How to Adjust the Carburetor on Bikes

by Contributing Writer

Your Yamaha bike carburetor is responsible for keeping the mixture of fuel and air in your engine Yamaha bike the proper level. If the carburetor stops doing its job, your Yamaha bike engine will either have a lean mixture -- not enough fuel for the air involved -- or a rich mixture -- too much fuel for the air involved. If you have a lean mixture, you'll hear your engine sounding like it's revving higher, but you won't see a similar increase in power. If it's too rich, your engine will make a sputtering sound, or act like the power is coming and going.

Under The Hood:

 How to Adjust the Carburetor on a 1970 Honda Trail

Low Speed Operation from Idle to One-Eighth Throttle

Start the engine and let it idle in place for five minutes.

Clamp an inductive tachometer over the spark plug cable at the top of the engine. The tachometer should indicate an idle speed of 1,300 to 1,500 rpm. Alternatively, listen to the engine to roughly estimate the idle rpm. The engine should be idling consistently without hesitation or racing.

Turn the throttle stop screw, located on the right side of the carburetor's throttle valve cylinder, to adjust the idle speed, as needed to adjust the engine idle speed. Turn the screw clockwise to increase the idle speed, or counterclockwise to decrease the idle speed, until the engine is idling between 1,300 and 1,500 rpm.

Turn the air screw, located below the throttle stop screw, clockwise until it is lightly seated, using a flat-head screwdriver. Count the amount of turns needed to seat the screw. Ideally, the screw should take one and three-quarters turn before seating for a CT 70 carburetor, or one and three-eighths of a turn for a CT 90 carburetor. Turn the screw counterclockwise the appropriate number of turns until it has been reset.

Turn the air screw counterclockwise one-eighth of a turn at a time until the engine idle speed begins to increase. Stop, then turn the air screw clockwise one-eighth of a turn.

Check the engine idle speed again, using the inductive tachometer. Readjust the idle speed as needed to return the idle speed to between 1,300 and 1,500 rpm. Remove the tachometer from the spark plug cable.

Take the motorcycle for a test ride, but do not apply more than one-eighth throttle. The motorcycle should accelerate smoothly from a complete standstill and return to an idle speed between 1,300 and 1,500 rpm when stopped.

Intermediate Speed Operation between One-Eighth to Half Throttle

Start the engine and let it idle in place for five minutes until warm.

Twist the throttle grip open to between one-eighth to half throttle. Watch the exhaust pipe for black smoke, indicating that the air-to-fuel mixture is too rich, meaning that there is more fuel than air. Close the throttle completely.

Take the motorcycle for a test ride. Accelerate from a standstill to one-eighth throttle, then accelerate to half throttle. The motorcycle should accelerate smoothly and without hesitation. Any hesitation is an indication of a lean air-to-fuel mixture, meaning that there is more are than fuel. Stop the engine upon completion of the test ride and let it cool for 10 minutes.

Unscrew the throttle valve cylinder cap from the top of the carburetor by hand. Pull the throttle valve and spring out from the throttle valve. Pull the spring out of the throttle valve, then pry out the jet needle retaining clip from the inside of the valve, using a flat-head screwdriver. Push the jet needle out of the throttle valve. Skip this step if the motorcycle did not hesitate during acceleration or did not emit black smoke.

Pull the E-shaped clip out of the grooves at the top of the jet needle, using needle-nose pliers. Push the clip into a grove one step closer to the top of the jet needle to reduce smoking, or one step closer to the bottom of the needle to enrich the mixture and reduce hesitation. Reinstall the jet needle, retaining clip and spring into the throttle valve, then reinstall the throttle valve and cap onto the carburetor.

Take the motorcycle for another test ride. Move the clip into a higher or lower groove until it no longer hesitates or emits black smoke from the exhaust pipe between one-eighth to half throttle.

High Speed Operation at Full Throttle

Start the engine and let it idle in place for five minutes until warm.

