'71 911T popping on decel only when in gear

[Groosh]

Hi. I'm having problems tuning out exhaust backfire when decelerating in gear. I'm running:

- Newly rebuilt Zenith's with 130 main, 57 idle, 195 air correction, 30 venturis
- Carbs appear to be in synch on drawing down air
- Marelli distributor with Pertronix

When I pull the car out of gear coming to a stop it does not backfire. In gear, it scares small children.

The engine is not fresh so compression is down in one or two cylinders but it pulls really nice to red line.

Any advice?

Thanks very much, Paul

[1QuickS]

Decelerating in gear causes engine to develop maximum intake manifold vacuum which draws air past throttle valves which can cause rich mixture in muffler to explode.

Zeniths had a solenoid valve attached to flange at cylinder #6 that would draw fuel into the intake tract in this situation. The plumbing included a "barnacle" on the side of the carb body & brass lines running from that to the flange at each intake tract.

Typically this system is disabled without issue but if you have loose throttle shaft/bearing interface then air can be sucked through the clearances. Jetting sounds typical for your application.

You might want to try opening mixture screws 1/2 turn to see if that helps.

[Groosh]

Yes, the solenoid has been removed and all the barnacle plumbing is closed off. I'll try backing out the mixture screws. Thank you.

[T77911S]

I think exhaust leaks can do it too

[ossiblue]

Originally Posted by T77911S

I think exhaust leaks can do it too

Yes^^. Very common in 912s with carbs so a 911 carbed engine would also be subject to this. If there is no air leaking past the throttle linkage, there would still be air in the exhaust, causing a backfire.