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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. History 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. System
components 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. Vacuum spring 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. |