Twist the throttle grip to a full throttle position. Close the choke, using the lever on the left side of the carburetor. Listen to the engine for an increase in rpm. The main jet is too small, creating a lean air-fuel mixture, if the engine speed increases with the choke closed. Release the throttle immediately and open the choke.

Stop the engine and let it cool for 10 minutes if the engine speed increased with the choke closed.

Turn the fuel valve under the right side of the gas tank to the "Off" position. Loosen the float chamber drain screw on the bottom of the carburetor, using a flat-head screwdriver. Empty the gasoline from the float chamber into a gas can, using a funnel. Remove the float chamber from the carburetor, using a Phillips-head screwdriver.

Remove the main jet, located at the end of the center tower on the bottom of the carburetor, using a 90-degree angled flat-head screwdriver. A CT70 will have a No. 58 main jet, while a CT90 will have a No. 72 main jet.

Screw a larger No. 60, or No. 75 for a CT90 model, main jet into the center tower. Reinstall the float onto the carburetor and turn the fuel valve on.

Restart the engine and let it warm. Open the throttle completely and close the choke. Replace the main jet with another jet one step larger than the No. 60, or No. 75 jet installed previously if the engine speed increases with the choke closed.

Items you will need

  • Inductive tachometer

  • Flat-head screwdriver

  • Needle-nose pliers

  • Gas can

  • Funnel

  • Phillips-head screwdriver

  • No. 60 or No. 75 main jet

  • 90-degree angled flat-head screwdriver

 How to Adjust the Carburetor on a Honda Elite 80 Scooter

Adjusting the Carburetor Jet

Locate your Honda's carburetor, which is on the left side of the bike and has a yellow plastic fuel line connecting it to the gas tank.

Remove the carburetor with a Phillips-head screwdriver. Unfasten the two screws holding it in place and put them in a plastic bag for safe keeping. Gently tug on the carburetor to dislodge it from the moped. It is necessary to remove the carburetor in order to access the brass fuel jet inside of it.

Strip down the carburetor to its bare parts. This process involves unscrewing the plastic "float-bowl" as well as the metal cap holding the throttle assembly in the carburetor. Keep all removed parts with the carburetor.

Locate the brass screw that has a hole in the middle of it. This is the carburetor jet, which controls fuel flow to the piston head. Unscrew this using your fingers, and replace it with a larger jet for better performance, and a smaller one if you live in a high-altitude area.

Replace the carburetor parts, and reattach the carburetor on the bike.

Locate the air filter on the right side of the moped. Remove its plastic cover using a flat-head screwdriver.

Keep the cover off of the air filter. This will increase air flow to the engine, helping to balance the increased fuel flow caused by the bigger carburetor jet. If you do not remove the air cover, the engine will stall out due to a rich fuel source.

Adjusting the Idle Screw

Locate the idle screw which is on the side of the carburetor and has a metal spring attached to its neck.

Start the engine and place the moped on its kickstand. Twist the idle screw to the right, 1/4-turn at a time.

Wait for the moped to react after each turn. The ideal idle speed is reached when the back wheel is barely turning on its axis, but the engine is not stuttering or stalling.

Items you will need

  • Phillips-head screwdriver

  • Brass jet set

  • Flat-head screwdriver

 How to Adjust the Carburetor on a Kawasaki Mule

Remove the screws that hold the black metal cover to the carburetor. Take off the metal cover and set it aside in a safe location.

Locate the idle adjust screw, or accelerator stop screw on the side of the carburetor that is above the top of the rear axle of the vehicle. Loosen the idle adjust screw by moving it a 1/4 turn, counter-clockwise.

Start the engine and see if it runs smoother. If not, keep adjusting the screw, 1/4 turn at a time until it does. Stop adjusting when the engine idles at a speed somewhere between 850 and 950 RPM.

Items you will need

  • Screw driver

 How to Adjust the Carburetor on a 1986 Yamaha ATV

Identify the faulty circuit in your carburetor by noticing where the throttle is when you notice signs of trouble. The pilot circuit controls the carburetor between zero and one-quarter throttle; the mid-range circuit controls it between one-quarter and three-quarter throttle; and the main circuit controls it from three-quarter to wide-open.

