back fire

c.routhier · 05:06 PM

hello, i just had my '66 912 tuned up and it came back not running quite right, however, the mechanic seems to think it's fine. here's what's going on: it back fires lots when i use the engine to slow down and when i pull off the gas at higher speeds, and it doesn't idle well at all when cold (I know it's the 'nature of the beast' but...) and when warm it doesn't seem to have a consistent idle either (sometimes 1000 or 200). anyway, the mechanic feels that the carbs are fine...

so, other than the idle and backfire it's running great (fast, smooth, etc.).

is the back firing due to too rich a mixture? to fix this is it a simple adjustment to that black knob (idle mixture) on the Solex?

i've checked the plugs and they actually seem quite clean, not carboned over - which is what I thought would have been consistent w/ a rich carb.

any suggestions? and, what are your experiences tuning up the car? I'd love to do it myself but am a bit intimidated.

thank you for your thoughts,
Chuck

ron69912t · 08-28-2002, 04:43 AM

cr:

My 69 backfired, popped and had very eratic idleing along with hesitation on accelleration. I rebuilt the Solex carbs and even took the car to a local garage for a carb adjustment so I could pass the state emissions test. I failed the test. The state gave me one more try.

Solution..I replaced the original Solexex's with Weber 40IDF carbs, now it idles cold at 700-800 rpm, and 900 when warm, no hesitation, flat spots, popping or backfiring. Runs like a new car, strong and smooth. Hard to tell it's even ideling it is so smooth.

Consider the same change.

Good luck,
Ron D

Green 912 · 08-28-2002, 06:14 AM

Ron is right, Webbers are nice, but you might be able to fix things without replacing the carbs. Did the car run OK when you took it in?

Did the mechanic change the timing? Is the distributer in good shape?
Be sure to check that the shaft bushing in the distributor is not worn or it will wobble around causing the point gap and timing will wander all over the place. Grab the rotor move side to side to check. There should be little if any movement. Rotate the rotor it should move some and the advance weights should move in and out without binding and snap back via the springs.

Does the backfire come out the tail pipe or are the carbs popping back into the air cleaners?

Out the tail pipe, start with the timing and distributor then carbs. Out the carbs, then check the idle mix, too lean will make the carbs fart at idle and when you lift out of the throttle.

C. Clark · C. Clark's Garage

Chuck,
Ron is probably correct, my car did not run very well with Solex carbs. The butterfly shafts become oblong after 30 years of use and air can be sucked into the body of the carb. causing mis-firing. You said you just had it tuned, did it run bettter before the tuner got a hold of it? If so the timing could be wrong, that will cause popping also, take it back, make the guy drive it, or move on and learn by doing and or find a good mechanic.
Good luck,
Chas.

h.beard · 08-28-2002, 05:13 PM

Chuck, I'm with Chas. If this mechanic is telling you everything is OK and its poppin' and spittin', you don't need a new car - you need a new mechanic. In what area do you live? Maybe somebody out here can make a recommendation.
I had my Solexes rebuilt by a carb guy. He looked them over and told me they didn't look to worn and should be good after a rebuild. Paid $120 to do both (plus parts), but they came back looking like they just came out of the box. This work fixed a serious fast idle that I had. It was running much better but still poppin' a little. Just had someone help me fine tune them yesterday. Car is running great now. no poppin', no hesitation.
Howard

Duke · 08-29-2002, 09:59 AM

Backfiring during deceleration can be caused by the following:
1. Vacuum leaks ( check gaskets for proper fit)
2. Check the accelerator pump circuit for proper operation.
3. Mixture control screws could have a damaged tip due to over tightening during the adjustment process.
4. Check for leaks at the exhaust ports.
The vacuum leaks are the hardest to find if this is causing your problem. Just takes time.

I agree with a previous post. You need a second opinion from someone who knows the proper procedures to follow in adjusting the Porsche engine.

c.routhier · 03:25 PM

thanks all for input. the car was running pretty good before the mech got to it, still 'burbled' a bit upon engine braking, but ran super at speed and did idle okay. My charge to him was to 'fix' this burble, and boy did he fix it.

The burble is coming from the exhaust, not from the carb. for sure.

I will take it back to re-do and will ask him about the vacuum; timing; worn distributor; etc..

If it comes back still not to my satisfaction I will try myself and/or look for another mech. I am in NY, north of the city, most Porsche mechs are on Long Island and not that easy for me to get to.

thanks again,
Chuck