MyVincent.co.uk

Carburetters

Carburetters

Early 'Monobloc' Design

Early 'Monobloc' carburetters just occasionally had float needles prone to sticking. Erratic tick-over with any 'Monobloc', coupled with ragged opening-up is frequently caused by the standard petrol level being too high. A 3-BA nut will cut its own thread on the nylon needle spigot and, if one thread is left protruding, the float will be lowered about 1/8 in. which will improve matters until a later-type needle is fitted.

Starting Drill

A good throttle opening for both cold and hot starting, single or twin cylinder, is about one-eighth open with an Amal carburetter of the separate float chamber pattern. The 'Monobloc' instrument, as fitted on the 'D', may call for even less opening.

The twins should be started on the rear cylinder. The routine is otherwise as for the single except that, of course, there are two float chambers to tickle. Rear cylinder compression is determined by turning the motor over with the exhaust valve lifter just open, when both compression phases can he detected. It will be found that one pair is closer than the other as the engine is rotated. The second compression of the close pair is that of the rear cylinder.

By starting up on this cylinder, use is made of the long dwell before the subsequent compression comes along and so the rider can build up by means of the kickstarter, good momentum in the flywheels. It is a fact, though an inconvenient one, that the twin-cylinder engine, if stopped of its own accord or by a cut-out button, will come to rest so that the first compression to be felt is the wrong one for re-starting. This does not, of course, apply to stopping the engine with the valve-lifter, as this cuts both power and pumping resistance within the cylinders and the engine then turns over until the kinetic energy of the flywheels is dissipated in friction.

If an engine persistently kicks back-due to over-advanced ignitioncheck that the automatic timing control has not stuck in the fully-advanced position. Two causes of this fault are corrosion and weak bob-weight return springs.

On Series 'D' machines, if starting is difficult, try flooding the 'Monobloc' carburetters until fuel issues from around the ticklers. This will give a sufficiently rich mixture for a cold engine to fire without the air levers being closed. These models have coil ignition which must, of course, be switched on!

The singles answer to normal starting technique. Turn a cold motor over once or twice, with the exhaust valve lifter in use, tickle the float chamber, close the air lever fully and then turn the engine over on to compression. A swinging kick on the starter, along with a swift snatch at the valve lifter - just long enough to release some of the compression - will do the trick. A motor that starts immediately and then cuts out will require a little more flooding; and, naturally, the air lever is to be fully closed. Hot starting should not call for mixture enrichment.

The starting of magneto-equipped models seems to be very sensitive to any trouble with the contact-breaker points which should be renewed if they are in any way faulty. They should be inspected frequently; say every 2,000 miles, perhaps even less. Note that the special points fitted to the 'Black Shadow' can be usefully installed in other models. The fitting of a non-recommended radio-interference suppressor into each H.T. lead will often cause difficult starting as well. About the highest resistance that the Vee-twin magneto can stand is 5,000 ohms; most suppressors are rated at 15,000 ohms. Also-and this is a point well worth bearing in mind-some plugs are fitted with a resistance of 2,000 ohms moulded into the insulators.

Some riders use an FF80 in the hotter rear cylinder and a softer FF70 in the cooler front one. The recently introduced FF75 grade may well prove to be many people's answer. Plug gap is critical and should be kept between 0.015 in. and 0.018 in. for magneto ignition and in the region of 0.030 in. for coil.

The temperature gradient sometimes (but not always) obtaining between the two cylinders of the Vincent twin can also affect the plugs. On the standard range of K.L.G. plug installed as original equipment, an FF70 (14 mm. long-reach) for normal work is about right with an FF80 (harder) for faster riding. More serious motoring may call for FFlOOs to resist the extra heat, while puttering about might require FFSOs. Really oily motors have been known to want FF3Os but these are extreme cases indicating need for a top overhaul at least.

Some engines need a richer part-throttle carburation on the front cylinder than on the rear. For instance, a "Black Shadow" fitted with 29/4 slides as original equipment may respond to a 29/3.5 in the front cylinder and a 29/4 in the rear. But as, at one time, 3.5 slides were not made, many engines were built up with 4s whereas 3.5s would have been better in both cylinders, though at the expense of fuel economy.

