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If your car has a carburetor it will need the idle speed adjusted from time to time. For the person who prefers to do maintenance on a vehicle themselves, a simple instrument called a tachometer and a few simple tools will help you do the job. A tachometer is an instrument that reads the engine's speed.

You can begin by checking under the hood of your car to find out what the curb idle specifications for your vehicle are. This will be listed on your tune up decal. Many of the later model cars will have a soleniod to control the idle speed. If the car has an air conditioner you will need to determine which soleniod is controlling the air conditioner and which is controlling the idle speed. To do this, have a helper hold down the gas pedal to the floor and then turn the ignition switch to on. Do not start the vehicle. Watch under the hood of your car while this is being done. If you see a solenoid plunger move out you will know that it is the one which controls the idle. In case nothing happens, have the helper turn on the air conditioner. Leave the ignition on, but again do not start the car. If you see a plunger move you will know that it is not the curb idle control. There are some solenoids that are made by General Motors that have an additional internal adjuster which controls the curb idle speed when the air conditioning is turned off on your car. On a none air conditioned car, this one solenoid will also control a second low idle speed.

When you can find no solenoid that controls the idle, you will need to look for a spring loaded adjusting screw bearing that can usually be located against the throttle linkage. When doing this be very careful that you do no confuse the screw that controls the fast idle when the engine is cold. This screw bears against a curved part that is stepped and is known as the fast idle cam. With the exception of the General Motors solenoid, if the decal specifies curb idle and low idle, and you can find an idle solenoid, it will be that soleniod that controls the curb idle alone. Once you have determined if and where your curb idle adjustment is, you need to look for a throttle linkage screw. When you find this screw disconnect the solenoid wire and set it to low idle. To adjust the idle, warm up the engine of your car and read any special instructions for the idle speed adjustment given on your tune up decal. Most will say things such as hoses to disconnect and plug.

Attach your tachometer according to instructions, which in most cases will mean that you attach the negative wire to an electrical ground like an engine bolt and the positive lead to the negative terminal of the ignition coil. On some models such as the General Motors cars that have high energy electronic ignitions, with the coil built into the distributor cap, you will need to connect the positive lead to the TACH terminal in the flange of the cap. In case this confuses you, it will be the one without the pink wire. Make your adjustments in very small increments. With each turn you should push down on the throttle and wait until the solenoid plunger has extended before releasing the throttle. Wait about 10 to 15 seconds until the idle has stabilized before taking the reading on your tachometer. Continue to repeat these steps if necessary until the reading is correct.

To adjust the screw against the throttle linkage or the plunger of a non adjustable solenoid you will need to turn the screw with a wrench on the screw's hex end. When there is no solenoid you should turn clockwise to increase the speed and counter clockwise to decrease it. If you do have a solenoid simply revere this action and turn in the opposite directions. When you have a square or hex tip on the solenoid plunger, take your wrench and turn the plunger counter clockwise to increase the idle speed. To reduce the idle speed make a clockwise turn. With a two piece solenoid bracket, which has one part moveable along the screw and one part fixed, you should turn the long bracket screw clockwise to increase the idle and counter clockwise to reduce it. If you find that the adjuster is on the back of the solenoid simply turn it clockwise to increase the idle speed and counter clockwise to reduce it.

When the car is not air conditioned but has a General Motors solenoid, you will need to use a wrench to turn the solenoid body on the hex at the back in a clockwise motion to increase the idle speed and counter clockwise to reduce it. Be sure when you do this that you disconnect the wire from the solenoid and use an 18 inch allen wrench to turn the internal adjuster until the engine speed matches the low idle specifications. If the car is air conditioned and has a General Motors solenoid use a 1\8 inch allen wrench and turn the internal adjuster clockwise to a stop. Never force the turn, but instead use a slow steady action until you reach the stop. Next you should disconnect the wiring at the air conditioner compressor and turn the air conditioning dash controls. Then set the idle to the exact specifications given for the solenoid. When you have finished this, turn off the air conditioner and reconnect the compressor wire. Carefully back out the allen wrench adjuster until the speed is the same as the specifications for the low idle.
As with any self repairs made on your car, if you have any problems finding the proper specifications you should check with your local auto repair shop before beginning the repair. Once you have determined the type of controlling solenoid used in your vehicle and know the exact specifications you are ready to begin.