Knocking and backfiring at decelleration

Hi all!

On my 1975 911S, when decellerating, I experience a LOT of knocking, popping and backfiring. Is this due to a bad decel valve? And if so...where is this valve? It isn't referenced in my CIS troubleshooting book. Any ideas?

Thanks in advance
Matt
1975 911S

Decel valves rarely go bad but vacuum lines and connections to the decel valve are more likely the problem.

Joe

My 70' 911T had the same impressive problem during deceleration. For quite a while, my trusted mechanic kept saying the engine had an exhaust leak. Well one day I decided to play with the fuel mixture ( on the rich side ) and it got much better. I took the car to another independant Porsche shop where they made the final tune-up of the carburation : they made it even richer and now it runs even better and smoother ( Mileage even better ). the bottom line is that a too lean of a mixture can cause serious backfiring !!!

Thanks!

Thierry,

Were you experiencing an "exhaust" backfire or an "intake" backfire? Mine sounds like an exhaust backfire, which would indicate a too-rich mixture...but who knows!!

Matt

Well, It always sounded coming from the exhaust ( Same as my old 500cc 4 stroke motorcycle but " six " times louder ). My understanding is that if too lean, the mixture will not burn efficiently and completely and under heavy deceleration unburned fuel will ignite in the exhaust and it will go BaBaBoum down the pipe....
Thierry