The carburetor is the fuel-mixing device and is paramount to good economy. A four-barrel design when driven on the primary barrels alone will usually return better mileage than a two-barrel. This is because the primary barrels are smaller. A two-barrel has throttle plates that are more compromised since the design needs to serve the full range of engine rpm.
Tune The Entire Vehicle
With your driving skill mastered, your attention should be placed on maximizing the stock efficiency of your Pontiac before any modifications are considered.
A systematic approach needs to be used, and every aspect of the car fine-tuned and calibrated. Even though the engine consumes the fuel, other areas such as the brake system, wheel alignment, tire pressure, and body cleanliness all impact fuel usage.
The carburetor needs to be at peak efficiency, along with the distributor and ignition system. Check for and repair any vacuum leaks and make a habit of tightening the intake manifold bolts and carburetor to proper torque spec.
On EFI Pontiacs, the fuel injectors and throttle body need to be kept clean, and the oxygen sensor should be considered a consumeable. A degraded oxygen sensor will kill fuel economy while producing no adverse effect on driveability. If your car has more than 70,000 miles, it would not be a bad idea to install a fresh oxygen sensor. Always use an AC-Delco oxygen sensor as a replacement.
The proper ignition advance curve will improve fuel mileage. Most Pontiac engines can gain economy with a few degrees more of initial timing.
A slightly more aggressive ignition timing curve will usually pay dividends at light load and improve fuel mileage without causing ping. See a pro for this modification. The cooling system should be at full potential, and the proper concentration of coolant should be maintained for the chemical employed (there are now coolants such as the Evans NPG+ that use no water).
The engine should be chemically decarbonized since any deposits will impact fuel usage. Carbon on the backside of the intake valve will absorb fuel as the mixture travels past and then slowly leeches out, richening the air/fuel ratio. Deposits on the piston crown and combustion chamber will create detonation and require the ignition rate of advance to be less aggressive than desired to eliminate the carbon induced knock.
CAT Products Inc. produces a carbon removal chemical (Run-Rite brand, (800) 872-8921) and applicator kit that does an extremely good job of removing both IVD and CCD, which stands for intake valve deposits and combustion chamber deposits, respectively. The product is available only through the company and is sold to professional shops. In the past when mentioned in this magazine, CAT Products has retailed to Pontiac hobbyists. If the Run-Rite product is not available, then GM Top Engine Cleaner is a good substitute. It is sold in both liquid and spray form at almost every GM dealership.
If the GM product is going to be used, we suggest two cans each of the liquid and spray, especially if your Pontiac has higher mileage and is older. First, follow the instructions on the back of the can. After using the GM procedure, employ a more aggressive protocol.
With the engine cold, remove the spark plugs and pour about 2 ounces of chemical into each plug hole. The can is 15 ounces, so it will just about do a V-8. Then loosely install the spark plugs and let the cleaner sit for 12 to 24 hours. Next, remove all of the spark plugs and disable the ignition primary so there is no arcing at the plug wire. With a wrench on the balancer bolt, turn the engine over slowly to spit out the carbon-removal chemical. Two revolutions should be all that is required. Reinstall the spark plugs and ignition. Do not start the engine yet.