On the four-cylinder Honda CB750 motorcycle, each cylinder has its own carburettor. Adjusting the throttle slides to open exactly the same amount in unison is called synchronisation. Improperly synchronised carburettors may cause one or more cylinders to work harder than the others and reduces the performance of the engine. Synchronising the carburettors is done using a mercury manometer that gives a visual reading of the vacuum demands of each cylinder of the motor. Periodic synchronisation of the carburettors will keep your motorcycle operating in peak performance.
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Things you need
- Multi-port carburettor synchronisation tool
- Carburettor wrenches
- Flat tip screwdriver
Place the motorcycle on the centre stand. Start the engine and allow it to warm up to normal operating temperature.
Shut down the engine. Remove the seat and the side covers. The side covers are a press fit into rubber grommets and will pop off without the use of tools.
Turn off the fuel supply valve on the gas tank and remove the fuel line to the carburettors. Remove the hold down bolt at the rear of the gas tank. Remove the gas tank by lifting it up and to the rear.
Remove the vacuum plugs from the carburettor bodies. Connect the vacuum lines from the synchronisation tool to the ports on the carburettor bodies. Connect each line in order from one to four, starting on the left side of the motor as you would look at them from sitting on the bike.
Start the engine and observe the readings on the synchronisation tool. If the difference in the readings between carburettors is greater than 2.4 inches of mercury, then the carburettors require adjusting. The number 2 carburettor is the base piece and is not adjustable. The other three carburettors will be adjusted to match the readings of the number two carburettor.
Loosen the locknut on the number 1 carburettor adjusting screw. Turn the screw until the reading on the number 1 gauge is less than 2.4 inches difference from the number 2 carburettor. Tighten down the locknut, making sure not to disturb the adjustment. Open and close the throttle and recheck the adjustment for accuracy.
Replace the fuel tank and turn the fuel supply valve on. Refill the carburettor bowls with gas for the next step. Do not use an external fuel supply tank due to the risk of fire.
Repeat Steps 6 and 7 to adjust the number 3 and 4 carburettors.
Shut down the engine and remove the synchronisation tool. Replace the vacuum port plugs on the carburettors. Replace the gas tank and side covers.
Restart the engine and adjust the idle to 1,000rpm if necessary, by using the idle speed adjusting screw located between carburettors 2 and 3.
Tips and warnings
- Engine exhaust fumes are toxic. Work only in a well ventilated area.
- Avoid overheating the engine. Allow the motor to cool down between adjustments or use a shop fan to circulate fresh air over the motor.
- Ensure the carb-sync tool is equipped with restrictors. These restrictors prevent the mercury from being drawn into the motor. If the mercury is drawn into the motor, no harm to the motor will occur but the tool will have to be replaced.
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