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Fuel System Service Continued

       The first step in adjusting the carburetor is to check the idle speed and adjust it as necessary.  Assuming you can adjust the speed to specification, then turn the mixture screw in (leaning the mixture) by 1/8 increments while watching for the idle speed to drop.  On two barrel carburetors, always turn both screws in the same direction and the same amount.  Once you see the idle speed drop, then back the mixture screw out (richening the mixture) by 1/8 turn increments to achieve the highest idle speed.  Adjust the idle speed back to specification and repeat the process of first leaning the mixture and then richening it for the highest idle speed.  It should not idle beyond specification the second time if everything is correct.  Now that the speed is stable at the specified rpm, slightly raise the engine speed by moving the throttle linkage at the carburetor, or by having a helper step on the accelerator, and abruptly release the throttle.  Does the idle return to the specified rpm immediately?  If the engine stalls, the idle mixture is too lean, if it stays fast or slowly returns to the proper speed, the idle mixture is too rich.  Adjust the mixture screw(s) in 1/8 increment and repeat the speed-up test.  The idle mixture is adjusted correctly when the engine immediately settles to the specified rpm and idles smoothly with a steady vacuum reading between 17 & 21 in. hg. (inches of mercury).

  If you are unable to achieve a smooth steady idle, begin by turning one mixture screw in (lean) until the engine stalls, then back to its original setting, and do the same with the second screw.  It should stall with either screw lightly seated, if it kept running then fuel is getting into the engine somewhere that it shouldn’t be.  Remove both mixture screws and examine them.  The tip of each screw should have a smooth taper to a nearly sharp point, and both screws should be identical.  Assuming you do not find any damage to either screw, you could try squirting aerosol carburetor cleaner into each mixture screw passage to clean out any varnish or dirt that might be there.  There is one other situation you can check before removing the carburetor for repair.  It is possible that the float level is slightly too high.  Remove the air cleaner and look into the carburetor with the engine idling, you should not see fuel dripping from the nozzle in either venturi.  If you see fuel dripping, you could carefully remove the top of the carburetor and adjust the float level to specification.  Be aware that setting the float level too low is as bad as having it too high!  The float level is a critical adjustment for proper operation of the carburetor at every engine speed and must be set correctly.

  Now that I have explained how everything is supposed to work, we will consider the effect of improper adjustments.  If the ignition timing is “late” or retarded from the proper specification, the engine will idle slower and by necessity, the idle speed screw will have to hold the throttle plate open further to achieve idle.  If the mixture screws are set too lean, the idle will slow and the same situation occurs with regard to the idle speed screw.  If the ignition adjustments are correct, but you adjust the idle speed higher then the specification, you have created the same problem yourself.

  The problem is this; at idle all of the fuel mixture necessary should be supplied by the idle mixture screw only, if the off-idle passage is exposed because the speed screw is adjusted too fast, some mixture will naturally come from this source, and the mixture screw will be set artificially lean to achieve the best idle quality.  The engine will idle just fine, but when you attempt to accelerate the engine and put the car in motion, it is very likely to hesitate due to a momentary lean condition.  Since the off-idle passage was already supplying fuel for idle, there will not be enough fuel to accelerate the engine smoothly up to the point that the main metering system takes over.  The car will also be “flat” when driving at very low speeds, when the off-idle passage is essential for smooth performance.  This whole problem will be compounded if the base timing was adjusted while the vacuum advance was working.  With vacuum applied, you would naturally be retarding the timing to get the marks aligned and retarded timing also results in sluggish performance.

  The relationship between the throttle plate(s) and the off-idle passage(s) and the distributor vacuum port is essential for smooth acceleration and responsive low speed driving; so it is essential that the idle speed and mixture be adjusted correctly.

