I have a 100% 81 RX with the carbed 12a. I have read some about the coasting valve, shutter valve, and the afterburn valve.
There is no vacuum coming from the hole in the air cleaner for the coasting valve, so that appears to be ok, however I have not seen the coasting valve actually operate.
The issue is back firing, and of course high HCs (1000 or so where I need to be a 220 to pass) The CO is about in the zone, so I am not sure what is going on.

Recent tune-up
NGK plugs/wires
Bosch cap/rotor.
Its getting pretty darn good spark.

The car does have synthetic oil in it so I'm wondering if that has a part to play.
I would rather not start throwing parts at it. So far I'm thinking coasting solenoid,
Or afterburn valve.
Thoughts?