Idle Screw Adjustment

• The carb's idle screws control the engine's RPMs and can be tuned to help the engine run more smoothly. With the engine on, turn the two idle mixture screws all the way to the right (you need a screwdriver to do this). After the screws are tight, loosen each one. The goal is for the engine to idle smoothly -- the RPM should be from 800 to 900 at this point, although this can vary with each carb. Each screw should be turned to match its counterpart to ensure proper adjustment.
  
  

Float Level Adjustment

• The float level directly affects your carb's performance. First, locate the float bowl in your carb -- the bowl typically is located at the bottom of the carb and has angled valves pointing inward at the top of the float bowl casing. The float bowl is contained in a metal casing (the casing can be made of brass, silver or bronze to resist high temperatures). On the side of the float bowl there is an adjuster for the float level (in the form of a hex bolt) and a sight plug to observe the level inside the float bowl. The engine should run for 10 minutes and then the sight plug must be removed to begin adjusting the float level. Before you remove the sight plug, place a rag around the plug's opening to catch any fuel that spills. The adjuster is then rotated to lower the fuel level. After the fuel level goes below the sight on the float bowl, the plug is replaced.
  
  

Choke Adjustment

• The carb's choke is essential for starting a cold engine; for this reason, issues with the choke are most often apparent during cold weather. Some signs of a choke that needs adjustment include a rough idle and black smoke coming from the exhaust pipe. You can adjust the choke by rotating the choke cap (located at the top of the carb) one notch at a time while your engine's running and then observing the idle for improvement.
  
  

Accelerator Pump Tuning

• The accelerator pump determines whether your carb runs lean or rich. These two terms indicate the level of fuel mixed with air inside the engine. When a carb runs lean, there is too little fuel in the mix; when the engine runs rich, there is too much. Both make an engine perform poorly. If your engine is running too lean, you will notice lower gas mileage and hesitation in the acceleration. If the engine is running too rich, you might notice an unresponsive throttle and excessive engine vibration. To solve this issue, the accelerator pump is tuned by accessing its circuit at the bottom of the float bowl; as the circuit is tuned, the vehicle's idle is analyzed to determine whether it is still running too rich or lean. This can be a minute process (slight adjustments in the accelerator pump can affect the engine's performance) -- unless you have significant experience tuning carburetors, it is best to let a professional mechanic tune your accelerator pump.