Carburetor Quandary
I race an '81 Camaro on a half-mile sealed clay, D-shaped oval in the Pure Stock class. The engine is a 350 outfitted with flat-top pistons, stock intake, and exhaust manifolds, .390-/.410-lift cam, 993 heads with stock valves, and a 750-cfm Quadrajet. A three-speed manual transmission and 5.57 rear gear get the power to the rear wheels.
A veteran racer installed a set of used DA secondary metering rods in the Quadrajet, which is a rebuilt unit intended for a '72 Camaro. After this simple swap, the carb was deemed ready to race. Today I inspected the rods and found significant wear on one rod where it contacts the hanger, and the other rod has a slightly bent tip.
The car performs up to my expectations, but the air/fuel mixture seems to be rich. Spark-plug and exhaust-pipe color is consistently black. The plugs are also slightly damp.
Do you have any advice on secondary metering rod and primary jet selection?
Lee Atkins
Jacksonville, FL
Let me see if I can help you here. On the basis of the information you've provided, several thoughts come to mind.
First, the installation of used metering rods and a hanger assembly runs the risk of parts being previously damaged. Bent metering rods may hang in their respective jets, possibly causing improper fuel enrichment, and mixtures that are either too rich or too lean. In fact, depending upon the extent of metering-rod damage, rod movement may be sufficiently impaired to cause extreme rich/lean conditions somewhere in the rpm range.
You should replace the metering rods and hanger assembly with new parts right away. Next, make certain the vacuum piston and its bore are free of any nicks or burrs that may impede movement of the piston. Then, see if you can obtain a couple of different sets of metering rods. The recommendation would be to opt for ones that provide leaner intermediate steps. This will help part throttle torque and response in your race car. Keep in mind that the idle enrichment is a source of fuel that never stops once it is activated. The idle circuit will reach its maximum delivery point and remain there throughout the rpm range.
For calibration (including jet selection), a good plan is to jet and select corresponding metering rods for the best power. Then you can use a spark plug heat range that allows reasonable plug life. Stated another way, this means calibrate fuel for power and put a plug in the engine that will live. However, do not put a plug in that will run so hot as to encourage detonation.
In any case, the suggestion is that you use only new metering rods, new hanger assemblies, and new jets. Experimenting with the hanger "cam" can also be a fine-tuning method. As a starting point, see how well a "K" cam will work in your Quadrajet.
I hope this will help you with your issues in the carburetor.
Jim McFarland
AutoCom
Austin, TX