This Supplement to the Viper/Venom Service Manual deals exclusively with the Venom Thruxton 500 and the variations affecting the specification, running instructions and maintenance.

The general instructions given for the Venom Clubman in the Viper/ Venom Service Manual can be applied as far as practical to the Thruxton 500 with the exceptions given in this supplement.


Identification Markings

The engine number prefix letters are V.M.T.

Tappet Clearances

Running clearance : Inlet .006’. Exhaust .008’

Compression Ratio. 9 to 1.



Size 1 3/8”. Main Jet 280. Needle Jet 109

Pilot Jet 25. Air Jet "125. Slide No.3.

Needle position 5 (counting from top).

Later Thruxton models were fitted with Amal type carburetter 1035/4:

Main Jet 320. Pilot Jet 30. Needle Jet .106.

Needle position 1, Throttle valve 3.

For maintenance and running instructions apply to the makers.


Lucas Type K.1.F.C. with hand control.

Coil Ignition.

Lucas type

Lighting Equipment.

Lucas Headlamp Type No. M.C.N. 60 fitted with Sealed Beam Unit 700 H. Later models fitted with Headlamp type SS700P.

Gear Ratios.

With 20T Gearbox Sprocket.

First 10.1 : Second 6.97 : Third 5.3 : Top 4.4 to one.

T.T. Close Ratios : First 8.4 2 Second 6.3 2 Third 4.83 : Top 4.4 to one.

Oil capacity. Front Fork. Each strut carries 120cc of oil.

Unladen Weight. 375-lbs. 170 kilograms.

Wheels and Tyres. Front Tyre 19" x 3.00” Ribbed.

Rear Tyre 19" x  3.25’ or 3.50’ Studded.


 Front Hub and Brake Assembly

FIG. 44 Front Hub and Brake Assembly
1. Brake Torque Bolt 5. Fork Damper Tube Adaptor Nut
2. Brake Cable Adjuster 6. Air scoop
3. Brake Adjuster Lock Nut  7. Shoe Setting Control Rod
4. Front Fork Oil Drain Bolt 8. Greasers for Top and Bottom Operating Cams (where fitted)

Note.—The brake cable adjuster and lock nut is positioned on the brake plate and is concealed behind the triangular steel plate attached to the mudguard stay.


Front Brake Adjustment (see illustration). A twin leading shoe front brake is fitted on the Thruxton with a 7.5” drum diameter and the hub incorporating an air scoop is drilled for cooling. Adjustment is carried out by the cable adjuster as detailed in the Venom Manual.

The Front Hub is supported on a hollow spindle by two non-adjustable journal ball bearings and the process of dismantling and re-assembling the bearings detailed in the Venom Manual is equally applicable. The instructions on re-fitting the brake plate and shoe assembly also apply with the exception that the brake plate has two operating cams.

Re-Setting the Brake Shoes

The brake shoes are set when the brake is assembled at the factory, but renewal of the brake shoes will necessitate re-setting.

To do this, remove the split pin and withdraw the clevis in from the oke end at the top of the adjuster rod. Operate the front brake handlebar liever which will bring one brake shoe in firm contact with the drum and retain the pressure on the lever by securing it to the handlebars by the use of a strong rubber band or other means. Slacken off the two lock nuts on the adjuster rod and apply firm pressure to the top cam operating lever which will bring the second brake shoe in contact with the drum. Hold the cam lever in this position and turn the adjuster rod to a position where the clevis pin can be inserted through the cam lever and yoke end of the adjuster rod. Then re-fit the clevis split pin, tighten up the lock nuts on the adjuster rod and release the pressure on the handlebar lever. Finally, check the brake control cable and carry out" anv adiustment which may be necessary. 


The telescopic front forks are Scrambler type with two-way damping and fitted with rubber gaiters. The instructions given in the Venom Manual for the Scrambler fork will apply to the Thruxton but it should be noted the 3/8" dowel screw is no longer fitted and that adjustable clip-on handlebars are positioned on top fork tubes.

