Handy930

My sport exhaust(Bisshof) has always popped and gurgled when I left off, but now it seems to be more pronounced. If I am too lean or rich on the mixture will that make this more obvious? Thanks as always for your shared expertise.

1979 930

911rudy

The crackle and pop you hear from your exhaust is fuel igniting. It can be caused by too rich and timing. Either one or both can cause backfire.

fullquack

It can also mean that you now have a new leak in your exhaust system, that is letting in fresh air on decel, thus giving oxygen to the mix and causing greater ignition, thus causing a louder popping.....
Mark

Handy930

Thanks for the input. What about the decel valve? I think I plugged it several years ago. Should I put it back in service?

KeithC2Turto

Not sure but I suspect is is because CIS keeps suppling fuel with throtle lift. I belive the 930's vac retard also retards timing such that with the then low effective compression ratio it dose not fully burn. This then sends unburned or burning fuel into the exhaust system. With the deaccel valve extra air is supplied and there is a better chance it will burn off in the area before the turbo. If not, when the unburned gasses get to the turbine they get supper heated and when they hit the muffler section I suspect they find air and burn very fast which is the poping sound.

That is my guess.

Erik@GBox

When was the last time you had your timing checked? You may be a bit too much advanced. I have this same issue currently. If timing is perfect then it is probably an exhaust leak.

Mark Houghton

That's contrary to what Keith just hypothesized:

I belive the 930's vac retard also retards timing such that with the then low effective compression ratio it dose not fully burn. This then sends unburned or burning fuel into the exhaust system.

I run with 12 degrees advanced timing at idle...with all distributor hoses connected. Decel valve is disconnected. I have barely a burble when winding down off the throttle.

Just another piece of information to consider.