Fuel System Service Continued
The only carburetor adjustments we are going to cover are the basic tune-up settings for the automatic choke, idle mixture and speed. Speaking of tune-ups, think about the fact that electronic controls have made them obsolete. New cars have no valve, ignition timing or idle mixture/speed adjustments. An electronic control module monitors engine parameters through sensors and makes corrections in real time, plus in most systems it has the ability to learn and maintain the corrections to keep the engine in peak tune. Even the sensors lack adjustment, once installed on the car, the module reads the data from the new sensor and accepts it so long as it falls with a pre-determined range. Electronic controls are basically on or off, the car either runs or it doesn’t, although there is usually some provision for limp-home in the event of a component or system failure.
Contrast this with our ETC-era cars; we have the ability to adjust the valves, ignition timing and the fuel delivery at every engine speed due to initial and tune-up adjustment provisions of the carburetor. The ability to adjust is a double-edged sword; if the adjustments are merely adequate the car will probably run, but not very reliably. If on the other hand, the adjustments are properly “tuned” the car will run very well and quite reliably. A measure of good performance is achieving the fuel economy the factory claims possible for a given model year. This is the reason that Pontiac provided that information in their service manual, fuel economy is a quantifiable measure of automobile performance readily available to the car owner.
An excellent tune-up is not only dependant upon making every available adjustment precisely; it is also possible to finesse the results by knowing how the adjustments interact and applying that knowledge with how the car will be driven. If your goal is a Pontiac for parade use and minimal driving to and from car shows; then “tight” (minimum setting) adjustment of the valve clearances will provide near silent idle and low speed running. You would also increase the spark plug gap from the recommended .025” to .030” or .032” as the wider gap will give you a smoother idle. The ignition timing could also be advanced with the octane selector; additional advance improves the idle quality and throttle response during low speed, part-throttle driving.
If however you are going to drive your car on road trips, the previous adjustments are poor choices. The valve clearance, especially on the exhaust valves, needs to be set “loose” (maximum clearance) to insure adequate cooling. The plug gap needs to be .025” or you are likely to experience miss-fire at high speed or upon heavy acceleration to highway speeds. You are also likely to hear excessive spark knock on acceleration because the timing is too far advanced.
I want to briefly mention the correct sequence for performing a tune-up. Assuming that no major repairs are necessary, the valves should be adjusted first. Then perform the ignition system service, beginning with the spark plugs. The contact (ignition) points can initially be set with a clean dry feeler gauge, but ideally you should adjust them to obtain the correct reading on a dwell meter. Finally you can zero the octane selector and adjust the base ignition timing. Disconnect the vacuum pipe at the distributor to insure the advance unit is not working. Always remember that changing the dwell changes the ignition timing, but changing the timing will not change the dwell. The final tune-up adjustments will be the idle mixture and speed.
There is a critical relationship between idle speed and proper idle mixture adjustment, and also between the carburetor adjustments and base ignition timing / vacuum advance operation. All ETC-era Pontiac engines with vacuum advance use a “ported” vacuum source at the carburetor. The vacuum advance port is positioned just above the throttle plate when it is in the idle position; vacuum is only present under the throttle plate. As the accelerator is depressed and the throttle plate opens, the advance port is exposed to engine vacuum under the throttle plate. If the idle speed is not correct, neither is the relationship with the vacuum advance port. A similar relationship exists for the carburetor during idle and off-idle operation.
To better understand these relationships, it will helpful to refer to the cut-away carburetor diagram. Note the idle mixture screw passage well below the throttle plate and also the port opening (off-idle passage) near the edge of the closed throttle plate. When the tune-up adjustments are correct, all of the fuel mixture necessary for engine idle is being fed past the mixture screw. When you step on the accelerator, the throttle plate opens along the off-idle passage exposing it to engine vacuum, which draws additional fuel to supplement the amount still coming past the idle mixture screw. This off-idle fuel is necessary for smooth acceleration and engine operation at very low speed. Naturally the vacuum advance port is also exposed to engine vacuum in the same manner and the ignition timing advances along with the increase in engine speed.
If you continue to accelerate, the air flowing through the venturi will become sufficient to cause fuel to flow from the discharge nozzle and the “main” or high-speed fuel circuit takes over to supply all of the engine’s fuel. Since the throttle is still only partially open, there is sufficient vacuum for the distributor advance unit to continue working. At wide-open throttle there isn’t any measurable engine vacuum, but the engine doe not need additional advance under this condition, so the system works very well as designed.
Now its time to adjust the carburetor, the automatic choke housing is marked for rich and lean, you merely have to loosen the retaining screws and rotate the cover to the correct setting. If your car starts well, I would leave the setting where it is. The engine needs to be at normal operating temperature, with the air cleaner installed, to properly adjust the idle mixture and speed. Before starting the engine I turn the idle mixture screw in until it is lightly seated, I count by 1/2 turns so I can turn it back out to its original position. Naturally on a two barrel carburetor you have to do this with both mixture screws, and they should both take an equal number of turns! If you find more than ¼ turn difference between them, it is possible there is a carburetor problem. A vacuum gauge is a great help in adjusting the idle mixture screws, if you have one available. You can connect it at the vacuum port for the windshield wipers.
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