An internal combustion engine including a carburetor (60, 70) having a first fuel ejection port located in a venturi section for feeding fuel to an intake passage, the carburetor (60, 70) feeding the fuel to the intake passage by sucking out the fuel from the first fuel ejection port to the intake passage with negative pressure around the first fuel ejection port generated by an airflow in the venturi section, the internal combustion engine comprising: a metering chamber for accumulating the fuel pumped up from a fuel tank; a first fuel feeding passage for feeding the fuel in the metering chamber to the first fuel ejection port; a second fuel ejection port arranged in the venturi section for ejecting the fuel to the intake passage with negative pressure around the second fuel ejection port generated by the airflow in the intake passage, wherein the second fuel ejection port and the first fuel ejection port are configured by a common ejection port; a first additional fuel feeding passage coupled to the common ejection port for feeding the fuel in the metering chamber to the common ejection port; a flow rate adjusting valve provided in the first additional fuel feeding passage for adjusting a flow rate of the fuel flowing through the first additional fuel feeding passage; engine speed detecting means for detecting engine speed; and electronic control means for receiving a signal from the engine speed detecting means to control an opening degree of the flow rate adjusting valve; characterized in that the engine further includes a manual adjustment valve provided in the first fuel feeding passage, wherein a flow rate of the fuel flowing through the first fuel feeding passage can be adjusted by operating the manual adjustment valve; and in that the flow rate adjusting valve is set such that a reference opening degree thereof is a median in an adjustment range of the flow rate adjusting valve. a metering chamber for accumulating the fuel pumped up from a fuel tank; a first fuel feeding passage for feeding the fuel in the metering chamber to the first fuel ejection port; a second fuel ejection port arranged in the venturi section for ejecting the fuel to the intake passage with negative pressure around the second fuel ejection port generated by the airflow in the intake passage, wherein the second fuel ejection port and the first fuel ejection port are configured by a common ejection port; a first additional fuel feeding passage coupled to the common ejection port for feeding the fuel in the metering chamber to the common ejection port; a flow rate adjusting valve provided in the first additional fuel feeding passage for adjusting a flow rate of the fuel flowing through the first additional fuel feeding passage; engine speed detecting means for detecting engine speed; and electronic control means for receiving a signal from the engine speed detecting means to control an opening degree of the flow rate adjusting valve; characterized in that the engine further includes a manual adjustment valve provided in the first fuel feeding passage, wherein a flow rate of the fuel flowing through the first fuel feeding passage can be adjusted by operating the manual adjustment valve; and in that the flow rate adjusting valve is set such that a reference opening degree thereof is a median in an adjustment range of the flow rate adjusting valve.