Every carburetor has a transfer...
Every carburetor has a transfer slot in each bore with an idle circuit that allows the fuel to be pulled from the main metering circuit. Next to it (the round hole) is the idle discharge port. The trick is to have as little of the transfer slot exposed during idle by closing down the throttle plates. This provides more authority to the idle mixture screws. That is why it is a good idea to try different ignition timing settings so that the transfer slot area can be reduced.
When the highest idle speed is obtained via the mixture screws, the air/fuel ratio is perfect. Now the engine idle speed will be too high. If need be, remove the air filter and back out the throttle stop screw to get a slower idle speed. When you have reached the slowest speed that you feel the engine is happy with, reinstall the air filter and try tweaking it with the mixture screws following the same procedure as before. You may find that the engine wants a slightly different mixture screw setting than before, since the throttle plates are closed more now.
9. If you’re happy with the results and how the engine runs, you don’t need to play with the base ignition timing, but it may not be a bad idea to try.
The mixture screw on a Rochester...
The mixture screw on a Rochester carburetor has the needle valve at the end of the discharge port, a much better design for fuel control and a smooth, slow idle.
Remove the tune-up tachometer and take the Pontiac for a drive. It should run great.
If your Pontiac has a performance camshaft then you might want to consider installing a CD ignition, such as an MSD, Crane, and others. These systems have a multi-strike feature that will help mask the fuel distribution issues in the engine if reversion is taking place. If the CD box steadies the idle and allows a lower speed then you know that distribution is an issue.
The Q-jet has a transfer slot...
The Q-jet has a transfer slot too. You can see the tip of the idle mixture screw in the idle discharge port. The Q-jet is an often misunderstood but excellent carburetor.
The process for obtaining bragging rights of a slow and steady idle speed is not difficult but requires a careful touch and listening to what the engine is telling you through the tachometer, feel, and the sound of the exhaust. A mildly cammed Pontiac sounds and runs the best when the idle speed and mixture is adjusted properly. A high idle speed masks a lot of the rumpty-rump we all love to hear.
To create this primer, HPP worked with Classic Restorations in Sloatsburg, New York. There, technician Greg Schwark used a ’79 Trans Am with a 400 for us to photograph the proper steps.
Usually a few degrees more...
Usually a few degrees more ignition advance at idle will allow the throttle plate to be closed more and magnify the mixture control through the idle mixture adjustment.
In closing, a slow and stable idle to a Pontiac engine guy is just as impressive as tire smoking performance. If you follow the steps provided here, your Pontiac will be a kitten at idle but a tiger when you stomp on the go pedal. Isn’t that what it is all about?