The present invention relates to a variable venturi carburetor which is feedback-controlled with the air of an exhaust gas sensor.
FIG. 1 shows a construction of the conventional feedback type carburetor. In this carburetor, the signal from an exhaust gas sensor 4 is sent to a control circuit 7 where it is compared with the target air-fuel ratio. The control circuit 7 then supplies a control signal based on the comparison to an actuator 6 to control the amount of air bleed to be supplied to the main fuel system or the idling system of the carburetor 5 so as to maintain the air-fuel ratio of the mixture at an optimum value. The conventional carburetor has the main fuel system and the idling system and there is a substantial time lag after the two fuel systems have been switched over until a desired amount of fuel is supplied to the intake manifold. The conventional carburetor also has the following drawbacks: when the fuel begins to be supplied from the main fuel system, the vacuum pressure level at the main nozzle 5-3 is not sufficiently high so that the fuel is injected irregularly and non-uniformly and therefore the air-fuel ratio of the mixture fluctuates greatly; and the amount of air bleed for both fuel systems must be controlled simultaneously. In other conventional carburetors (not shown), the negative pressure derived from the intake manifold is regulated at a constant vacuum level and is further controlled at a desired level by the solenoid valve whose open-close time ratio (duty ratio) has a linear relationship with the controlled vacuum level and which is determined on the basis of the signal from the exhaust gas sensor. The controlled vacuum pressure is then admitted to a vacuum diaphragm chamber of an air-bleed control means to control the amount of air bleed and thereby control the richness of the mixture to be drawn into the engine. This type of carburetor requires a regulator means to regulate the negative pressure derived from the intake manifold at a constant level. The former type of carburetor has the problems of slow feedback response when the two fuel systems are switched over, and of uneven supply of fuel when the fuel begins to be delivered from the main fuel system.