  The float level setting is absolutely critical because it effects the fuel mixture throughout the entire operating range of the carburetor; too high and the mixture is rich, too low and the mixture is lean.  Regardless of engine speed or load, the float admits fuel into the float bowl at the same rate it is being used, so the fuel level within the carburetor float bowl remains at a constant level.  The level of fuel within the carburetor was carefully determined by Pontiac and Carter Carburetor engineers to provide the best all around compromise for smooth reliable performance and operating economy.

 For the sake of this discussion, let us make two assumptions with regard to the cut-away at left.  The first is that the correct fuel level is even with the words “main jet”, and the second is that the float level specification is 1/2 inch.

 Notice that all of the fuel must pass through that main jet to reach either the idle or main metering circuits of the carburetor.  This makes it essential to keep the main jet covered with gasoline, if the jet was to be uncovered, on a sharp turn for instance, the fuel flow would be interrupted and the engine could hesitate or even stall completely.

 Bear in mind that all of the carburetor passages will have fuel in them at the same level as the float bowl.  Note that the low speed jet, which meters the  total amount of fuel to reach the idle mixture screw and the off-idle passage, is above the fuel level.  Engine vacuum, even at cranking speed is sufficient to draw gasoline up through the low speed jet and the economizer restriction, from there vacuum is no longer needed for fuel to flow.

 The (idle) air bleed actually admits air into the idle circuit, this emulsifies the fuel with air even before it reaches the mixture screw and gives better idle quality.  The (idle) by-pass opening connects to the float bowl side of the economizer restriction, when the throttle is open to the point that the main metering circuit is supplying fuel, airflow back through the by-pass passage prevents fuel from flowing in the idle circuit.  Shutting off the idle circuit is an economy measure, hence the term economizer.  This is an example of the sophistication of the went into designing the Carter carburetor.
 
  Although it is not shown in any detail, the main metering passage is the one that runs upward at a 45 degree angle to the discharge nozzle at the top of the venturi.  As the throttle is opened beyond idle, air flowing down through the venturi creates the vacuum necessary to draw fuel up the main passage to the discharge nozzle.

  Our cut-away carburetor has a float level specification of 1/2 inch, and the service manual shows how this measurement is to be made.  If you set the float at 3/4 inch, you would be lowering the fuel level in the bowl, conversely if you set it at 1/4 inch you would be raising the fuel level.  I already mentioned the possibility of uncovering the main jet due to low fuel level, and interrupting the flow.  If set too high, fuel can slosh out of bowl vent passages on abrupt turns or stops and momentarily flood the engine with too much fuel.  Unfortunately the symptoms are nearly identical to insufficient fuel, a hesitation or possible stalling.  The one unique symptom of a high float level is the likelihood of gasoline odor.

  A low float level is very likely to cause a part throttle hesitation or surge.  Remember that the idle circuit is limited by the low speed jet and can only supply a fixed amount of fuel.  If the float level is low, it will take more vacuum, (airflow) to draw the fuel up main metering passage to the point of discharge from the nozzle.  This extra air is too much for the fixed amount of fuel from the idle circuit and the mixture suddenly goes very lean, affecting engine performance.

  Once you have your carburetor adjusted correctly, it is unlikely to need any attention for thousands of miles.  The fuel system is much more reliable then the ignition system, both in terms of required maintenance, and also with regard to potential failures.

 I keep record of the fuel mileage of our ’53 Chieftain; it is a reliable indicator of vehicle performance and any significant change in fuel economy needs to be investigated.  Your engine can be running perfectly, but one wheel with a dragging brake will definitely affect your fuel economy!

  I have not had any trouble using 87 octane gasoline, nor do I worry about ethanol blends.  I do not buy gasoline if the tank truck is there delivering fuel, because it is likely to agitate any sediment or water that might be lurking in the underground tank.  In general, today’s gasoline is significantly less likely to contain water or other contaminants, but the possibility will always exist.

  I hope I have not thoroughly confused you, but I prefer to explain how things work and not just tell you to do this, that and the other if you want your car to perform properly.
  
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Fuel System Service