What causes intermittent backfires, help

[Flank]

Hi all,
I'm working on a friends '75 911 with a 2.8 motor and the thing keeps backfiring. Sometimes they're mild, sometimes they're wild. Mostly it backfires through the exhaust. Is it always a timing issue or can worn carburetors cause this also. Should I look for dirt in the distributor? How does the advance work? Since it doesn't rely on vaccum. It seems that the advance does not return to the low idle position, although the backfires occur at all rpm's. Then sometimes after the engine is warm it doesn't do it as much. When I sit at a light, the thing is going blam-blam.
Thanks for your help.
Flank

[azasadny]

Flank,

Does the car have the original 2.7L with CIS (Bosch K-Jetronic fuel injection) or has it been changed to carbs? There is alot to troubleshoot, but we need to know what engine/fuel system you have.

[Flank]

It has 2.8 cylinders. 40 ida Webers with worn throttle shafts. Stock distributor, it's condition is unknown right now.
Thanks

[jluetjen]

The worn throttle shafts can cause back-fires on overrun or when coasting with the throttles closed.

[azasadny]

My newly installed Weber 40IDA's backfire a little on decel (poppity-pop-pop) and my mechanic told me it's probably the points or mixture, or both. I refreshed the gas in the tank, since the gas had been sitting in the gas tank for 5 months while the car was in the shop and that helped reduce the poppity-pop-pops quite a bit. I'll get the points adjusted and the mixture set in the spring when the weather warms up as it's in the 30-40's here now and I don't drive the car in cold weather as it's stored in my garage until warm weather and salt-free roads.

Start with the basics, check your ignition (points, plugs, wires), then check your mixture and that may be the problem. Hope this helps...

[ArtechnikA]

10 times outta 9, backfires through the exhaust on overrun are leaks in the exhaust. Start there.

[cfries]

Flank,
we have a similar setup but my 2.8 has larger cams (dont know which) but 46 webers. I also have the same problems with spitting thru the carbs and some backfires. the carbs must be in good shape (no warn parts) and the mixture has to be right. make sure you give it a full tune up and rebuild the carbs if necessary. Start with all parts in good condition before you start troubleshooting. A good tune up may be all it needs.

[unclebilly]

Is the car parked outside or has it been recently washed? Check for moisture in the distributor cap and be sure that the wires are dry. I blew the brand new muffler apart on my 55 Ford Pickup because of a damp dist cap. You can use WD40 to displace any moisture inside the cap.

[Flank]

Thanks you all for the great reply's. I'll look for the moisture in the cap, since the car can be either in the garage or carport there are mornings where everything is wet out. I'll check the points, then focus on the exhaust. Because I already know the carbs have worn throttle shafts.

[ArtechnikA]

Originally posted by Flank
I already know the carbs have worn throttle shafts.

worn shafts and other induction leaks lead to leanness and popping in the intakes - poor synchronization can produce that effect too, especially off-idle.

Exhaust backfires are almost always air leaks. Aside from (carefully!) running a hand near the pipes to feel for pulses, another trick is to turn off all the lights at night and look for blue flashes. lots of older exhausts have pinholes and it can look like a lightning storm under there. again - use caution when working around hot exhaust parts in the dark...

Exhaust backfires can be worsened in the presence of overly rich mixtures - as may be a side effect of trying to overcome induction leak leanness to improve driveability.