Carter WD-0 2 Barrel Carburetor High Speed Circuit

The intermediate and high speed circuit consists of the metering rod, main jet, nozzle, anti percolator, anti percolator arm assembly, vacuum piston & link assembly.

 

As the throttle is opened wide enough for a no load speed of a little more then 20 mph the velocity of the air flowing down through the carburetor throat creates a pressure slightly less than atmospheric at the tip of the main nozzle.

Since the gasoline in the float bowl is acted upon by atmospheric pressure, the difference in pressure between the two points causes gasoline to flow from the bowl, through the metering jet, and out the main nozzle into the throat of the carburetor.

As the speed increases from 20 mph the high speed system continues to cut in more and more and the idle or low speed circuit to cut out until at 30 mph the high speed circuit is carrying the entire load and the idle system is doing nothing.

At higher speeds, the area of the opening between the jet and the metering rod governs the amount of gasoline going into the engine. At top speed, the smallest section of the rod is in the jet. It requires more vacuum to operate teh low speed circuit than it does the high speed circuit, and the reason that the low speed system ceases to operate at 30 mph is that the vacuum in thethroat of the carburetor has decreased until it is no longer strong enough to draw gasoline through the low speed circuit.

Motors operated at part throttle on level road use a mixture of maximum leanness. The mixture for greatest power and acceleration is somewhat richer.

Under part throttle acceleration and for hard pulling at part throttle, the power mixture is required for a short time. This requirement of the power mixture always coincides with ta drop in manifold vacuum. The vacumeter carburetor meets this demand by allowing the drop in manifold vacuum to permit a spring to move the metering rod to the proper step and give the required richer mixture the instant it is required, independent of the throttle opening. As soon as the demand is passed as shown by the rise of manifold vacuum, the metering rod moves down against a tongue attached to the pump arm and it is then controlled mechanically until another such demand arises.

The vacumeter control consists of a small brass piston beneath which is a spring fitted into a cylinder bored into the carburetor casting. The lower end of the bore has a drilled passage opening into the carburetor throat below the throttle valve. the vacuum acting upon the piston through this passsage holds the piston down and compresses the spring as long as the vacuum is stronger than the spring. The spring under the piston is calibrated to force the piston up to a carefully determined point (not the top of the cylinder) when manifold vacuum drops below 3″ of mercury. This spring varies in length for different carburetors and the correct spring must be used. It should never be altered. With the manifold vacuum above 3″ of mercury, the piston is pulled down until the arm on the piston link rests on the tongue attached to the pump arm. When the piston is down this tongue lifts the metering rod in direct ratio to the throtle opening without regard to vacuum.

 

Servicing the High Speed System

Main Nozzle – Can become restricted. Ethanol will leave deposits behing that regular cleaning cannot remove. Run thin wire through the tube, being careful not to make the hole any bigger. A restricted jet can cause poor performance above 20 mph. If the hole is too large, increased gas consumption will occur above 20 mph. A leak at the seat can cause a loping idle and poor performance at all speeds.

Make sure there is 1 and only 1 gasket under the main nozzle.

Metering Rod – If the metering rod is too large, flat spots will be experieneced on light acceleration from any speed. Where the linkage is adjusted so that the metering rod is lower in the jet than it should be, the steps of the metering rod will not be timed properly as they are raised out of the jet.

A metering rod too rich (small) will give poor gasoline economy at all speeds above 20 mph. If the metering rod linkage is adjusted too high, it will affect gasoline mileage at all speeeds above 20 mph. Changing the adjustment of a metering rod 1/16″ will definitely affect gasoline economy. A worn metering rod will have the same effect upon gasoline economy as a metering rod that is too rich and must be replaced. Measure the rod with a micrometer at different steps. If worn, the metering rod jet will also be worn and both should be replaced. Metering rods are no longer available, so you will either have to custom make a rod, or change the main jet to accomodate the difference.

Metering Rod Jet – A rich, lean, or worn metering rod jet has the same effect as a rich, lean, or worn metering rod. If a metering rod jet is not correct, it should be replaced.

Anti Percolator Valve – Which may open early or not at all. An Anti Percolator Valve that opens early upon deceleration may allow a quantity of air to be dreawn into the high speed system which will result in a flat spot as the car is accelerated immediately after deceleration. If the valve does not open at all, it will fail to relieve the vapor pressure and hard starting will result with a hot engine.

The metering rod controls the flow of gasoline and if adjusted too high or too low, the mixture will be too rich or too lean at certain speeds. To adjust the metering rod: Back out throttle lever adjusting screw. Insert gauge T109-113 in place of metering rod, seating tapered end in metering rod jet. Hold gauge vertical to insure seating. Bend tongue on anti-percolator arem, use T109-105, so that when the vacuum piston shaft is pushed down all the way, with throttle valve seated, metering rod pin will rest lightly on shoulder in notch of gauge. Remove gauge, install metering rod disk and metering rod spring. Be sure metering rod is in jet. If metering rod shows wear replace it. (note: metering rods are not available, find a NOS rod, or custom make one). Care must be taken that the vacuum piston is clean and dry, shows no wear, and moves freely. A spring of proper length must be under the piston.

 

When you don’t have the Carter adjustment tool – Metering Rods: I make sure that the “finger” lifts them all the way to the top of their travel (or very nearly so) with throttle wide-open and the accel. pump link adjusted. Just be certain that they don’t contact the bottom of the pot metal cover.