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Carter WDO Carburetor Idle and Low Speed Circuit
Carter WD0The idle and low speed circuit completely controls the supply of gasoline to the engine during idle and light load speeds up to 20 mph. and it partially controls the supply for light load speeds between 20 and 30 mph.

The idle and low speed circuit consists of the low speed jet, bypass, air bleed, port opening, idle adjusting screw, throttle valve, carburetor throat and economizer.

During idling and low speed operation of the engine, gasoline flows from the float bowl through the idle speed jet to the point where it is combined with a stream of air coming in from the carburetor throat through the by pass. The combining of the stream of air with the stream of gasoline tends to atomize or break up the gasoline into a vapor.

This mixture of air and gasoline continues on through the economizer and is further combined with a stream of air coming in through the air bleed. This, again, tends to break the gasoline particles into a finer vapor. The gasoline and air mixture that flows downward in the passage from the air bleed is still richer than an idle mixture needs to be, but when it mixes with the air which has come past the throttle valve, it forms a combustible mixture of the right proportions for idle speed.

Be sure your air bleed is clean by running thin wire down the passage.

This mixture of air and gasoline continues on through the economizer and is further combined with a stream of air coming in through the air bleed. This, again, tends to break the gasoline particles into a finer vapor. The gasoline and air mixture that flows downward in the passage from the air bleed is still richer than an idle mixture needs to be, but when it mixes with the air which has come past the throttle valve, it forms a combustible mixture of the right proportions for idle speed.

The idle port is made slotted so that as the throttle valve is opened, it will not only allow more air to come in past it. but will also uncover more of the idle port, allowing a greater quantity of the gasoline and air mixture to enter the carburetor throat from the idle mixture passage.

The idle position of the throttle is such that at an idle speed of 7 to 8 mph, it leaves enough of the slotted port as reserve to cover the range in speed between idle and the time when the high speed system begins to cut in.

The idle adjusting screw varies the quantity of the idle mixture entering the carburetor throat which mixes the air coming past the throttle valve. In this way, it varies the richness of the mixture finally entering the engine.

Since the throttle lever adjusting screw not only regulates the idle speed by opening or closing the throttle valve, but also regulates the amount of air coming past the throttle valve during idle speed, it partially regulates the richness of the mixture finally entering the engine and for this reason, must be adjusted in conjunction with the idle mixture adjusting screw since the two adjusting screws must be balanced in their adjustment.

All the gasoline flowing from the float bowl during the idle period and at no load speeds up to 20 mph flows through the small metering hole in the low speed jet. This hole is held to a manufacturing tolerance of .00025" and is a very important factor in controlling the flow of gasoline into the engine during these speeds. In the past it was recommenced to not clean the passage with anything other than compress air, but ethanol leaves deposits behind that only thin wire can get out. Be very careful not to widen the hole.

Servicing the Low Speed Circuit

If the low speed metering hole in the end of the jet is too large because of improper cleaning, or is clogged so that it cannot be cleaned with compressed air, the jet should be replaced.

The black carbon deposit which forms in the throats of carburetors may restrict the air bleed holes to the extent that insufficient air will be supplied to mix with the gasoline before it reaches the idle port. This condition will generally be indicated by having to screw the idle mixture adjusting screw in closer than the minimum limit. If the condition is bad, a "loping" idle may continue even after the idle mixture adjusting screw is screwed entirely in against the seat. These air bleed holes may be cleaned by wire or proper drill bits.

The idle mixture passage - which may be restricted. This condition is quite unusual but it may be found as the cause of an unsteady idle when no other remedy seems to correct the trouble. The passage should be cleaned by removing the aluminum plugs from the carburetor body and using a wire and compressed air.

Idle port - must be clean and unrestricted. If damaged, motor will not perform properly at low speed.

Idle adjustment screw - point of screw must be smooth and free of burrs. Spring must have proper tension to hold adjustment.

Throttle valve may be installed incorrectly. A capital "C" enclosed in a circle is stamped on the face of the valve. When installed in the carburetor, the "C" should be on the side of the valve toward the idle port, and facing the manifold. To properly center the valve in the throat of the carburetor, back out throttle lever adjusting screw so that it does not contact its seat, start the valve screws into the shaft and then while holding the carburetor with the air horn down, and the valve seated tight, tap valve on upper side. This will centralize the valve in the carburetor throat. Pressure should then be maintained with the finger until the screws are tightened. If the valve is worn or bent, replace, as it affects the port relation. Valves are made with two opposite edges beveled so as to fit the carburetor throat when valve is closed. Always make sure the valve is not installed wrong as poor performance at low speed will result.

Throttle shaft and lever - If the shaft is badly worn it will affect the port opening or if lever is loose on shaft, carburetor will idle slow one time and fast another. Replacement would be best, but you are not likely going to find a new one. When the lever is loose, you can drill and tap a hole in the shaft, then use a screw and washer to clamp it. You can also braze the lever to the shaft.

Valve screws - before removing the valve screws grind the screw end flush with the shaft. This will make it easier to get them loose. When installing new screws add a small amount of red permatex thread lock.

Carburetor throat - which may be restricted with a carbon deposit. This will make it necessary to open the throttle wider than the specified opening to obtain the proper idle speed. Opening the throttle more than the specified amount in order to obtain the proper idle will uncover more of the slotted idle port than was intended in the calibration of the carburetor. This will result in leaving an insufficient amount of the idle port as reserve to cover the period between idle and 20 mph when the high speed system begins to cut in. A flat spot will be the result. Clean by scraping or with emery cloth. I have found the Purple Power parts cleaner does a good job on the black marks. Use a tooth brush to get at the small places. Do not soak your carburetor in this cleaner for any extended period of time.




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