backfiring during deceleration
 

I just put 83 motor back in car after repairing broken head studs and went for test drive. During downshifting or rapid decline in RPM's engine rapidly backfires. This does not occur during acceleration only during downshifting and not at idle.

Any suggestions or ideas would be appreciated.
Thanks

Mike
1974 911S

Lean mixture? Air leaks in the intake?

Timing? Vac leak, mixture.

mixture check
 

On the CIS systems how do you check the mixture or change the mixture.
Thanks

CIS needs a few special tools, Wayne has 'em.

My experience is that this condition is caused by a leak in the exhaust (hole in muffler or leak in the heat exchange to muffler connections). Lifting off the throttle rapidly causes a rich mixture leading to unburnt fuel in the exhaust stream that gets ignited when exposed to outside air at the exhaust system leak.

Check you muffler connections for carbon trails.

more then likely lean mixture...not necessarily a bad thing on deceleration......just too lean to fire in the c chamber....backing off is an other way to put it