1972 240Z Automatic, misses?sputters, coughs, through rear carb at low rpms

[Namvet6970]

1972 240Z, automatic, with aftermarket AC.    ISSUE:  Between approx. 1100 to 1800 rpm when the motor is at operating temp., the engine will; cough, sputter, miss through the back/rear  carb and it makes it hard to leave stop signs.  As soon as it hits 1900 to 2k rpm it takes off like crazy and runs perfect. it is a fast car for what it is. 

This is my fourth Z but my first three were brand spanking new with no troubles except for my 74 260 that hated cold weather in South Dakota.  I also had a 71 240Z, automatic and a 76 280Z 5 speed, all back in the 70s new.

I got this Datsun 240Z in November 2016.  I have had this trouble all along only so much worse I when I got it, I could not drive it anywhere for fear of dying.   So here is what I have done;

PARTS:  new plugs,  new rotor,  new, distributor cap, new dual points, new condensers, new plug wires, new ignition coil, new mechanical fuel pump, new air cleaner filter.  Put oil in the top of the dash pots.  

I took it to a mechanic who said he used to work on them years ago and raced datsuns.  He said he pulled the distributor checked it all out, set carbs and timing and and put in new plugs, and reset the points gap, and said one of my new condensers was bad so he replaced it, and did whatever for 300 bucks, it ran great for one day then started doing it again the next day.  Took it back and paid him 80 bucks more for whatever he did, got home 30+ miles away and it started doing it all over again.  He said come back and I haven't because;  it used to cough through the carb so hard it would fall flat on its face and sputter in the  back  carb, and keep doing it so that it was almost dying until I hit the 2k rpm.  Then I did all the work I said and it seems the added oil in the carbs brought it to a milder miss/ backfire or cough thru the rear carb.   It runs so much better now at the low rpm but it is still there.

 

When it is a cold engine start, I choke it and it starts up, sputtering a lot and i right away ease up on the choke. it coughs thru the rear carb giving it gas in park at the low end until it warms up, After engine warms up, it only misses or coughs through the rear carb when the car is in drive under load or reverse. Park or N it runs great at low rpm on up to any rpm. It sounds super and runs super.  

CHRIS,  Florida

19,500 OBO  tired of dealing with it and don't have an easy time getting in and out of it. I am old and too heavy, But I love these Z cars.