The figure quoted after the oblique stroke on a throttle valve refers to the height of the cut-away, in sixteenths of an inch, above the base of the slide. The higher the cut-away, the larger the number, and the weaker the mixture. It is possible to hand file an extra 1/32 in. of cut-away on a 3.5 slide to turn it into a 4; the marking on the top of the slide should then be amended. Slides that are slack in the mixing-chamber body produce a weak mixture, spitting back, erratic starting and running and a chattering noise.

Some 1,000 c.c. models have a heavy twistgrip action, caused by double the normal spring pressure. An immediate palliative is to release the junction box from its fixing and merely to let it dangle on the cable; the slides will have to he resynchronized.

Another method is to insert a contra-spring into the twistlirip. Cut about 1/2-in. from the end of the handlebar and drill the bar some 6-in. in to take a pin. File a slot in the end of the grip sleeve to take another pin and attach a suitable spring between these pins, pre-tensioning it in the opposite sense to the carburetter springs; that is so that the contra-spring tries to open the grip.

'Monobloc' carburetters are not so unpleasant in the respect of grip action owing to the use of larger, barrel shaped springs with better characteristics. Softer springs are available but make sure that these can, and do, return the slides. A 'Black Shadow' jammed fully open is no laughing matter. In such an emergency, close the choke levers and turn the petrol off if no ignition switch is fitted. Do not use the valve lifter if this can be avoided.

Final Adjustments

Carburetter synchronization on the twin is all important While synchronization may not offer any appreciable improvement in starting, pick up, smooth running and acceleration over a reasonably well set pair, it certainly does make the exhaust note spot-on and, therefore, more pleasant. Bad synchronization will result in the above factors being either poor or, so far as smoothness is concerned, nearly absent.

A 'leading' front cylinder, that is one in which thee carburetter throttle valve is more open than that of its companion at the rear, will certainly produce a 'thump'. Due to the firing order of the twin, with its long/short gaps between pulses, the converse does not hold true. It cannot he over stressed that, rather than tinker with a little adjustment here and there, it is better to start from scratch and get it completely right with the adjuster, lock up, and then leave well alone!

Before commencing the tuner must be certain that everything else about the machine that could have an adverse effect is properly set. For instance, the valve clearances muust be right and the contact-breaker points correctly gapped: too much gap will advance the ignition - the setting is critical to within +- 0.002 in.

One method of synchronization, carried out with the engine thoroughly warm and the machine on the rear stand, necessitates making the twin work as a single by immobilizing each cylinder in turn. Remove one sparking plug and take its H.T. lead safely out of the way because of the fire risk from mixture being exhausted through the plug hole.

Start the engine, which will then run on the other cylinder Operate the slow-running mixture screw in conjunction with the throttle stop screw until that cylinder settles down to a slow, steady tick-over with clean response to a gently opened throttle. There must be a little slack in both cables to avoid a slide being hung on a tight inner.'' Repeat the drill on the other cylinder. The result is two 'singles', in a manner of speaking, each set to idle properly and to open up well.

With both plugs in use, the engine will, naturally, tick over too rapidly. Ease down the throttle stops until the firing is slow enough, reliable and sounds equal at the exhaust.

Do not alter the mixture screws. Lock the throttle-stop nuts (not used on 'Monoblocs') and stop the engine; leaving the motor running while the nuts are locked will show up any accidental movement of the screws. By means of the adjuster fitted into the rear cable, obtain a rough synchronization. This is done by placing a finger gently against one slide and watching the other as the grip is twisted to lift both off the stops; make corrections as necessary.

Now listen to the slides closing A slight click will be heard as each hits its stop when the grip is shut off. The two clicks should occur in unison; modify the setting of the rear cable adjuster to obtain this condition. If it fails to provide sufficient adjustment, resort may have to be made to the threaded ferrules at the top of the mixing chambers; lock-nuts are not used on these, Beware of 'Monobloc' top-retaining rings (the large, milled ones), backing off and allowing the tops to lift slightly under spring pressure.

