43-991 Tillotson Low Speed or Idle Mixture Screw is commonly found on HR Tillotson carburetors. 

The low speed controls the fuel that enters the low side fuel well. This low side adjustment screw is sometimes called an idle mixture screw but it does more than control the idle. The low side screw also controls the low speed, and midrange and can be viewed as the main circuit of the carburetor. 

On 2-cycle engines, the original engines for the Tillotson carburetors, these carburetors do idle. On 4-cycle engines, the carburetors are too rich at idle, especially on methanol. This condition causes a dead spot or hesitation and leaning the low side will cause the engine to run flat and not accelerate. Both conditions are fixed by not setting the low side mixture or idle speed screw to idle the carburetor. 

43-991 OE Tillotson HR Idle Mixture Screw (Positive stop type). Fits HR-181B, HR-184A, HR190A, HR-191C, HR-191G, HR-196A