Personally I prefer to use a vacuum gauge.
Connect vacuum gauge to the inlet manifold & external tach to the ignition system, then start engine. [allow engine to reach full operating temp first & this is probably best done by going for a drive BEFORE connecting the Vac gauge & tach]

Restrict the airflow in the vacuum gauge tubing by using either a restrictor or by mechanically squeezing the tube [vyce grips?] untill the needle on the gauge stops jumping about. The needle must give a nice steady reading.

Turn MIXTURE either way & watch vacuum gauge. You are looking for the highest available reading.

When highest reading achieved, then adjust idle speed by turning the IDLE SPEED adjustment screw untill correct idle speed is achieved.

Repeat idle MIXTURE adjustment in attempt to improve vacuum reading.
If reading can be improved, then re-adjust idle speed & repeat idle mixture adjustment.

Once the mixture & speed are right on the money the polution cops want you to lean out the mixture by a specific value, often a quarter turn clockwise on the mixture screw but I usually richen up to the tune of a drop of about 20-25 rpm then readjust the idle speed.

Once the idle is correctly adjusted with this method, then a fully serviceable engine will tick over like a swiss watch.

once my ride puffed black smoke. not because of the idle mixture, but the power valve wont close.

The idle mixture screw should have no effect on smoke, if it's anywhere near correct. I suppose if it's way off it make cause black smoke.

Nissan specifies the "best idle" method of setting the idle mixture. Then lean it a bit per the process. But it can be too lean or too rich and still not smoke.

I had this issue the other day but silly me didn't bother to put a torch up to the weber to see if there was an idle mixture screw, I thought it was blanked off like the other ports on this particular carby (it turns out it's just recessed inside the carb, the other 32\36 I have it is quite noticable because it's a large external screw) so I took it to my mechanic to figure out why it was blowing smoke like a chimney. Not even a 1\4 turn and problem solved.

turning the mixture clockwise screw leans the mixture. air flow still the same, the fuel decrease.

Hi DD

So then turning it clockwise should reduce smoke ? is it ?
so means Idle Air Mixture screw controls fuel flow, i thought it controls the air flow into the carb , i am bit confused now . if you can explain it me clearly please ?

thanks
Regards
sumith

That is not correct for a Datsun engines.
The correct procedure is here on Datsun1200.com. See Tune-up, engine section, step 7.

Clockwise DEcreases fuel flow in the Datsun carburetor. Some other kinds is just the opposite.

Hi Guys

I was going through some web automotive forum on how to setup best idle air mixture for carburetor and found this

Keep turning air mixture screw clockwise until engine dies or just to die and turn it anti clock wise half a turn and set the accelerator pedal screw to correct rpm .

and also i have few questions ?

turning idle air mixture screw clockwise will increase the fuel flow or decrease it ?

is this the correct way ?

thanks
Regards
sumith