How the Zenith 267 Works

The Zenith 267 carburetor is a 1 barrel updraft type of carburetor. They are made with a selective fuel inlet, with or without a back suction economizer and a main jet adjustment.

The fuel bowl is designed so that the carburetor can be used at extreme angles without flooding or starving.

The mounting bolt pattern on the Zenith 267 can vary a bit. 2 3/8″ and 2 11/16″
The information below illustrates each circuit in the Zenith 267 carburetor. This can help when diagnosing any problems you may be having.

Fuel Supply System

The fuel supply system is made up of the fuel inlet, needle & seat and fuel bowl. The level of fuel in the float bowl is regulated by the float. The needle, or float valve stays open regulated by the float so that a sufficient amount of fuel is being delivered to maintain the correct fuel level. The fuel bowl is vented from the air intake through the fuel bowl. Be sure this passage is clean and clear.

Idle System

The idle system consists of 2 idle discharge ports, idle air passage, idle air bleed, idle adjusting needle, idle jet, and fuel passage.

The fuel for the idle is supplied through the main jet to a well directly below the main discharge jet. The pick up passage is connected to this well by a restricted drilling at the bottom of this passage. The fuel travels through this channel to the idle jet calibration. the air for the idle mixture originates back of (or from behind) the venturi. The idle air bleed controls the air to be mixed with fuel for the idle mixture.

This system is referred to as a two hole idle system. the idle fuel air mixture is discharged into the air stream of the carburetor through two calibrated holes located in the throttle body. The location of these holes is in direct relation to and is determined by the position of the throttle plate when completely closed.

The discharge of the idle fuel mixture into the air stream is controlled directly by the idling adjusting needle located in the throttle body at the upper idle discharge hole.

Turning the idle adjusting needle in, clockwise, creates a richer idle fuel mixture because more of the fuel mixture is discharged into the air stream through the idle discharge hole.

The fuel is metered by the idle jet and is mixed with air admitted by the fixed idle air bleed located in the air channel. This fixed air bleed meters air to the idle jet where it mixes with the fuel metered through the idle jet calibration. This idle fuel mixture is drawn through the idle mixture passage, and its discharge into the air stream is controlled by the idling adjusting needle.

Note that in this type of idle system the idling adjusting needle controls the amount of the idle fuel mixture that is discharged into the air stream.

High Speed System

The high speed system controls the fuel mixture at part throttle speeds and at wide open throttle. This system consists of a venturi, controlling the maximum volume of air admitted into the engine, the main jet calibration, which regulates the flow of fuel from the float chamber to the discharge jet, the well vent, which maintains uniform mixture ratio under changing suction and engine speeds, and a discharge jet, which delivers the fuel into the air stream.

The main jet controls the fuel delivery during the part throttle range from about one quarter to full throttle opening. To maintain a proper mixture ratio a small amount of air is admitted through the well vent into the discharge jet through the air bleed holes in the discharge jet at a point below the level of fuel in the metering well.

The passage of fuel through the high speed system is not a complicated process. The fuel flows from the fuel chamber through the main jet calibration and into the discharge jet where it is mixed with air admitted by the well vent, and the air fuel mixture is then discharged into the air stream of the carburetor.

Economizer System

The economizer system consists of a milled slot in the throttle shaft, which acts as a valve to open or close the system; a vacuum passage from the throttle bore to the slot in th throttle shaft, and a vacuum passage from the slot in the throttle shaft to the bowl vent channel.

The economizer system performs its function by establishing a back suction on the fuel in the fuel bowl during most of the part throttle range of operation. This back suction is created by manifold vacuum, through the channels connecting the throttle bore with the fuel bowl. This retards the flow of fuel through the metering systems and thus permits the carburetor to operate on leaner part throttle mixture ratios.

The rotation of the throttle shaft controls the economizer system. During part throttle operation from about one quarter to three quarters throttle, the passages are open and the pressure in the fuel bowl is lowered. This retards the flow through the main jet and a leaner mixture is supplied. On full throttle opening the passages are closed and the main jet flows to full capacity to supply the richer mixture required.

It is important to make sure these small passages are fully open. They could be caked up with ethanol residue.

Choke System

The choke system consists of a valve mounted on a shaft located in the air entrance and operated externally by a lever mounted on the shaft. The choke valve is used to restrict the air entering the carburetor. This increases the suction on the jets when starting the engine. The choke valve is of a semi automatic type, having a poppet valve incorporated in its design, which is controlled by a spring.

The poppet valve opens automatically when the engine starts and admits air to avoid over choking or flooding of the engine. The mixture required for starting is considerably richer than that needed to develop power at normal temperatures. As the engine fires and speed and suction are increased, the mixture ratio must be rapidly reduced. This change is accomplished through adjustment of the choke valve and the automatic opening of the poppet valve to admit more air when the engine fires.

Updated on 12/15/2020

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