engine backfiring on decelleration

[halgb]

My 1977 911S has some engine issues I cant solve. The car was converted to Webber 40s from original CIS by the original owner and was running fine when I bought the car 3 years ago. Recently had the engine worked on for an unrelated issue and once back in the car, we're experiencing popping and backfire from what seems like the muffler but also hearing what sounds like there are electrical sounds, perhaps a voltage/or short problem on 1,2,3 cylinders. We have new insulated plug wires but this hasn't solved the issue. Also, noted, the tach is jumping around and this too is a new development since the engine-out work was done. The coil seems ok and the capacitor discharge unit is the same one that's been in the car....so we're totally stumped as to what may be the cause?

Sure would appreciate advice if anyone has experience on or recognizes any of the symptoms....

Thanks much,

Hal B
Roswell, GA
halgb@yahoo.com

[john walker's workshop]

All engines burble through the exhaust on decel. I hear that mentioned a lot after installing a performance muffler. The carbs are a lot less precision than the CIS they replaced, so some unburned fuel mix popping off in the hot exhaust would not be unusual.

[Paulporsche]

Do you still have points? Have you checked them?

[RWebb]

points

crud in fuel

jetted too rich

plugs

normal...

and other things

you are going to need to work your way thru it step by step; start with ignition if the grounds to the tach are fine...

[rwest]

With the tach bouncing around, I would suspect a bad /loose power connection and seeing that the engine was out, I would check the connections on the starter and then maybe the alternator and 14 pin connector in the engine compartment.

[NeedSpace]

+1 for rwest. I get that bouncing tach issue.

I think you have a few issues and not 1 thing causing all of it. For the tach issue, these are the 2 pieces of knowledge you need. My wiring harness is in poor shape so I have added a wire that circumvents the cable. While I am not recommending you do this, it will help diagnose the problem.

In this pic you can see the small stray black wire that goes into the 6th top pin in the connector (counting from top to bottom in the pic). It looks like it doesn't belong, because well, it doesn't. That is my tach hack.



This wire then travels around the wiring harness to where 1 wire separates out and clips into the distributor. Here you can see the blue spade connector attached to the distributor. This is where the tach signal emanates from.

[7783911]

contact timmy from the forum, he can offer advice and possible a new harness

[NeedSpace]

Originally Posted by 7783911

contact timmy from the forum, he can offer advice and possible a new harness

LOL, Know him well, in fact im pretty sure he is the one who told me about the hack. Thanks. Too busy driving it. Respray and rebuild won't be for a while!