Speedometer and Rev. Counter

These instruments are mounted independently on brackets attached to the top fork yoke. The speedometer which is illuminated when any of the lights are switched on is on the left-hand side of the machine and the Rev. Counter, which is not illuminated, is on the right-hand side.

Engine Lubrication System

The lubrication system has been improved and made more efficient by the incorporation of a crankcase breather.


Removal of Fuel Tank

It will be noted the rear end of the fuel tank on the Thruxton is now mounted on a thick rubber pad positioned on the top frame tube, eliminating the use of a rear fixing strap.

Rocker Box Assembly

It will be noted the cups at the top end of the push rods are now fixed, being pressed into the rods.

Re-adjustment of Tappet Clearances

The running clearances should be Inlet .006”. Exhaust .008”. The adjustment procedure is identical to that described in the Venom Service Manual under the heading “Re-adjustment of Tappet Clearances" except that the clearance has in this instance to be checked between the cam and the bottom rocker. It is of course necessary to remove thetiming case cover to carry out the check. It is also important to see if the bottom rockers are removed for any purpose they are replaced correctly. They are not inter-changeable, the inlet rocker being marked “In” and the exhaust rocker marked "Ex".

To avoid discrepancies in the tappet settings when checking and readjusting (this also applies to the checking of the valve timing) we suggest that the engine is rotated until the bearing face of the exhaust cam follower is located on the base of the exhaust cam (see page 27, para 3, 4). Then carefully remove the timing gear steady plate by removing the four securing bolts and camwheel spindle. (The camwheel oil jet can be left in the plate). DO NOT ROTATE ENGINE UNLESS STEADY PLATE IS REFITTED. UNLESS INTENDING TO REMOVE THE TIMING GEARS, CARE SHOULD BE TAKEN NOT TO DISTURB THESE.

Check exhaust clearance between base of cam and follower. This should read .008-in. for the running clearance. Adjust if necessary. (For checking valve timing see page 100.) Replace timing gear steady plate, etc., and follow similar procedure for checking inlet clearance which should be .006-in. These readings, of course, are taken between the inlet cam and cam follower.


All Thruxton models are equipped with a Revolution Counter and IT IS MOST IMPORTANT THAT AT NO TIME MUST A MAXIMUM OF 6,200 R.P.M. BE EXCEEDED. Obviously greater reliability and a much longer engine life will be obtained if the r.p.m. is kept down as much as conveniently possible below the maximum stated.


It will be appreciated driving on the long and fast stretches of the new motorways demands a different driving technique. The natural tendency is to travel at a constant abnormally high speed and if this is done it is advisable to momentarily close the throttle at periods during the run.


This is an Amal Racing-type 5GP2 with remotely mounted float chamber and detailed maintenance instructions can be obtained upon application to the carburetter manufacturers : Messrs. Amal Ltd., Holford Drive, Witton, Birmingham 6.

Amal T5GP2 Carburetter with remotely mounted Float Chamber
FIG. 43 Amal T5GP2 Carburetter with remotely mounted Float Chamber

In the GP2 carburetter the pilot adjuster screw controls the volume of air ad the petrol is metered through a detachable pilot jet giving much more flexible tuning over the pilot range and at the same time this arrangement has been so designed that the carburetter can be used at an increased downdraught angle.

It is inadvisable to tamper with the carburetter but occasional adjustment of the control cables and cleaning out may be required.

For your guidance we are including in this supplement an illustrated and numbered sectional view of the carburetter and separate float chamber and give below some general details of settings and operation which we hope you will find helpful.

Amal T5GP2

 1 3/8" choke bore.

Main ]et


Needle ]et


Pilot jet


Needle Postion No.

5 (counting from the top)

Air ]et


Slide No.


