1. When the air filter element of the intake system is filtered, the front of the filter element is in direct contact with the air, and the rear of the filter element is the filtered clean air. But not all impurities can be completely filtered out, there will be fine particles of dust or other dirty things, through the throttle and the throttle surface contact and residual, accumulated over time accumulation needs to maintain the throttle. 2, the installation of the air intake system is not in place, the pipeline interface displacement, leakage, clamp position is not correct or not screwed, resulting in unfiltered air entering, and the dust particles in the air accumulate in the inner wall of the throttle valve. 3, the working temperature of the summer engine is relatively high, the oil will produce oil steam, at idle speed and low speed, the oil steam will take the gum in the fuel and condense on the throttle. 4, the engine valve seal is poor, resulting in air leakage, pollution throttle, the inner wall behind the throttle black, serious dirt. 5, mixed air dust and oil steam mixed together, burning black. 6, the intake air filter has not been maintained for a long time, or caused by the use of substandard air filter elements.