away totally.

Wild,