Another difficult starting cause is an incorrect fuel/air mixture ratio. The function of the carburetor is to mix fuel and air in a fog, which is then absorbed by the engine to burn it. A mixing screw adjusts a needle valve/jet, which controls the flow of gas to optimize the air/fuel mixture, maximizing the output power (some motors have no set screw and the mixture is factory set). The jet works exactly like the nozzle in a garden hose, producing a fine pulverization of gas particles. Unless someone has turned this screw since the last time you used your lawn mower, this is unlikely to be a problem. However, sometimes the dirt or gums may clog the valve. To adjust the mix:
- If you are not sure if the screw was tampered with, turn it fully clockwise and then unscrew 1 1/2 turns counterclockwise.
- Allow the engine to run for five minutes until it is completely hot.
- Using a flat-head screwdriver, turn the screw clockwise, 1/4 back at a time, pausing for about 5 seconds after each adjustment until the motor starts to jam. Do this carefully, because if the spout is exceeded and screwed firmly into your seat, there is always a danger of damaging the soft tip of the needle valve.
- Now repeat this procedure, but this time turn the screw counterclockwise until the motor begins to jam again.
- Finally, place the screw halfway between the two loss positions. If the mixture is too poor, this can cause a phenomenon called "hunting", where the engine is put "brmmm brmmm brmmm" and accelerates and decelerates repeatedly. Hunting can also be caused by a stretched Governor's Wharf or an obstructed carburetor that restricts the flow of fuel. A poor mixture makes an engine run excessively hot and "spit". If the mixture is too rich, the symptoms are black smoke and an unburned gas odor.
- Make sure that the fiber or rubber gaskets on the shaft of the mixing screw are in good condition. If poorly sealed, it reduces the engine's ability to absorb fuel.
