Warning: Petrol (gasoline) is extremely flammable, so take extra precautions when you work on any part of the fuel system. Don't smoke or allow open flames or bare light bulbs near the work area, and don't work in a garage where a natural gas-type appliance is present. If you spill any fuel on your skin, rinse it off immediately with soap and water. When you perform any kind of work on the fuel system, wear safety glasses and have a fire extinguisher suitable for a Class B type fire (flammable liquids) on hand.
Warning: Take great care not to burn your hand on the hot engine unit when accessing the gauge take-off points on the intake manifolds. Do not allow exhaust gases to build up in the work area; either perform the check outside or use an exhaust gas extraction system.
1. Carburettor synchronisation is simply the process of adjusting the carburettors so they pass the same amount of fuel/air mixture to each cylinder. This is done by measuring the vacuum produced in each intake duct. Carburettors that are out of synchronisation will result In decreased fuel mileage, increased engine temperature, less than ideal throttle response and higher vibration levels.
2. To properly synchronise the carburettors, you will need a set of vacuum gauges or calibrated tubes to indicate engine vacuum. The equipment used should be suitable for a twin cylinder engine and come complete with the necessary adapters and hoses to fit the take-off points. Note: Because of the nature of the synchronisation procedure and the need for special instruments, most owners leave the task to a Honda dealer.
3. Start the engine and let it run until it reaches normal operating temperature, then check that the idle speed is correctly set, and adjust it if necessary (see Section 2).
Warning: The engine and carburettors will be hot. With the restricted access to the screws, great care must be taken not to bum yourself while synchronising the carburettors.
4. Remove the fuel tank (see Chapter 4).
5. On XL600V models, release the clamp and detach the fuel tap vacuum hose from the take-off point on the rear cylinder intake duct (see illustration). Undo the blanking screw from the front cylinder intake duct and thread a suitable hose adapter in its place (see illustrations).
17.5a. Detach the fuel tap vacuum hose from the rear cylinder take-off point
17.5b. Undo the blanking screw (arrowed)...
17.5c ...and thread a suitable adapter in its place
6. On XL650V models, release the clamp and detach the fuel tap vacuum hose from the take-off point on the rear cylinder intake duct (see illustration 17.5a). Release the clamp and detach the PAIR solenoid valve vacuum hose from the take-off point on the front cylinder intake duct (see illustration).
17.6. Front cylinder vacuum take-off point - XL650V models
7. On XRV750 models, undo the blanking screw from each cylinder intake duct and thread suitable hose adapters in their places (see illustrations 17.5b and c).
8. Connect the gauge hoses to the vacuum take-off points (see illustration). Make sure they are a good fit because any air leaks will result in false readings.
17.8. Connect the gauge hoses to the vacuum take-off points
9. Arrange a temporary fuel supply, either by using a small temporary tank (a two-stroke motorcycle oil tank works very well as it has an outlet union on its base to which a link hose can be attached). Similarly an old oil container with a nozzle type cap can be used (see illustration), or by using extra long pipes to the now remote fuel tank, making sure on XL models the tank is sitting higher than the carburettors, or the fuel will not flow (XRV models have a fuel pump). On XRV750-P models onward (1993-on) the tank can be set in its normal place on the bike as the carburettor synchronisation screw is accessed from below the carburettors - on all other models this is not possible as the screw is accessed via a hole in the top of the air duct. On all XRV750 models, make sure that any auxiliary lank or hose is connected into the hose that normally attaches to the fuel tap so that the filter and pump are included in the supply system.
17.9. One way of setting up an auxiliary fuel supply
10. Start the engine. If using vacuum gauges fitted with damping adjustment, set this so that the needle flutter is just eliminated but so that they can still respond to small changes in pressure.
11. The vacuum readings for the cylinders should be the same, or at least within the maximum difference specified at the beginning of the Chapter. If the vacuum readings vary, adjust the carburettors by turning the synchronising screw situated in the throttle linkage between the carburettors until the readings are the same. On XRV750-P models onward (1993-on) the screw is accessed from below the carburettors - on all other models it is accessed using a long screwdriver inserted through the hole in the top of the air duct (see illustration).
Note: Do not press hard on the screw whilst adjusting it. otherwise a false reading will be obtained.
17.11 Adjusting carburettor synchronisation. Note how on XL models the adjustment screw is accessed via the hole in the air duct or filter housing
12. When the carburettors are synchronised, open and close the throttle quickly to settle the linkage, and recheck the gauge readings, readjusting if necessary.
13. When the adjustment is complete, recheck the vacuum readings, then adjust the idle speed by turning the throttle stop screw (see Section 2) until the idle speed listed in this Chapter's Specifications is obtained. Stop the engine.
14. Remove the vacuum gauges and the hose adapters, then replace the blanking screw(s) and/or attach the vacuum hose(s) to the takeoff points as required by your model (see Step 5, 6 or 7). Install the fuel tank (see Chapter 4).