Design Features

The GP2 carburetter has been designed with a view to obtaining the maximum possible power from the engine, at the same time maintaining a progressive and consistent acceleration throughout the throttle range.

Float Chamber

Remotely mounted with bottom feed and_ incorporating a lever-type operated float. It is mounted on rubber and is adjustable for position.

Petrol Level

After removing the toolbox, the petrol level in the float chamber can be checked by the use of a small length of clear plastic tubing. Turn off both petrol taps and remove the rubber pipe from the float chamber to the carburetter.Replace this on the float chamber with an 8” length of 3/8' bore clear plastic piping. Hold the piping upright on the nearside of the carburetter. Turn on one or both of the petrol taps when the petrol will flow into the float chamber and up the plastic piping. The level of the petrol in the piping should correspond with the lowest point of the circular scribe mark on the air jet plug (3) and to achieve this the float chamber can be raised or lowered by means of the adjusters provided.

When the correct level is obtained turn off the petrol taps, remove the plastic piping and re-fit the rubber pipe, being careful not to alter the position of the float chamber in the process.

Locking Devices

A spring blade locking device (18) held in place by the air tube lock ring (19) engages with serrations on the mixing chamber cap (16), which positively prevents the unscrewing of same due to vibration. The jet plug (20), banjo bolt (43), plug screw (42), jet block holding screws (13), float chamber cover screws (46) and the float/hinge spindle head (not illustrated) are drilled to enable them to be lockwired up.

Tuning, General

The tuning sequence of the GP2 carburetter follows the well-established Arnal principles, in as much as there is a main jet (15) controlling the fuel supply at full throttle, a needle jet (1), the emission from which is controlled by the position of a taper needle (11) in the same and at the lower throttle openings by the cut—away of the throttle valve (23), a detachable pilot jet (24) and a pilot air adjusting screw (27) controlling the mixture strength for idling; an air jet (2) controls the amount of air which primarily atomises the fuel as it comes out of the needle jet (1) before going into the spray tube (12) and thence to the heart of the choke.

The Needle

The needle control covers a range of the throttle opening from about one-third throttle up to seven-eights throttle opening. The needle grooves in the GP needle will be found to number five instead of seven as previously on the TT instruments, due to the fact that the needle control of the GP carburetter is rather more sensitive than on other types. Two types of needle (11) are available, what we call a standard taper needle and a much weaker taper needle. The standard taper is known as Type 5GP and the weaker taper needle is designated Type SGP6.


Always bear in mind, however, that whatever the type of needle used, or the position in which it is fitted, there will be no affectation of the main jet (15).

The main jet (15) can be very readily removed by taking off the hexagon cap (20) at the base of the Carburetter Mixing Chamber. The jet size is marked on the side of these jets and represents the flow in c.c. per minute on the Amal Calibrating Machines at the Works. These jets are made in 10 c.c. increments, that is, for instance—250,260, 270, etc.

The Throttle Valve (23) which surrounds the choke adaptor (22) in the carburetter, controls with its leading edge the velocity of air entering the throttle bore and consequently the depression on the spray tube at the lower throttle opening with a diminishing effect up to a point where the cut-away disappears from the cross bore. 

The trailing edge of the throttle valve, of course, controls the volume of mixture passing to the engine.

The Needle Jet (1), which is of stainless steel to prevent wear, has been found for the best all round usage on petrol or petrol benzole to require a diameter of .109’. For alcohol fuel, of course, larger needle jets are necessary.

Pilot System

This gives a supply of metered fuel through a detachable pilot jet (24) which mixes with air regulated by the pilot air adjusting screw (27) and passes into the mixing chamber through a small hole on the engine side of the throttle slide.

Compensation on this GP2 carburetter is obtained through the medium of the primary air which passes through a slot (4) in the mixing chamber and then, via the air jet (2) previously mentioned, atomises the liquid fuel passing from the needle jet (1).

As the engine supply increases or decreases at a given throttle opening with a varying load, so compensation will take place.


