Backfiring

[ereed27]

My '70S pops and crackles when I lift the throttle, especially after aggressive acceleration. If I just blip the throttle, it doesn't pop as badly.

I've been told that a degree of this noise is of little concern. But how much is too much? Could it be a timing and/or spark plug problem?

[reed]

The first thing I would check is the MFI RPM Transducer/Switch. This device cuts off the flow of fuel to the engine when lifting off the throttle. The transducer is very prone to failure which can cause the crackles and pops you are describing.

[spydr32000]

You need to check throttle shaft clearances, mixtures, exhaust for leaks, manifolds for vaccuum leaks, and throttle valve balance as well. Again how much is too much as this is almost a characteristic trait.

[ereed27]

My backfiring was diagnosed as throttle bodies that need rebuilding. Air is slipping through when I lift. Not an urgent fix... at least it's not causing any damage. When I get an extra few grand I'll fix it. (Just in time for Christmas, for the man who has everything...)

[RTincher]

I'd bet on the throttle bodies also, I used to have a '73S with completely factory engine but well worn that popped and ocassionally backfired especially on decelration - and it was diagnosed as throttle bodies. My 73E, in much better mechanical condition, never pops or backfires even on hard deceleration.

[schmidt]

First check to see if the transducer is operational, most are not working. If the backfire is one loud bang and not a burbbly backfire then it is likely the transducer and the setting of the transducer function position and the driving style should be modified.

Jeff