Honda produced the CL450 motorcycle from 1970 to 1973. Engines are twin-cylinder, overhead valve with dual carburetors. The CL450 scrambler and its cousin CB450 road bike were manufactured in an era before oxygen sensors and catalytic converters, making carburetor adjustment relatively uncomplicated. As long as the air filters are clean and the carburetors are not leaking gas, adjusting the dual carbs improves throttle response and overall performance. You adjust the carburetors on a Honda CL450 with the motorcycle parked in a ventilated area.
Start the motorcycle and allow the engine to warm up for three minutes. As the engine is warming, locate the throttle stop screw at the upper side of the throttle levers on the outside of each carburetor. If necessary, reach up and twist the throttle hand grip a few times and you'll see the levers go up and down.
Look at the tachometer gauge and notice the rpm's while the engine is idling. Normal idle speed is between 1100 rpm's and 1200 rpm's. If the idle is above 1200 rpm's, turn the throttle cable adjustment coupler at the throttle twist grip counterclockwise with a metric open-end wrench as you watch the tachometer. Stop when the tachometer reading is in the normal idle speed range.
Push the rubber nozzle on a backpressure gauge into the end of one exhaust pipe. Hold it firmly for two seconds and pull the gauge away. Note the backpressure reading on the gauge. Zero the gauge and repeat this with the other exhaust pipe. Note the backpressure.
Compare the backpressure readings and determine whether the right or left exhaust has the lower backpressure. Turn the throttle-stop screw on that carburetor one quarter-turn clockwise and recheck the backpressure. If necessary, turn the screw counterclockwise a bit and recheck the backpressure. The idea is to make small adjustments until the backpressure is equal in both exhaust pipes.
Locate the pilot-air screw on the right carburetor, just below the throttle lever. Adjust the pilot-air mixture by turning pilot-air screw clockwise or counterclockwise with the screwdriver until the engine is at the maximum idle speed. Listening to subtle changes in engine speed as you adjust the idle mixture is the easiest way to do this. Repeat the procedure to adjust the idle-air mixture on the left side carburetor.
Recheck the backpressure on each exhaust using the gauge as before. If necessary, adjust the appropriate throttle-stop screw so the backpressure is equal in both exhausts.
Locate the throttle cable adjuster and lock nut at the top of each carburetor. Loosen the lock nuts at the base of each throttle cable adjuster by turning them counterclockwise with the metric open-end wrench.
Take a position where you can reach the throttle twist grip and observe the throttle cable adjuster on the right side carburetor. Roll the twist grip open slowly until you see the throttle cable begin to tighten. Repeat this as you observe the throttle cable on the left side carburetor. Determine whether one throttle cable has more tension and engages before the other.
Adjust the throttle cable that needs more tension by turning the throttle adjuster coupler clockwise with the metric open-end wrench. Repeat the observation test and make any final adjustments. Tighten the lock nuts on each throttle cable adjuster clockwise with the metric open-end wrench.
- Ask an experienced motorcycle mechanic to assist with carburetor adjustment of necessary. Watching the procedure is helpful the next time carburetors need adjusting.
- Retrofit a straight-stem pressure gauge with rubber washers when a backpressure gauge is not available.
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