The idle mixture screws on the Edelbrock Performer carburetor are easy to access right in the front of the carburetor. The first thing we did was balance the two adjusting screws so they were turned out the same number of turns from fully seated. Then, as we adjusted the idle mixture, we made sure to keep the two adjustments consistent. After a few adjustments, we had cut the idle emissions down to 630 PPM and around 1 percent CO.
We felt that this engine could idle much cleaner, so we added a Pertronix Second Strike unit and a distributor to create a hotter fire under this small-block. After wiring the box in place, we fired the Camaro up, and after the engine warmed up to 190 degrees F, we noticed that the HC and CO emissions jumped radically. In fact, for a while, the HC skyrocketed up to over 1,000 ppm! This didn't seem right, so we decided to take the Camaro for a quick testdrive. After a 20-minute boogie where we took the Camaro out on a nearby freeway, a retest of the idle emissions now revealed much lower HC and CO readings.
We talked to our pal Norm Brandis of Westech Automotive in Silver Lake, Wisconsin. He has extensive experience with emissions testing and performance engines, and he told us that it's common to see an initial spike in the HC emissions after installing a strong ignition system. Brandis says the level will stay high until the ignition has completely burned away the top layer of carbon on the pistons. It's this carbon layer that spikes the HC levels. We now had reduced the HC down to between 410 and 500 PPM and a CO reading of 0.19 percent. Using our CO conversion table, this placed the little small-block at roughly 14.5:1 air/fuel ratio.
Next, we welded in an oxygen sensor bung in the collector of the driver-side header so we could use the Innovate Motorsports air/fuel ratio meter. We plugged the meter into the cigarette lighter for power and went for a part-throttle cruise to measure the air/fuel ratio. The carburetor we were using is the leanest of the Edelbrock carbs, and a quick check of the calibration made it appear that very few changes will have to be made.
One of the advantages of the Edelbrock carburetor is that this is a metering rod style carburetor, much like the system used in a Q-jet. In addition to the primary jets, there are also two primary metering rods that fit inside the flow orifice of the jets. At light throttle, these rods are buried deeply into the jets, reducing the flow area created by the jet, in essence making the jet flow area very small. As load increases, intake manifold vacuum drops off and the spring under each metering rod pushes up on a piston connected to the metering rods. The power tips of the metering rods are smaller, allowing more fuel to flow through the fixed orifice jet, richening the mixture. It's a simple system that works well, especially for part throttle tuning.
This 600-cfm Edelbrock carb is the leanest of the carburetors and is set up with a number 98 primary and a 95 secondary jet. The primary jet also has a metering rod with a 0.075-inch major diameter with a power tip that measures 0.047 inch. We experimented with leaning the stock 98 primary jet down to a 95 jet with the same primary metering rod.
With all these changes, we were able to improve the part-throttle cruise and fuel mileage, reduce the exhaust emissions, all while improving the throttle response and driveability. Not bad for a couple of afternoons' worth of tuning.