The Carburetor Shop LLC
204 East 15th Street
Eldon, Missouri 65026
Carter Metering Rod Technology
What is "metering rod technology"? It is a method of changing the calibration of the carburetor in response to different stimuli by inserting a movable tapered rod through the main metering jet. The effective calibration at any time would be the area of the jet less the area of the portion of the rod currently in the jet.
As best I can determine from the Carter records, Carter began using metering rod technology in 1929 on some models of their brass bowl updraft carburetors. The first use I can find was the carburetor used on the 1929 Chevrolet 6-cylinder, Carter number 125s. The early installations were mechanical in operation; that is, the metering rod was directly connected to the opening of the throttle shaft, and the position of the rod in the jet was determined by the amount of opening of the throttle valve.
In 1932, Carter started using another method of calibration change, based on vacuum, rather than throttle opening. This method, consisted of a vacuum piston held in a certain position by vacuum, which would change position when vacuum was reduced by additional throttle opening by the tensioned spring pressing against the piston. This method was used on the Carter BB updraft (BB does NOT stand for brass bowl, as in the paragraph above, the BB was a new series released in 1932). The piston pushed against a pushrod, which in turn would press open a ball bearing valve, which was normally held closed by another spring. Additional fuel would then flow through a fixed auxiliary jet. Thus enrichment occurred, but only at a fixed rate.
By 1940, Carter combined the tapered metering rod with the vacuum piston/spring combination on their more expensive carburetors, while leaving the mechanical only metering rods in their less expensive models.
The system components are: (1) the main metering jet, (2) the metering rod (or in the case of some later carburetors, the step-up rod), (3) the vacuum spring, and (4) the vacuum (or step-up) piston. Considering each of these components:
Main metering jet
The main metering jet resembles a main metering jet from other carburetor manufacturers, with an orifice that is larger than a competitor's jet from a comparable sized carburetor because of the rod. The size of the jet must be sufficiently large to flow enough fuel for WOT with the smallest diameter of the rod in the jet.
Metering rod (or step-up rod)
Carter used metering rods with a single step, 2-step, 3-step, and 4-step. The single step rods were used on some of the price leader carburetors (i.e. BBS) and were basically an on/off condition similar to the function of a power valve. Most of the single step rods were steel. The jets for these rods are much taller than normal, and have a steel insert in the top (higher than the metering orifice) of the jet to position the rod so the rod would slide back into the jet after it was raised out of the jet. For street use, changing the number of steps on a rod is asking for trouble. For a trailered race car, it is often permissable.
The multiple step rods are machined from brass. Each series (i.e. WCD, AFB, WCFB, etc.) have their own group of rods. Some series have multiple rod lengths; all series have variable step diameters and step lengths.
Probably the most common question I am asked about metering rods is can I substitute "clone" rods into my original Carter AFB. The short answer is "probably not!!!". The longer answer is "maybe, but you probably WON'T like the results". That brings about question number 2 "why not?". There is no short answer here. Part of the reason is the physical rod profile (NOT the calibration profile). To date, I have identified 19 different physical profiles for the step-up rods used by Carter in their AFB carburetors. There are two different physical lengths, but within especially the shorter length, there are many different step lengths. As an example, two common power tip lengths are used: (1) 0.250 inch, and (2) 0.400 inch. There are others. So if a rod with a 0.400 tip is replaced by a rod with a 0.250 tip, the power calibration is going to be late at WOT, possibly contributing to hesitation, or worse, detonation. The second reason is trueness. Carter diameter tolerances for the rods were +- (plus or minus) 0.00002 inch. The lathe which is used to cut the rods is quite special in its construction. The stock is turned at a much higher than normal rotational speed, and the cutting service is a 4-blade cutter with the blades separated by 90 degrees which is contained in the tail stock. The tail stock with cutter slides over the rod, cutting the diameters necessary. Even when replacing rods with original Carter rods, one should always compare the physical profiles of the two rods.
The vacuum springs come in a variety of tensions, and are placed beneath the vacuum piston, and hold the vacuum piston in the up (fully rich) position when there is no vacuum. Vacuum springs DO fatigue with time and use, and SHOULD be replaced during a carburetor rebuild. Vacuum piston springs are included in the better rebuilding kits for Carter. As the movement of the piston (hence the rod) is determined by the opposing forces of vacuum and spring tension, change in vacuum values because of a change in camshaft profile can cause the calibration of the carb to change. Thus if a camshaft with a different profile from stock is placed in the engine, the carburetor builder needs to retune the carburetor by changing the tension of the springs.
Vacuum piston (or step-up piston)
The vacuum piston is a cylinder, generally with a fluted outer diameter, and a hole on the inside sufficiently large that one end of the vacuum spring could be inserted into the hole; and some physical means of attaching to the metering rod(s). The piston is then placed in a cylindrical chamber from the top, where the bottom of the chamber is exposed to engine vacuum. The piston will physically move up and down inside the chamber due to the opposing forces of the spring tension and engine vacuum.
Early vacuum pistons were machined from brass, later pistons were machined from aluminum. THIS IS IMPORTANT!
A plug here for Carter marketing philosophy. It was found that the brass pistons were harder than the zinc alloy cylinder (aluminum in later series) in which they rode, wearing the cylinder to the point where high mileage carburetor actually needed to be machined for a brass sleeve to maintain the piston to cylinder tolerance. Thus the brass pistons were discontinued (a superceding aluminum piston was designed to replace each of the existing brass pistons) on carburetors constructed from this point in time forward. Carter wanted their product to be serviceable, rather than throw-a-way (well, maybe not on their very cheapest carbs i.e. model AS). Thus the aluminum pistons were machined from an alloy of aluminum SOFTER than the zinc alloy or aluminum carburetor castings. In other words, the pistons were/are sacrificial. AFB pistons are contained in the better carburetor rebuilding kits.