GM Prizm/Nova 1985-1993 Repair Guide

Carburetor - Part One

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The carburetor is the most complex part of the fuel system. Carburetors vary greatly in construction, but they all operate the same way; their job is to supply the correct mixture of fuel and air to the engine in response to varying conditions.

Despite their complexity, carburetors function on a simple physical principle known as the venturi principle. Air is drawn into the engine by the pumping action of the pistons. As the air enters the top of the carburetor, it passes through a venturi or restriction in the throttle bore. The air speeds up as it passes through the venturi, causing a slight drop in pressure. This pressure drop pulls fuel from the float bowl through a nozzle in the throttle bore. The air and fuel mix to form a fine mist, which is distributed to the cylinders through the intake manifold.

There are 6 different systems (fuel/air circuits) in a carburetor that make it work; the Float system, Main Metering system, Idle and Low Speed system, Accelerator Pump system, Power system, and the Choke System. The way these systems are arranged in the carburetor determines the carburetor's size and shape.

It's important to remember that carburetors seldom give trouble during normal operation. Other than changing the fuel and air filters and making sure the idle speed and mixture are proper at every tune-up, there's not much maintenance you can perform on the average carburetor.

The carburetor used on the Nova model, 1985-88 VIN Code 4 engine, is a conventional 2-barrel, downdraft type similar to domestic carburetors. The main circuits are: primary, for normal operational requirements; secondary, to supply high speed fuel needs; float, to supply fuel to the primary and secondary circuits; accelerator, to supply fuel for quick and safe acceleration; choke, for reliable starting in cold weather; and power valve, for fuel economy.

ADJUSTMENTS



See Figures 1, 2, 3 and 4

Before making any adjustments to the carburetor, ALL of the following conditions must be met:



All accessories switched off
 
Ignition timing correctly set
 
Transmission in neutral, parking brake set, wheels blocked front and rear.
 
Fuel level (float level) correctly set; view the fuel level in the small window on the right side of the carburetor.
 
Tachometer correctly connected. Please refer to for detailed instructions.
 



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Fig. Fig. 1: Front view of the Nova carburetor



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Fig. Fig. 2: Rear view of the Nova carburetor



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Fig. Fig. 3: Left side view of the Nova carburetor



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Fig. Fig. 4: Right side view of the Nova carburetor

When adjusting the carburetor, please resist the temptation to randomly turn screws hoping for the best. The many adjustments on a carburetor interrelate; if you change 1 setting you may affect three other things. A detailed, reasoned approach to driveability problems will yield much better (and quicker) results than guessing and groping.

Curb Idle (Warm Idle)

See Figures 2 and 3

The curb idle is adjusted by turning the idle adjusting screw located on the rear of the carburetor. The knob has a knurled plastic head to make grasping easier. Turn it clockwise to increase idle speed. Correct idle speed for the carbureted Nova is 650 rpm w/manual transmission and 750 rpm w/automatic transmission.

Fast Idle

See Figures 2, 5 and 6

  1. Stop the engine and remove the air cleaner housing.
  2.  
  3. Disconnect and plug the hot idle compensator hose to prevent rough idling.
  4.  
  5. Disconnect the hose from the Thermovacuum Switching Valve (TVSV) port M and plug the port. This will shut off the choke opener and EGR systems.
  6.  



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Fig. Fig. 5: Adjusting the fast idle screw



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Fig. Fig. 6: M port on the TVSV

  1. Hold the throttle slightly open by either moving the linkage on the carburetor or pull lightly on the throttle cable. Push the choke plate closed and hold it closed as you release the throttle.The carburetor is now fooled into thinking it is performing a cold start - the choke is set and the various external controls are not functioning. These conditions duplicate cold start conditions.
  2.  
  3. Start the engine but DO NOT touch the accelerator pedal or cable. (If you do, the choke will release and Step 4 will be needed again.)
  4.  


CAUTION
The engine will be running; be careful of moving parts and belts! Keep loose fitting clothes and long hair well away from the engine area!

  1. The correct fast idle speed is 3000 rpm. If adjustment is necessary, turn the fast idle adjusting screw at the lower rear of the carburetor.
  2.  


WARNING
Do not allow the engine to run on fast idle any longer than necessary. Once the correct fast idle is achieved, release the fast idle by depressing the accelerator and releasing it. Allow the engine to run at curb idle for about 30 seconds and switch the engine off.

