A carburetor start pump circuit, for starting an engine has an auxiliary fuel pump mounted on a relatively standard carburetor body, a start pulse passage extending through the carburetor body to the auxiliary fuel pump, and a fuel circuit having an intake side which extends from a metering chamber of the carburetor body to the auxiliary fuel pump and a discharge side which is interconnected to the intake side and extends from the auxiliary fuel pump to a throttle bore in the air intake of the carburetor body. To prevent the engine from stalling, a restriction jet is placed within the start pulse passage to prevent the auxiliary fuel pump from discharging fuel into the throttling bore when the engine transmits pulses at high frequencies.