A Lucas lighting set is fitted to the Thruxton and a wiring diagram is included in this supplement. Other constructional details :

Headlamp and Parking Light

The headlamp incorporates the Lucas Light Unit, which consists of a combined reflector and front lens assembly A “prefocus" bulb is used, ensuring that when the bulb is ‘fitted, the filament is correctly positioned in relation to the reflector and no focussing is necessary. The parking light bulb holder is a push fit in the rear of the Light Unit reflector. The headlamp main bulb is 6-volt 30 x 24 watt, double filament bifocal and the parking light bulb 6-volt. 3-watt single filament MSS cap.


Set the headlamp so that when the motor cycle carries its normal load the main or driving beam is projected straight ahead and parallel with the road surface.

Removing Headlamp Front

Slacken the rim securing screw located on top of the headlamp shell. It will then be possible to detach the front rim complete with Light Unit assembly. To re-place, locate the Light Unit assembly in the lamp body, press the front on at the lower edge first, then at the top and secure in position by tightening the securing screw.

FIG. 45 Wiring Diagram for Lamps and Dynamo with A.V. Regulator

Colour Code































 Lucas Lighting Set Wiring Diagram

Lucas Lighting Set Wiring Diagram
Wiring Diagram for Coil Ignition
FIG. Wiring Diagram for Coil Ignition

Replacement of Bulbs

When replacement of a bulb is necessary, it is important not only that the same size bulb is fitted, but also that it has a high efficiency and will focus in the reflector. Cheap inferior re-placement bulbs often have the filament of such a shape that corrrect focussing is not possible for example, the filament may be to one side of the axis of the bulb, resulting in loss of range and light efficiency.

Lucas Bulbs are specially tested to check that the filament is in the correct position to give the best results. To assist in identification, Lucas bulbs are marked on the metal cap with a number. When fitting a re-placement, see that it has the same number as the original bulb. 

To gain access to the headlamp bulb remove the front rim and Light Unit assembly as previously described. Push on the adaptor and twist it in an anti-clockwise direction to take it off. The bulb can now be removed from the rear of the reflector. Place the correct replacement bulb in the holder, engage the projections on the inside of the adapter, press onand secure by twisting to the right.

To gain access to the parking light bulb, remove the front rim and Light Unit assembly and withdraw the bulb holder from the reflector in which it is a push-fit.

Rear Lamp

The rear lamp and stoplight are combined in one body and the bulb bulb used is a 6-volt 1S>< 6 watt off-set pin stoplight or 6-volt 6-watt S.C.C. for rear light only when stoplight is not fitted.

Miller Dynamo.

The instructions given in the Venom Service Manual for the dynamo and automatic voltage regulator apply equally to the Thruxton equipped with a Miller dynamo.


A Lucas Competition Magneto Type KIFC with manual control is fitted on the Thruxton and the general maintenance instructions given in the Venom Manual can be applied To clarify the previous instructions we give the following additional information.


The cam ring is supplied with lubricant from a felt strip contained in a recess in the contact breaker housing. Oil reaches the inner surface of the cam ring by way of a small circular wick passing through the thickness of the cam ring.

Remove the contact breaker cover. Take out the central hexagon-headed securing screw and carefully withdraw the contact breaker from  the tapered magneto spindle.

Withdraw the cam ring. It is a sliding fit in the contact breaker housing. 

Note.—Withdrawal and re-fitting of the cam ring will be made easier if the handlebar control lever is moved to the half-retard position, thus taking the cam ring from its stop peg.

Clean the cam and lightly smear the inside and outside surfaces with light grease. 

Add a few drops of thin machine oil to the felt strip and to the circular wick.

Remove the contact breaker lever and smear the pivot with light grease, applying sufficient grease to fill the annular groove. The method of removing the contact breaker will be apparent from Fig. 8. Since the push-on retaining ring may need renewal after removal, an alternative form of lubrication for this pivot post is to apply a spot of clean engine oil to the tip of the post.

