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This controls the engine's idle speed or rpm. For 4-cycle racing applications, we do not recommend idling the carburetor as it makes the fuel mixture too rich at idle or sacrifices acceleration performance. We only use the idle speed screw as a stop for the throttle assembly to prevent the throttle shutter from damaging the body. For more information about idling 4-cycle Tillotson carburetors check out our blog.
15B-338 OE Tillotson HR Idle Speed Screw Fits HR-181B, HR-184A, HR190A, HR-191C, HR-191G, HR-196A