How to Adjust the Carburetor on a 1970 Honda Trail

by Chris Gilliland

Honda's 1970 CT-series, consisting of the Trail 70 and 90, were small single-cylinder motorcycles that were intended to be used both on and off of paved roads. Both motorcycle styles used a simple carburetor to provide a combustible mixture of air and fuel to the engine. The carburetor allowed for the adjustment of low speed operation through an air screw along the side of the carburetor, as well as an adjustable needle in the throttle valve to accommodate changes at intermediate speeds. High speed operation, such as full throttle applications, could only be adjusted by replacing the main fuel jet.

Low Speed Operation from Idle to One-Eighth Throttle

Step 1

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

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

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.

Step 6

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.

Step 7

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

Step 1

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

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

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.

Step 6

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

Step 1

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

Step 2

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.

Step 3

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

Step 4

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.

Step 5

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.

Step 6

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.

Step 7

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.

References

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