Another method is to listen to the exhaust note at about 25 m.p.h. in top gear on a slowly falling throttle. It should be even; if it is not then one cylinder is pulling more than the other. Detection is simple. With the left hand, grasp the outer of the rear cable near the carburetter and pull gently. If the note becomes more pronounced then clearly the rear cylinder was doing most of the work anyway; shorten the outer-cable adjuster on the rear cylinder and retest. If the note equalizes between the cylinders then the rear outer needs lengthening a shade. Some riders favour this system, as it takes into account performance irregularities between the two cylinders, and no two 'pots have the same efficiency.

On some models it will be found that, when the slides are opening and closing in unison, they do not disappear from sight in the choke orifice together as full throttle is approached. Bad siting of the cable to the front instrument is the usual cause. Though it may look untidy, it is a good idea to release the junction box from its clip and allow it to hang from the cables. Or it can be clipped to the front offside cylinder-head holding-down nuts, above the head bracket. Such rerouting calls for resynchronization.

An Amal twin-rotor twistgrip is available; no junction box is required as two full-length cables are used.

Ultimate Performance

Even with twin carburetter and ignition settings dead right, it is hard to suggest a maximum obtainable performance for three reasons. First, speedometers vary in optimism, especially with different front tyre pressures; secondly, the Vincent engine is run for exceptionally long periods as a rule without decarbonizing and while satisfactory general performance is nevertheless forthcoming, the maximum is gradually reducing. Thirdly, for a machine in the 110 m.p.h. 'Rapide' or the 125 m.p.h. 'Black Shadow' classes, the amount of road needed to reach a true top speed is in excess of that usually found in Britain. But the 'Comet is good for a genuine 90 m.p.h. in standard trim.

Both the singles and the twins are slightly over-geared for sheer performance and this overdrive effect is deliberate and desirable factory policy. For instance, the standard 'Rapide' gearing is 46t. at the rear wheel; 41t. will produce more speed at the expense of fuel and longevity of working parts.

Fuel consumption ought to be about 70 m.p.g. with the 'Comet', 60 to 65 with the 'Rapide', and 50 to 55 with the 'Black Shadow'. But the latter may fall to 40 m.p.g. with a heavy sidecar.

A wide range of sprockets is available to sidecar owners. They range from 46t. (standard solo) to 60t., with the 56t. a good all-round compromise for most sizes and weights of sidecars. The gearing can also be varied at the gearbox output sprocket by fitting a 22t. component in place of the usual 21t. item. Sprockets smaller than 45t. are available but call for special adapters to mount them.

The standard 21/46 gearing produces a top of 3.5:1 for the twin, which needs an engine speed of 4,600 r.p.m. for 100 m.p.h. On the single, the standard 48t. rear sprocket gives a 4.64:1 top which would produce 6,100 r.p.m. at 100 m.p.h. if such a road speed were attainable on this gear. In both cases, the calculations are based on a 3.50-in. by 19-in. rear tyre as fitted to all 'B' and 'C' models. To all intents and purposes, the 4.OO-in. by 18-in. 'D' rear cover produces the same result.

Riders searching for the ultimate in longevity have been advised by the factory not to exceed 5,000 r.p.m. with the standard uncaged roller big-end. Little point is gained by exceeding 5,800 r.p.m., anyway, as the power definitely begins to tail off by then: 5,000 r.p.m. permits 55 m.p.h. on the 'Black Shadow' 7.2 bottom gear and 44 m.p.h. on the 'Rapide' 9.1 ratio. In both cases, the second and third gear speeds corresponding to 5,000 r.p.m. are 70 and 95 m.p.h. respectively, achieved on 5.64 and 4.16:1 overall. Equivalent 'Comet' speeds are 32 (12.4 1st), 48 (8.17 2nd), 66 (5.94 3rd) and 85 m.p.h. (4.64 top gear).