Adjust the pilot screw using your screwdriver, if the symptoms of carburetor trouble happen when the throttle is between zero and one-quarter. Let the engine idle and turn your pilot screw clockwise (or in) until the idle gets rough. Turn the screw counterclockwise (or out) through the range of smooth idle, until it starts to idle roughly again. Find a place roughly halfway between those rough idles as your ideal adjustment. If the symptoms of carburetor trouble indicated that the mid-range or main circuit was at fault, move to Step 3. If you have to give the screw more than two turns in either direction, it may be time to change the pilot jet.

Move the jet needle if the carburetor's trouble happens when the throttle is between one-quarter and three-quarters. Lower the needle to enrich a lean mixture; raise the needle to thin out a rich mixture. If adjusting the needle does not fix the situation, it may be time to change the mid-range jet. If the symptoms of carburetor trouble indicated that the main circuit was at fault, move to Step 4.

Change the main jet if the symptoms of carburetor trouble occur between three-quarters and full (wide-open) throttle. If you want to thin a rich mixture, replace your main jet with a smaller one; if you want to enrich a lean mixture, replace it with a larger one.

Items you will need

  • Flathead screwdriver

 How to Adjust the Carburetor on a 2000 Yamaha Big Bear

Park the ATV in a well-ventilated area and engage the parking brake. Crouch to the left of the ATV, near the left front fender. Reach around the fender to the engine cylinder head to locate the spark plug cable. Clamp an inductive tachometer over the spark plug cable.

Crouch to the left of the ATV, near the left floorboard, and look into the engine bay to locate the carburetor. Turn the pilot air screw -- located on the bottom of the carburetor and directly below the engine intake flange -- clockwise with a flat-head screwdriver until it is lightly seated. Count and record the amount of turns needed to seat the screw. Ideally, the screw should take 2 1/2 turns to seat. Turn the screw counterclockwise 2 1/2 turns to reset the pilot air mixture.

Start the engine and let it idle in place for three to five minutes to warm the engine to operating temperature. Do not stop the engine.

Turn on the inductive tachometer. Ideally, the tachometer should indicate an engine idle speed between 1,450 and 1,550 rpm.

Look for the black idle adjustment knob attached to a cable on the left side of the carburetor. Turn the adjustment knob clockwise to increase the idle speed, or counterclockwise to decrease the idle speed, as needed to bring the idle speed to between 1,450 and 1,550 rpm.

Turn the pilot air screw counterclockwise a half-turn at a time, while watching the tachometer. Stop when the idle speed begins to increase, then turn the screw clockwise a quarter-turn. If the idle speed does not increase after more than two full turns, reset the screw to its 2 1/2 turn position; repeat this step and turn the screw clockwise instead of counterclockwise.

Remove the inductive tachometer and release the parking brake. Mount the ATV and accelerate to a moderate speed from a standstill. The ATV should accelerate smoothly without hesitation. Stop and turn the pilot air screw clockwise a quarter turn, then repeat the test ride. Turn the screw another quarter turn if the hesitation does not improve.

Park the ATV and stop the engine once the ATV smoothly accelerates from a complete standstill.

Items you will need

  • Inductive tachometer

  • Flat-head screwdriver

 How to Adjust the Carburetor on a Yamaha Road Star

Open the choke by pulling the choke lever on the left handlebar control toward you. Set the bike on its kickstand, put it in "Neutral" and start it up. Allow the Road Star to warm up for about 10 minutes before continuing with the adjustment procedure.

Locate the idle adjustment screw on left side of the bike just below the rider's seat. Twist the adjuster screw clockwise one full turn. This will ensure the bike stays alive so you can adjust it to the proper setting.

Close the choke lever and listen to the engine carefully. Twist the idle adjustment screw slowly counterclockwise. You should hear the engine speed start to slow.

Twist the idle screw counterclockwise until you hear the engine start to sputter and die. When this happens, stop and turn the screw one half turn clockwise. Your bike should now be idling at the correct speed.