Automotive Systems

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Carburetor Troubles


Some of the engine troubles that can usually (but not ALWAYS) be traced to some fault in the carburetor system are as follows:

  • EXCESSIVE FUEL CONSUMPTION can result from a high float level, a leaky float, a sticking metering rod or full power piston, a sticking accelerator pump, and/or too rich of an idling mixture.
  • A SLUGGISH ENGINE may be the result of a poorly operating accelerator pump, a low float level, dirty or gummy fuel passages, or a clogged air cleaner.
  • POOR IDLING, often characterized by a stalling of the engine, is usually due to a too rich idle mixture, a defective choke, or an incorrectly adjusted idle speed screw at the throttle plate.
  • FAILURE OF THE ENGINE TO START may be caused by an incorrectly adjusted choke, clogged fuel lines, or air leak into the intake manifold.
  • HARD STARTING OF A WARM ENGINE could be due to a defective or improperly adjusted throttle link.
  • SLOW ENGINE WARM-UP may indicate a defective choke or defective radiator thermostat.
  • SMOKY BLACK EXHAUST indicates a very rich air-fuel mixture.
  • STALLING OF THE ENGINE AS IT WARMS could be caused by a defective choke or closed choke valve.
  • A BACKFIRING ENGINE may be due to an incorrect, often lean, air-fuel mixture reaching the engine. In turn, this condition could be caused by a clogged fuel line or a fluctuating fuel level.
  • An ENGINE RUNS BUT MISSES, the most likely cause is a vacuum leak at a vacuum hose or the intake manifold. In addition, it could be an improper air-fuel mixture reaching the engine due to clogged or worn carburetor jets or an incorrect fuel level in the float bowl.

Several quick checks can be made to see how well the carburetor is working. More accurate analysis requires test instruments, such as an exhaust gas analyzer and an intake manifold vacuum gauge. The quick checks are as follows:

  1. FLOAT LEVEL ADJUSTMENT. With the engine warmed up and running at idle speed, remove the air cleaner. Carefully note the condition of the high-speed nozzle. If the nozzle tip is wet or is dripping fuel, the float level is probably too high. This could cause a continuous discharge of fuel from the nozzle, even at idle.
  2. IDLE SYSTEM. If the engine does not idle smoothly after it is warmed up, the idle system could be at fault. Slowly open the throttle until the engine is running at about 3,000 rpm. If the speed does not increase evenly and the engine runs roughly through this speed range, the idle or main metering system is probably defective.
  3. ACCELERATOR PUMP SYSTEM. With the air cleaner off and the engine not running, open the throttle suddenly. See if the accelerator pump system discharges a squirt of fuel into the air horn. The flow should continue for a few seconds after the throttle plate reaches the wide, open position.
  4. MAIN METERING SYSTEM. With the engine warmed up and running at 2,000 rpm, slowly cover part of the air horn with a piece of stiff cardboard The engine should speed up slightly, since this action causes a normal operating main metering system to discharge more fuel.


Do NOT use your hand to cover the air horn when performing this test.

Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

Copyright 2001-2004 SweetHaven Publishing Services
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