Gone Outdoors

How To Troubleshoot a Carburetor Fuel Leak

by Chris Stevenson

An auto carburetor has several circuits that work in conjunction with each other. The float bowl, along with its needle valve, determines how much gas gets held in reserve. A power circuit, or accelerator pump, plunger sends atomized fuel down into the carburetor throat, also called the venturi. Leaking valves, needle seats and gaskets can cause raw gasoline to leak internally or externally from the carburetor. A vehicle owner can spot the leak and determine the component failure by knowing where and when the leak occurs in or around the carburetor.

1. Set the shift selector to park or neutral, depending upon the make of your transmission. Turn the engine off. Apply the emergency brake and raise the hood. Remove the air cleaner housing by unscrewing the wing nut, or use a screwdriver to remove the fastener. Use a screwdriver and socket to remove the cold air intake hose and air box from the top of the throttle, if so equipped.

2. Use a rag to wipe down all sides of the carburetor, the carburetor base and top. Hold a shop light close to the top of the carburetor so you can see any leaking fuel. Look at the fuel intake line where it enters the top side of the carburetor body, called the "air horn." Fuel leaking from the line connection will indicate a clogged carburetor screen or cone filter, or a loose flare nut line connection.

3. Tighten the fuel line with fuel line wrench. If the leaks persists, remove the line with a fuel line wrench and clean the screen with a tooth brush or compressed air. Look down inside the carburetor throat for any dribbling gas on top of the throttle plates. This indicates a check valve that will not operate when the fuel line has pressure in it, allowing the fuel level in the bowl to rise and spill over into the intake manifold. A leaking check valve must be replaced.

4. Refer to your owner's manual for the location of the accelerator pump plunger. The accelerator pump plunger will have a small rod and spring protruding up through the carburetor top case (air horn). If you see fuel leaking here, it shows that the carburetor float has sunk or its adjustment setting has stuck too high in the bowl.

5. Start the engine and let it idle until normal operating temperature. Look at the top carburetor plate, or air horn. Look for any leak between the air horn case and the major carburetor body. A leak around the seam between the two indicates a bad air horn case gasket. Tighten the top screws on the air horn with a screwdriver to reseal the mating surface between the case bodies.

6. Examine the throttle linkage shaft where it travels through the base of the carburetor. The shaft controls the opening and closing of the throttle plates. Any leak at the shaft ends on the outside of the carburetor indicates a worn throttle shaft and seat. The carburetor must be disassembled to replace the shaft, or the lower carburetor throttle body case must be replaced.

7. Look for any fuel exiting the bottom gasket seam under the carburetor base. The throttle body, or base gasket, keeps a seal between the intake manifold and lower carburetor base. Tightening the bolts with a socket and wrench can stop a minor leak. If the gasket allows to much fuel to escape, it means it has worn and deteriorated. Loosen and remove the the bolts with a socket. Clean the mating surfaces and install a new gasket on the intake manifold. Tighten the bolts with a socket.

8. Refer to your repair manual for the location of the mixture screw, or screws, just above the base of the carburetor. The mixture screw will not be attached to the linkage, like the idle speed screw. Look for any leaks at the screw base. A leak at the idle mixture screw indicates a defective rubber ring gasket. To replace the rubber ring, unscrew the mixture screw and pull out the old ring. Replace it with a new one and insert the mixture screw back into its seat. Adjust the screw according to your repair manual's specifications.

Items you will need
  • Owner's repair manual
  • Rags
  • Shop light
  • Screwdrivers
  • Socket set
  • Fuel line wrench
  • Tooth brush

About the Author

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.

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