Volkswagen Air Cooled 1949-1969 Repair Guide

Carburetor Troubleshooting

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The following are some of the problems and possible causes within the carburetor:

  1. Engine will not start (ignition system working properly and fuel in tank.)
  2.  
    1. Automatic choke not working properly.
    2.  
    3. Choke valve sticking.
    4.  
    5. Bi-metallic spring broken or unhooked.
    6.  
    7. Ceramic plate broken.
    8.  
    9. Float needle valve sticking, causing flooding of carburetor.
    10.  

  3. Engine runs at a fast idle.
  4.  
    1. Automatic choke not switching off.
    2.  
    3. Defective heater element.
    4.  
    5. Throttle positioner incorrectly adjusted.
    6.  

  5. Engine runs unevenly, with tendency to stall.
  6.  
    1. Incorrect idle speed adjustment.
    2.  
    3. Carburetor pilot jet blocked.
    4.  
    5. Incorrect idle mixture adjustment.
    6.  
    7. Throttle positioner incorrectly adjusted.
    8.  

  7. Engine diesels (runs on) after ignition is turned OFF.
  8.  
    1. Idling mixture too lean.
    2.  
    3. Idle speed too fast.
    4.  
    5. Electro-magnetic cut-off jet inoperative.
    6.  

  9. Banging in the exhaust when the engine is revved.
  10.  
    1. Idle mixture too weak.
    2.  
    3. Throttle positioner incorrectly adjusted.
    4.  

  11. Poor transfer from idle speed to normal running.
  12.  
    1. Defective accelerator pump system; sticking ball or blocked passages.
    2.  
    3. Accelerator pump diaphragm torn.
    4.  
    5. Incorrect idle adjustment.
    6.  
    7. Accelerator pump system injecting too much or too little fuel.
    8.  

  13. Engine stalls when accelerator is suddenly released.
  14.  
    1. Idle mixture too rich.
    2.  

  15. Engine runs unevenly at low idle speed; exhaust soots excessively at high idle speed; spark plugs tend to soot up. (i.e. the mixture is too rich.)
  16.  
    1. Excessive fuel pump pressure causing needle valve to remain open.
    2.  
    3. Float leaking.
    4.  
    5. Float needle valve defective, not closing.
    6.  

  17. Engine runs unevenly, misfires at high speeds.
  18.  
    1. Fuel starvation, due to dirty main jet, needle valve, fuel tank, or insufficient fuel pump pressure.
    2.  

  19. Excessive fuel consumption.
  20.  
    1. Incorrect jet sizes.
    2.  
    3. Excessive fuel pump pressure causing float level to be too high.
    4.  
    5. Float leaking, causing high float level.
    6.  
    7. Float needle valve does not close.
    8.  
    9. Improper operation of automatic choke.
    10.  


When checking and adjusting the carburetors one should keep in mind that the engine will respond to fine carburetor adjustments only after the electrical and mechanical parts of the engine are set to the proper specifications. If such things as the ignition timing, valve clearance, compression balance, and point gap are not in order, fine carburetor tuning will be of little or no avail.

 
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