Yamaha motorcycles with more than one cylinder usually have more than one carburettor for fuel delivery. Ever time these carburettors are removed for cleaning or rejetting they should be synchronised, or synched, when they are installed on the bike. Synchronisation is the process that ensures that all intakes are pulling the same amount of engine vacuum, so that they are also pulling an equal amount of the fuel/air mixture from the carburettors. A special tool and a little knowledge are all that's needed to properly sync Yamaha carburettors.
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Things you need
- External fuel bottle
- Basic hand tools
- Long Phillips screwdriver
- Vacuum gauges
Remove the fuel tank and connect an external fuel source, such as a remote fuel bottle. On some models you can loosen the rear tank bolt and use a block of wood to lift the tank high enough to connect the vacuum gauges. However, this often makes access to synchronisation screws difficult. If you do not have a remote bottle, remove the tank and strap it securely to a platform higher than your motorcycle. Use a long piece of fuel line to connect it to your Yamaha to provide fuel during the synchronisation process.
Remove the synchronisation plugs from the intake manifolds. This will be a little Phillips screw on each manifold that has a copper washer and faces away from the centre of the motorcycle. Install the vacuum adaptors and attach your gauges. Yamaha motorcycles primarily use the 6mm adaptor, but compare the plugs you removed to the adaptors to identify which to use. Be sure that the provided O-rings are installed with the vacuum adaptors so they don't leak and give inaccurate readings.
Start the motorcycle and let it idle for a couple of minutes until the engine is warm. Adjust the idle to factory specifications found in your manual. Look at the readings on the vacuum gauges. They should be within 1 psi of each other. If they are not, adjust the carburettor sync screws. On a two-cylinder Yamaha, there is one screw between the carburettors. On a four-cylinder bike, there are three screws. The carburettor on the left side is your base carburettor and has no adjustment. The other three carburettors must be syched to that one. The sync screw is between the carburettors.
Adjust each screw so vacuum readings are equal. Rev the bike slightly, letting it return to idle. Ensure that the readings stay the same. Adjust the idle screw so that the idle is around 2500rpm for a four-cylinder bike, around 1500rpm for a two-cylinder machine. Check the vacuum gauges and make sure they are still even. If not, adjust them so that they are within 1 psi, and then return the setting to factory idle specifications. Make sure the vacuum readings are still equal.
Disconnect the vacuum gauges and remove the vacuum adaptors. Install the synchronisation plugs with new copper washers so they do not leak. Disconnect the external fuel bottle or the long fuel line. Reinstall the fuel tank. Start your Yamaha. Check that the idle is smooth and that nothing has been left disconnected.
Tips and warnings
- There are two types of vacuum gauges for synchronising carburettors. Dial gauges must be synchronised to each other to work properly. Mercury gauges are more accurate, but these must be handled with extreme care. Mercury is a dangerous substance, and if gauges are dropped and mercury is spilt, special hazardous materials clean-up procedures must be followed.
- Fuel vapours are flammable, so work in a well-ventilated area away from spark and open flame. Also, exhaust gases can be deadly, so ensure that your work area is open or the you have an exhaust capture system attached if you are in a shop environment.
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