The engine suddenly dropped from 750 rpm to 500 rpm, and then returned to normal.

The throttle is dirty. The reason why the engine suddenly drops from 750 rpm to 500 rpm and then returns to normal may be that the throttle valve is dirty, resulting in low idle intake, and the engine detects that it is going to turn off, and then adjusts it. Engine is a machine that can convert other forms of energy into mechanical energy, including internal combustion engine (reciprocating piston engine) and external combustion engine (Stirling engine, steam engine, etc.). ), jet engines, motors, etc.