  1. Remove the plug and reconnect the hose to the M port of the TVSV.
  2.  

Throttle Positioner

See Figures 4, 7, 8 and 9

  1. Disconnect the hose from the Thermovacuum Switching Valve (TVSV) port M and plug the port. Disconnect the vacuum hose from throttle positioner (TP) diaphragm A .
  2.  



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Fig. Fig. 7: Vacuum hose at diaphragm A



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Fig. Fig. 8: Throttle positioner adjusting screw

  1. Check that the TP is set at the first step; correct engine speed is:

    Manual transmission: 800 rpm
     
    Automatic transmission: 900 rpm If necessary, adjust the speed with the adjusting screw.
     

  2.  

Make the adjustment with the cooling (radiator) fan OFF.

  1. Reconnect the vacuum hose to diaphragm A .
  2.  
  3. Disconnect the hose from diaphragm B and plug the hose end.
  4.  



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Fig. Fig. 9: Vacuum hose at diaphragm B

  1. Check that the TP is set at the second step. The correct engine speed in this position is:

    Manual transmission: 1400 plus or minus 200 rpm
     
    Automatic transmission: 1500 plus or minus 200 rpm
     

  2.  

  1. Reconnect the vacuum hose to diaphragm B and check that the engine returns to normal idle within 2-6 seconds.
  2.  
  3. Remove the plug and reconnect the vacuum hose to TVSV port M .
  4.  

Float and Fuel Level

See Figures 10, 11, 12, 13 and 14

The float level is not externally adjustable. Removal of the air horn assembly or top of the carburetor is required. The engine should be cold during this procedure. All work is performed with the engine off.



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Fig. Fig. 10: No float level adjustment is necessary when the fuel level falls between the line on the carburetor sight glass

The float in the carburetor controls the entry of fuel into the bowl of the carburetor. (The bowl is simply a reservoir which keeps fuel available at all times.) The function of the float is to react to the level of fuel in the bowl and open or close a valve, thus maintaining the correct amount of fuel. The principle is identical to the float in a toilet tank; when the correct level is reached, the flow is shut off.

The position of the float (and therefore the amount of fuel available) is critical to the proper operation of the engine. If too little fuel is in the bowl, the engine may starve on sharp corners or on hills; too much fuel can literally lead to overflowing and flooding of the engine. To adjust the float level:

  1. Remove the air cleaner assembly and disconnect the choke linkage.
  2.  
  3. Disconnect the accelerator pump connecting rod.
  4.  
  5. Remove the pump arm pivot screw and the pump arm.
  6.  
  7. Remove the fuel hose and union.
  8.  
  9. Remove the 8 air horn screws. Be careful to identify and collect the external parts attached to the screws, such as wire clamps, brackets and the steel number plate.
  10.  
  11. Disconnect the choke link.
  12.  
  13. Lift the air horn with its gasket from the body of the carburetor.
  14.  
  15. Disconnect the wires at the connector.
  16.  
  17. Remove the gasket from the air horn assembly. Invert the air horn so that the float hangs down by its own weight. Check the clearance between the float tip and the air horn. The correct clearance is 7.0mm. If necessary, adjust the float lip by bending it gently into position.
  18.  



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Fig. Fig. 11: Checking the float level with the float in resting on the needle valve



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Fig. Fig. 12: Float adjustment at point (A) while in the resting position

  1. Lift up the float and check the clearance between the needle valve plunger and the float lip. Correct clearance is 1.6-2.0mm. If necessary, adjust the clearance by bending the outer part of the float lip.
  2.  



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Fig. Fig. 13: Measuring the float in the raised position



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Fig. Fig. 14: Float adjustment at point (B) while in the raised position

If the float has become misadjusted, it may be due to the float filling with gasoline. Give the float a gentle shake and listen for any liquid within. If fuel is inside the float, replace the float and reset the levels.

  1. Install a new gasket onto the air horn.
  2.  
  3. Place the air horn in position on the carburetor body and install the choke link.
  4.  
  5. Install the 8 screws. Make certain the brackets, clips and steel tag are reinstalled as well.
  6.  
  7. Connect the fuel hose and union.
  8.  
  9. Install the pump arm pivot screw and pump arm.
  10.  
  11. Connect the pump arm connecting rod.
  12.  
  13. Attach the choke linkage.
  14.  
  15. Install the air cleaner. Start the engine and check carefully for fuel and/or vacuum leaks. The engine may be difficult to start; once it is running smoothly, recheck the fuel level in the sight glass.
  16.  

 
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