Synchronising carburettors

Uneven idling, poor throttle response and more vibration than necessary in multi-cylinder engines can be a sign that the carburettors are not in sync. Imagine a team of horses where one wants to gallop, another is moving at a gentle trot while two others are more inclined to walk. As a result, the galloper strains at the harness in vain, the walkers begin to stumble, the trotter doesn't know if it's coming or going and the driver has had enough.

Before you consider synchronising the carburettors, you must ensure that everything else is in order. The ignition and valves must be correctly adjusted, as must the play in the throttle cables. The air filter, intake manifolds and spark plugs should be in good condition. 

First warm up the motorbike, then put it on the centre stand and switch off the engine. Now remove the tank and any covers and fairings that may be in the way. The fuel bottle must always be positioned higher than the carburettors. Next comes the synchroniser. For ease of packaging, synchronisers generally come in parts, but they are easy to assemble – simply follow the instructions. Each time you use the synchroniser, always screw in the knurled screw (which regulates the air flow) hand-tight, but without crushing the hose. 

The reason: We are dealing with very low vacuums, so the indicator needles on the gauges react sensitively. If you connect up a gauge with the damping set too low and start the engine, the needle will flick violently from one extreme to the other with each stroke of the engine and the gauge may break. 

Now connect the tubes of the synchroniser to the motorbike. Depending on what bike you've got, these connection points may be on the cylinder head (see Fig. 1), the carburettors (generally at the top towards the intake manifold) or the intake manifolds (see Fig. 2). 

The manifolds often have connection nozzles, which are closed off with a rubber cap. On the carburettor or cylinder head, you will need to unscrew small screw plugs and replace them with screw-in nozzle adapters (synchronisers usually come with the most commonly used adapters). 

Before connecting up the gauges, calibrate them so that they all show the same reading. This will reveal if any gauge is giving inaccurate readings or if any tube connection is leaking. To do this, first connect up all the gauges with T- or Y-adapters (generally also included with the synchroniser) so that they all feed into a single tube. Connect this tube to a carburettor or intake manifold, and leave the other connections closed off.

Now start up the engine and adjust the gauges at the knurled nuts just enough so that the needles are still have "lively" movement, but are sufficiently damped. A pointer that doesn't move at all means a blocked gauge, so undo the knurled nut a little. All gauges should now show the same readings. Switch off the engine. Assuming the gauges are completely intact, you can now attach one gauge to each cylinder and place the gauges somewhere safe on the bike and secure them so that they do not fall off (be careful because the gauges tend to "wander" due to the engine vibrations). 

Start up the engine, rev it gently a few times to around 3000 rpm and then let it idle. Check the gauge readings and readjust, if necessary, at the knurled nuts until they are easy enough to read. Most manufacturers tolerate a variance of up to 0.03 bar. 

Depending on the particular motorbike, either identify the base or master carburettor, and then adjust all the others as accurately as possible to the same setting by means of the adjustment screws or, as already described, first synchronise the right and left carbs, then synchronise the two pairs to each other. From time to time, rev gently to check whether the engine returns to the correct idle speed; if not, readjust with the idle adjustment screw. If synchronisation proves impossible, the carburettors may be taking in air via an intake manifold that is either porous or leaking at the connection to the carb or cylinder head, or the base setting of the carburettors has become completely scrambled. In rare cases, a heavily soiled carburettor may be the cause. You should definitely investigate and remedy these potential faults, or else all further attempts at synchronisation will be pointless. You can find further information on cleaning a carburettor under the DIY tip Carburettor.

However, we will assume it has been a success and congratulate you on a considerably smoother-running bike and more spontaneous throttle response, making for an even more enjoyable ride than before. Now you can disconnect the synchroniser and take the pressure off the tubes by undoing the knurled nuts slightly. Put the rubber plugs back on (take the opportunity to check for porosity) or screw the screw plugs back on with care (soft material!). Then refit and connect up the tank and covers/fairings, and pour any fuel left in the fuel bottle into the tank. Job done!