robert, this is highly dependent on what engine model you have and what carb is installed. These instructions are for Vanguard Single Cylinder:
"Initial Mixture Adjustment"
1. Turn the idle mixture screw ([A] Fig. 1 2) in until you feel it just make contact with the seat.
2. Back the idle mixture screw out 1-1/4 turns.
3. Start the engine and let run at half throttle for 5 minutes to bring it up to operating temperature.
Adjusting Idle Mixture RPM
1. Move throttle to SLOW.
2. While holding the speed control lever (B) against the idle speed screw, adjust the idle speed screw (C) to obtain 1750 RPM, except as noted for the following models:
• Model 104700: 1500 RPM
• Models 161400, 260700, 261700: 1200 RPM
• Models 085400, 086400, 115400, 117400, 118400, 185400: 1300 RPM
3. Turn idle mixture screw (A) clockwise (CW) until the engine just begins to slow.
4. Turn idle mixture screw counterclockwis
5. Turn idle mixture screw back to midpoint and release the speed control lever.
6. Install limiter cap (if equipped) using knockout pin (#19135)."
For more precise instructions for your mower, it is imperative to have the engine number, model etc. but I do hope this helps. Good Luck.