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Idle Speed and Mixture Adjustment should be set to Lean Best Idle. This does not affect maximum power, but will result in a good idle with maximum fuel economy.
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Process
NOTE: Idle speed must be adjusted with engine fully warmed, choke butterfly fully open. Take off air cleaner lid and confirm the choke is fully open before proceeding.
- Adjust curb idle speed. Carburetor Specifications vary, but any stock Datsun engine should idle fine at 750 rpm
- Mixture adjustment. Specs vary, but this will make any stock Datsun run nice:
- Adjust idle to 800 rpm
- turn mixture screw out (anti-clockwise) until engine runs at maximum speed. Readjust idle speed screw to 800 RPM. As you keep turning the screw out, it will start slowing down (idling too rich). If you go even further out, the screw will fall out and the engine will die (stop running). Turn screw in or out to the point where it runs the fastest.
- turn mixture screw in (clockwise) to lean the mixture down to 750 rpm
- Just learning? To get a 'feel' for this adjustment, turn the screw by half turns clockwise and anticlockwise till you hear the engine speed up and slow down. Then fine turn with 1/4 turns. Do this repeatedly until you are confident of the results
See also: http://www.redlineweber.com/html/Tech/carburetor_set_up_and_lean_best_.htm