When the engine air filter is blocked by dust to the point that the intake air volume cannot meet the air required for the engine to work, the following abnormal phenomena will occur in the engine:

1. Make the roar of the sinking net. This is caused by the lack of oxygen during the fuel combustion process, which prolongs the combustion time, reduces the working pressure, and causes the pressure difference at the moment when the exhaust valve opens.
2. When the fuel supply increases, the speed increases slowly. This is because the intake air volume is insufficient, and the detonation pressure of the working stroke is not enough, so that the speed increases slowly.
3. Inability to work. This is due to insufficient intake air, the entire combustion process lags behind, and the piston working stroke does not match the fuel combustion time, resulting in poor energy conversion efficiency.
4. The water temperature is relatively high. This is because the entire combustion process lags, when the piston has entered the exhaust phase, while combustion continues, overheating the cylinder.
5. After the speed increases, the exhaust smoke becomes thicker. This is because the intake air is insufficient, the air-fuel ratio decreases, and the mixture concentration increases, resulting in incomplete combustion.

When the engine is working, if the above two or more abnormal phenomena appear in the net, it can be basically determined that the air filter is blocked.

When maintaining the paper air filter, the color of the filter paper on the inner and outer surfaces of the filter element should be carefully checked. In the used filter element, the outer surface of the filter paper on the side in contact with the atmosphere is gray-black due to the deposition of dust. After removing the dust, if the original color on the outside of the filter paper is revealed, the inner surface of the filter paper facing the air inlet is still in its original color, and this filter element can continue to be used. When the outer surface of the filter element is cleaned of dust, the filter paper is still gray or the inner surface of the filter paper is dark, it must be replaced. The essence of discoloration is that some dust particles are adsorbed on the surface of the filter element under the action of static electricity. The inner surface of the filter paper is gray-black, indicating that the dust particles have penetrated the filter paper, and the filter element has lost its ability to filter dust.