What is the reason why the third engine doesn't work?

Causes of engine weakness are: 1. The throttle of the engine is too dirty or faulty. Its function is to control the air entering the engine. The throttle plate is too dirty, and the valve plate opening is not accurate; 2. The spark plug of the engine is broken or the electrode is worn. The gap between the two poles of the spark plug is too large, the ignition quantity is insufficient, the mixed gas combustion is insufficient, and the power is reduced; 3. It may also be the fuel supply system. Because the vent hole of the fuel tank cover is blocked, the fuel tank switch, fuel filter and the oil pipe between the fuel tank and carburetor are partially blocked, which can not meet the required oil supply, resulting in the mixture being too lean and the engine unable to work.