Starting and idle

Brianellwood: Z car? From the lingo, you don't sound like you are from or are in the USA. From my experience with Nissan/Datsun since 1968 including the U.S.V.I, where I also had been in the British West Indies, the Blue Bird, if my schooling and recall are correct, is a Datsun 510 with an L-16 engine in it. (It's a bomb proof motor!) It uses a 2 barrel Hatchi down draft carburetor with a vacuum actuated secondary and an electric choke which get it's power supply when the charging system is charging. Most of them had a 110 main jet and a 150 secondary jet.. These carburetors were very simple to service on the car and well engineered in general.
The design is such that when the engine is cold, the choke closed, which in turn will mechanically raise the idle speed to about 1,000-1,200 RPM. Depending on how cold it is, will depend on how quickly the engine will return to a normal idle. It does require depressing the gas pedal in order to release the tension from the fast idle cam spring as the choke heats up. There is a component called a "CHOKE BRAKE" which is designed to keep the choke from opening prematurely when cold when the throttle is blipped.
The normal idle for this engine is 7- 800 RPM. There were some factory options with SU's but they were never brought into the USA. I have installed many SU intake manifolds and 45mm SU's and SSS cams. The 510's did not have electronic ignitions, they used points. The "DWELL" angle is between 49 and 55 degrees.
It's been years since I worked on one of these cars, but I believe the ignition timing was 12 degrees BTDC @700RPM for most of the cars in that time frame with that engine. The total timing with mechanical and vacuum advance was 38 degrees. (I still own a 1970 Datsun 521 pick up which I bought NEW for a dollar a pound in St. Croix USVI! $2,158.00 pea green with AM radio, heater and white wall tires).
It takes more fuel to run a cold motor than a warm one. However, if the engine is not in proper tune, it will not perform well no matter if the carburetor were new out of the box. The older systems were simple to work on but more prone to failures. Points wear out from running on the distributor cam and arcing. This in turn changes the time in which the coil is triggered and when it fires the spark plugs.
The ignition coils back then were much lower energy: 20,000 volts VS 60,000 + in today's standards. Stay away from cheap parts!
Plug wires made by NGK are good quality. The original made by Yazaki or Sumitomo are excellent. I have not seen much better in the after market which would surpass their quality. Points made by Hatachi or Daichi as well as the condenser , cap and rotors. Spark plugs by NGK (BPR6ES11) is the best or an equivalent in Denso spark plugs. I use Denso plugs in my Nissan road race cars because they don't blister or burn up.
Additionally, the valve clearances on this engine must be set roughly at 12,000 mile intervals. I set my valves "HOT" and set the exhaust first and intake second. I set the valves by firing order and set the clearances with the cam lobes pointed at 12:00 always to insure I am at the bottom of the base circle of the cam, thus insuring near perfect clearances. Even though the specifications are .10 intake hot and .12 exhaust hot, I set them .09 intake and .11 exhaust. To insure my accuracy in setting the valves and not to have set them too tight, I use an exhaust gasses analysis, checking HC's. They should be below 300 PPM.
I hope I might have shed some light on the subject. There has always been a soft spot in my heart for the older Datsun 510's, 1200's and other older Datsun's and Toyota's
I took a pot shot at this question because I am not quite sure what it is you are seeking an answer for. I hope some of what I have said has some value for you. >>>>>>>>mybunkey