911sc exhaust 'popping'

[dntlft]

This is something I've noticed since removing my CAT and installing a Bursch bypass pipe. I might add that I also had removed the distributor when trying to track down an ignition no start problem. Didn't reset the timing, just put a score mark where the dizzy mounts and reinstalled as precisely as I could (I know, I know, I should stop being cheap and buy a timing light, already).

What happens... when driving at higher rpm's (pretty much all the time) if I immediately let off the throttle the exhaust will burble, pop, sputter. I don't know how else to describe it. It's not really loud, like an explosion or backfire, just kinda noticeable. Thoughts??

[JQ911]

Is this similar to this? Fast forward to 2:15 and 3:08 you will hear some popping or burbling on decel.

Porsche 911SC Carrera with Supercup Exhaust with Catbypass Pipe - YouTube

[tazzieman]

If all's adjusted to spec , just enjoy! Burble , pop and sputter as opposed to backfire is what makes these cars special and fun. As long as the mixture's playing a tune with the timing , it's part of the symphony of air cooled sounds

[mca]

Decel valves help prevent this condition. Basically you are going lean off throttle. Decel valves ease the rev drop.

[schumicat]

see if the decel valve is plugged. a little burble is a feature not a bug.

[dntlft]

JQ911,
Sounds very similar to this 911SC on youtube. Must just mean my car is a healthy and fine specimen of German aircooled flat 6 engineering!! I plan on following my own advice, get a timing light and make sure it's set to spec..

[spuggy]

Originally Posted by mca

Decel valves help prevent this condition. Basically you are going lean off throttle.

Actually, the exact opposite. CIS keeps injecting fuel with the throttle closed (what the "C" standa for), and you go pig-rich on the overrun, with lots of unburnt fuel collecting after the exhaust port. Same reason CIS 930s throw flameballs when the turbo is hot enough to ignite the unburnt fuel in the exhaust when you lift.

The decel valve lets air in, so more of the mixture can burn off in the head.

Decel valves ease the rev drop.

You can simulate the same effect by not letting the throttle plate bang shut and just keeping 1/8" or so on the pedal. Won't stop it completely, but quietens it down a lot.

[mca]

Makes sense but something isn't adding up. I swear my LM2 reads way lean off throttle. Gotta ponder this a bit longer.

Ok. Lifting off brings a rush of air into/around my tailpipe where LM2 bung is attached. Reads lean.

Thanks for the correction!