Carburettor tuning

Carburettor tuning in this section strictly covers re-setting and synchronisation and does not discuss jet/slide needle fitting. I have assumed that the carburettors are to standard specification, or at least, that your specification has been proven to work. When and how often you tune your carburettors will depend on a number of factors These will vary from the state of wear of your instruments to your riding method. For most of the job discussed here, it helps to have help from another knowledgeable assistant. Before starting make sure your ignition setting is correct.

On the Twin, remove the inlet trunking if fitted and then fuel tank. Insert an index finger into the venturi of one carburettor. Feel the slide and then slowly turn the throttle twist grip. Assess how, and how far. the slide travels. Do this slowly so to gain the best assessment and to minimise the amount of fuel the diaphragm pumps in. Do this again for the other carburettor. In a nutshell, each slide must start at the same point, rise at the same speed and top out at the same point and time. Now set the throttle stop screws to give equal amounts of opening for each slide. Place an w index finger in each venturi and have your help twist the throttle slowly. If each slide does not move at exactly the same time, adjust the throttle cables on the top of each carburettor, although one at a time, until synchronisation does take place. Remember that there should be between 1 and 2mm free play in the throttle cables and 4mm on the chokes before any movement takes place.

Replace the fuel tank and intake trunking.

When vacuum gauges are not available, proceed as follows.

Now start the Twin and warm the engine to its working temperature, but do not let it overheat. Set both throttle stop screws (idle screws) so that the engine runs at 1500 rev/min. This avoids stalling while adjustments are made. Remove the plug cap from one cylinder with the engine running. However, you should try to earth that plug cap to avoid coil damage. On the side which is firing at a fast tickover, slowly turn the mixture (out to enrich, in to weaken) until the tickover is as smooth as possible. There should be no hunting or spitting back. Now slow the tickover at the same time retaining that smoothness. Stop once an optimum has been hit — low tickover and smoothness.

Now swop plug caps, and repeat the process for the other (firing) cylinder. As soon as you have that 'smoothness' consistent with a low tickover, push the second plug cap back on and assess the tickover. If it is still too high, bring it down by adjusting each idle screw by the same amount until it reaches about 800 rev/min. Don't expect a worn or tuned engine to so perform.

If vacuum gauges are available proceed as follows. Connect the gauges to each of the two vacuum inlets on the inlet stubs, having removed the balance pipe. Unscrew each mixture screw one full turn from the closed, or 'in' position. Start the engine and run it to its normal working temperature. Set the idle speed using similar adjustments on both idle screws so that the engine just keeps running (that'll be something like 700 + rev/min.). Adjust the idle screws so that the reading on each vacuum gauge is the same (and the engine keeps running, but only just). Now adjust each mixture screw to obtain the smoothest of tickovers. Recheck the reading on the vacuum gauges and juggle both idle and mixture screws to achieve a super smooth tickover — both readings being the same, the tickover at around 800 rev/min and the engine neither hunting nor spitting. (Mixture enriches with the mixture screw being undone, weakens by the screw being done up!). Now run the engine to 2000 rev/min using the throttle twist grip and check the readings on the vacuum gauges. Align them if they have 'moved' making sure that the running remains smooth.

The SFC carburettor tuning, including the Amal Concentrics, uses a similar technique to the other Twins. Do not expect a low tickover.

On the Triple things are a little more difficult because each carburettor is closely connected with the other using a single throttle cable straight onto mechanical linkage, linking each carburettor (except on 1973 machines which used three cables). I strongly advise that you use vacuum gauges for the Triple — the best you can get from a manual adjustment is very much second best! Obviously you can use a similar method to that of the Twins but it will not ultimately help. Once you have fitted the vacuum gauges complete synchronisation incorporates mixture/tickover setting. All three are described below.

Leave the fuel tank on the Triple, start the engine and allow it to warm up. Let the engine reach a hot, running temperature but do not allow it to overheat. Stop the engine, remove the vacuum balance pipes and then fit a set of three vacuum gauges to the inlet stubs. Unscrew each of the three mixture screws so that they are each one full turn from the fully closed position. Now briefly start the engine again and set the engine to rev at around 2000 rev/min by adjusting the long idle adjuster screw found beneath the centre and right hand carburettors.

Using the two screws between the two outer carburettors and the centre one which adjusts the throttle arms, with the engine running, align the readings of the vacuum gauges fitted to the two outer carburettors to the reading of the centre one. Then reduce the engine speed using the idle speed adjuster to 1100 rev/min. Now try to set the running of the three cylinders as evenly as possible by adjusting the three mixture screws. Remember unscrewing them, enriches; screw them tight, weakens.

Now repeat the operation of synchronisation of the readings and then finally bring the idle speed to as close as possible to 800 rev/min, except for the Jota and Mirage on which it is likely to be between 1000 and 1200 rev/min. Remove the vacuum gauges and replace the balance pipes to the intake stubs. Try the engine again and assess the tickover.

The right hand side of the Triple s carburettor set-up. First there's the balance screw for the right carburettor; behind it is the 'overall' tick-over screw and to the left of that is the single throttle cable

Laverda Triple

The left hand side of the Triple engine, looking dc* -on the carburettors. That's the balance adjustment screw for the left hand carburettor. Tachometer onve

NOTE: Once carburettor tuning has taken place, tighten the cable stop screws and any other fittings which are not self-tightening. Lightly oil all the external joints and cables for smooth operation. Also make sure all the cable rubber sleeves are fitted. Not only do they support the cables as they leave the carburettor bodies but also seal them off. Remember weak mixture overheats the engine and will probably blow it. Over rich mixture just clogs everything. Laverdas are very 'clean burners' as motorcycles go.

The left hand side of the Triple engine, looking dc* -on the carburettors. That's the balance adjustment screw for the left hand carburettor. Tachometer onve

The right hand side of the Triple s carburettor set-up. First there's the balance screw for the right carburettor; behind it is the 'overall' tick-over screw and to the left of that is the single throttle cable

The knurled nut is the 'fine' tuning adjuster for an Dell'Orto's mixture screw three carburettors together

PHF carburettor - D is the mixture adjustment screw, F is the throttle stop screw

Looking down the throats of the Twin's PHFs - F is the throttle stop screw, G is the accelerator pump jet plug, L is the accelerator pump adjuster screw, M is the choke cable adjuster and N is the throttle cable adjuster. Apart from the pump jet the VHB carburettors used on the early machines, and GTL, are very similar

The knurled nut is the 'fine' tuning adjuster for an Dell'Orto's mixture screw three carburettors together

The Triple carburettor set-up

The Triple carburettor set-up

Motorcycle Twin Choke Cable

The SFC's PHB carburettors - D is the mixture screw, F is the throttle stop screw, M is the choke cable adjuster and N is the throttle cable adjuster

The Triple's carburetter set-up - A is the vacuum gauge connection, B is the upper throttle stop screw, C is the cable adjuster

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