The Stovebolt Site Forums The Shop Area Engine Shop Carter carburetor metering rod technology

Carter first began producing carburetors with metering rod technology in 1929 with Carter number 136s used on the 1929 Chevrolet six.

Basically, metering rod techology allows the engineer, or later, the tuner, to provide different metering calibrations for different engine conditions. Most early Carters used linkage directly connected to the throttle linkage, such that the metering rod(s) were physically moved with the throttle linkage.

Later enhancements allowed for a vacuum control (a tensioned spring working against engine vacuum) with a mechanical backup. This enhancement allowed for more precise control of calibrations for a given engine condition.

Today's tuners can find metering rod technology is both a blessing and a curse. The blessing in the results obtainable by a persistant tuner; and a curse in that, while rods, jets, and springs are available (custom made) by mail order, they are not available at the local FLAPS.

Of course, in 1929, Carter engineering had to have a method of producing prototype metering rods; thus a custom lathe was hand-built for this purpose:

http://www.thecarburetorshop.com/Lathe_1.jpg

Specifications for the lathe included:

(A) a motor turning 3450 RPM instead of the normal 1725
(B) a special cutting head mounted in the tailstock that slides over the stock, with 4 cutting tools equally spaced at 90 degrees from each other thereby preventing excessive deflection of the small round stock:

http://www.thecarburetorshop.com/Lathe_3.jpg

(C) a special headstock accepting collets to hold the round brass stock:

http://www.thecarburetorshop.com/Lathe_4.jpg

And jigs were necessary to check the diameter and spacing of the pigtail, and the length of the rod. The picture below is the jig for the W-1 rod. Each carburetor style used different rods, some even used rods of different lengths:

http://www.thecarburetorshop.com/Lathe_5.jpg

The sizes of the rods were specified to the ten-thousanth of an inch so all of the above were necessary to keep the tolerances within range.

For some reason I never learned, several different rod diameters were used. Below is a picture of 0.072 rod stock:

http://www.thecarburetorshop.com/Lathe_2.jpg

Since the total calibration at any given engine condition is the diameter of the jet less the diameter of the step of the rod currently in the jet; simply changing the jet(s) MAY allow for a precise change at one engine condition, this will virtually always throw off the calibration at other conditions. Example: someone that NEVER operates the engine at wide-open-throttle may install a smaller jet to improve cruise economy, but runs the risk of the engine going very lean at WOT.

Rods, depending on the style of the carburetor, were produced in 2 step, 3 step and 4 step calibrations. Most were tapered between steps (try cutting a taper from 0.0455 to 0.05025 on a manual lathe).

Since Carter continued with this technology until the company was bought out, many enthusiasts that tour replace original carbs with Carters, as lean rods can be fabricated for use at various altitudes.

Jon.