[MaquinaVerde]

A Quiet Boom's guide is great, between that and the factory manual I was able to get my zeniths to run really well.

You have checked your ignition timing, but what is the condition of the distributor? I experienced symptoms very similar to yours with my Marelli distributor and worn (original?) cap/rotor. I swapped in a used Bosch unit with new cap/rotor/points and my problems cleared right up. It really was a night and day difference. Make sure your dwell is in spec too, setting it correctly helped the backfiring a lot. Before this I was almost ready to throw in the towel and take the car in for tuning.

My '69-71 spec book suggests an idle jet size of 47.5, but my car isn't available right now to check. I don't have my overrun circuit connected and don't really have any problems. I occasionally get a slight burble getting off the freeway in fifth, but nothing bothersome.

For mileage I get about 16-18 in town without really flogging it, and high 20s for long drives on the open road.