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How to Adjust the Idle on a Carbureted Car

by Contributor

Adjust the idle of your car if the engine runs too fast or too slowly, causing it to sputter and die.

Determine whether your car has a carburetor. Most (but not all) cars built in the past decade have fuel-injection systems. If unsure, consult your car's manual.

Turn on the engine and run the car until it's at normal operating temperature.

Turn off the engine.

Remove the air filter cover, base and filter to expose the carburetor.

Find the idle adjustment screw--typically, a regular slotted screw located below the fast idle cam (a small mechanism on the side of the carburetor) that rocks on a spring-loaded hinge. The adjustment screw is attached to the throttle cable. If you pull on the throttle cable (or have someone else step on the gas pedal while you watch) you'll be able to see which screw will affect the throttle operation.

Start the car.

Tighten the idle screw one-half turn (clockwise) if the car idles roughly or conks out: screwing it in increases the idle speed.

Wait 30 seconds and then use the car's tachometer (on the dash) to check the RPMs.

Loosen the screw half a turn or until the idle is where you want it if it idles too fast. Unscrewing the idle adjustment decreases the idle speed.

Replace the air filter assembly.

Close the hood.


  • Always adjust the idle when the engine is fully warmed up.
  • Most cars should idle happily at 850 RPMs. If you have to adjust the idle past 1,000 RPMs to keep the engine from stalling, you have another problem (first check for a vacuum leak or loose hose sucking in air).
  • Idling too high wastes gas.

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