The 1983 Yamaha Maxim uses two Mikuni CV carburettors to power the twin-cylinder, 650cc engine. Constant velocity (CV) carburettors are common on cruiser bikes because the design offers smoother acceleration than typical slide-valve carburettors. Carburettor adjustments require mechanical expertise and knowledge of the CV operation as it relates to the main jets, jet needles and fuel-air mixtures. Synchronising the carburettors for everyday riding is done using the motorcycle's tachometer to coordinate the idle and throttle operation.
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Things you need
- 1983 Yamaha 650 service manual
- Metric tools
- Main jets (optional)
- 2 Mikuni 34mm carburettor kits
- 2 air filter cartridges
Close the fuel petcock valve and remove the rider's seat from the motorcycle.
Detach the air cleaners from both carburettors and discard the filter cartridges. Disconnect the fuel lines and throttle linkages from both carburettors. Remove both carburettors and place them on a workbench.
Remove the fuel bowls from both carburettors. Empty any residual fuel in the bowls into a gasoline can. Discard the fuel bowl gaskets. Place each bowl upright on the bench with the floats and needle jets intact.
Turn the carburettors over. Note the size of the main jets stamped on the face of each jet. If the bike runs stronger when cold and weaker when warmed up, the main jet is too large. If the bike runs poorly when cold and slightly better when warmed up, the main jet is too small. Replace the main jets as needed.
Turn both carburettors over and remove the carburettor caps, diaphragm and throttle return spring from each. Note the position of the jet needle clips inside both carburettors. If the engine is strong in the mid-to-high range (5000rpm to 7500rpm) when cold, but poor when warmed up, the carburettors are too rich and the needle clips must be lowered. If performance is substandard at mid-range, the carburettors are too lean and the needle clips must be raised.
Lower or raise the needle clip one notch on each carburettors as needed to account for rich or lean operation. Put the new throttle return springs from the Mikuni carburettor kits in each carburettors. Install the new diaphragms and cap gaskets on each carburettor. Reattach the caps on each carburettor.
Hold the fuel bowls level and measure the float height from the lip of the bowl on each carburettor using your metric float gauge. Lower the float height 1-mm if you installed a larger main jet. Raise the float height 1-mm if you installed a smaller main jet.
Include the new float bowl gaskets from the kits and reattach the float bowls to each carburettor.
Rich and Lean Adjustments
Mount the carburettors on the engine. Connect the throttle linkages and both fuel lines. Put new filter cartridges in each air cleaner and reattach the air cleaners at both carburettors.
Open the fuel petcock valve. Start the motorcycle and allow the engine to warm up to normal operating temperature.
Take a position that allows you to operate the throttle twist-grip and observe the throttle linkages on both carburettors. Twist the throttle several times and allow the engine to idle. Observe the tachometer as you adjust the throttle-stop screws on each carburettor so both throttle levers are synchronised and the engine idles at 950 RPM.
Allow the engine to idle. Turn the fuel-air mixture screw on one carburettor each way until you find the point where the engine idles at the fastest speed. Repeat the procedure to adjust the fuel-air mixture on the opposite carburettor.
Observe the tachometer. Readjust the idle to 950 RPM by turning the throttle cable adjuster either way as needed. Reattach the rider's seat on the motorcycle.
Synchronising the Carbs
Tips and warnings
- Clean the carburettors using spray carb cleaner and a utility brush before reassembling. Wipe the cleaning residue away with shop rags.
- Lubricate the throttle cables using cable lubricant.
- Lubricate the throttle linkages with spray lubricant.
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