While this will obviate the necessity of removing the lever great care must be taken to prevent oil getting on or near the contacts.

Re-fit the contact breaker lever.

Re-fit the cam ring, taking care that the stop peg in the contact breaker housing and the spring loaded plunger engage with their respective slots.

If an earthing brush is fitted at the back of the contact breaker base plate, see that it is clean and can move freely in its holder before re-fitting the contact breaker assembly in the cam ring.

Re-fit the contact breaker assembly, ensuring that the projecting key on the tapered portion of the contact breaker base plate engages correctly with the spindle keyway

Every 3,000 miles

Checking Contact Breaker Gap

To check the contact breaker gap, remove the contact breaker cover and turn the engine over slowly until the contacts are fully open. A flat steel guide of thickness 0.012" to 0.015" (0.3-0.38 mm.) should be a sliding fit between the contacts.

The contact breaker unit is shown in Figs. 8 and 9. It has a fixed contact plate secured by a single screw passing through a slotted hole in the base. To adjust the gap, slacken this screw and, using a screw-driver in the manner shown in Fig. 9, move the fixed contact plate until the correct gap is obtained. Tighten the screw and re-check the gap.

CB Adjustment and exploded diagram
FIG. 8 & 9 CB Adjustment and exploded diagram

Every 6,000 miles


Remove the contact breaker cover and high tension pick-up mouldings. Thoroughly clean the inside and outside of the magneto using a clean dry fluffiess cloth, if necessary moistening it with petrol to remove any grease from the high tension pick-up mouldings and contact breaker contacts. Ensure that the pick-up brush moves freely in its holder. Renew the brush if it is worn to 1/8” above the shoulder. Clean the slip ring track and flanges by pressing the cloth on them while the engine is cranked by hand.

Ensure that the gasket between the pick-up mouldings and the magneto body is in good condition before re-assembling.

Examine the contacts when the contact breaker is removed for lubrication. If the contacts are pitted or piled, they should be trimmed with a carborundum stone, silicon carbide. paper, or very fine emery cloth.

Contacts do not retain a polished appearance when in use and, if operating correctly, will have a dull grey appearance.

Every Two Years

About every two years, or when the engine is given general overhaul, the magneto should be examined at a Lucas Service Depot. 

Renewing High Tension Cables

When the high tension cable shows signs of perishing or cracking it must be renewed. To re-place the high tension cable on machines fitted with 7mm. p.v.c. or neoprene covered cable, proceed as follows :--

Remove the metal washer and moulded terminal nut from the defective cable. Thread the new cable through the moulded terminal nut and cut back the insulation for about 1/4". Pass the exposed strands through the metal washer and bend them back radially.

Screw the moulded terminal into the pick-up moulding.


Engine will not start or difficult to start

(a) See that the controls are correctly set for starting, petrol turned on, etc.

(b) Turn off the petrol tap. Remove the sparking plug and place on the cylinder head. If a spark occurs regularly at the plug points when the engine is slowly hand-cranked, the magneto is in order. Look for engine defects and check ignition timing.

(c) If a spark does not occur in (b), disconnect the high tension cable from the plug and hold the cable end about 1/8” from a metal part of the engine. If a spark occurs regularly when the engine is cranked, the plug is faulty. If there is no spark, disconnect the high tension cable at the magneto, re-place with a new length of cable and test again as before.

Coil Ignition

Check for a fault in the low tension wiring, i.e. from battery to switch, coil and contact breaker. If the wiring proves to be in order, examine the contact breaker; if necessary, clean the contacts and adjust the gap setting. Check capacitor by substitution. If, after carrying out these checks, the ignition system is still inoperative, have it examined by a Lucas Service Depot or Agent.

(d) Should there still be no spark, possible causes of trouble are : contact breaker gap out of adjustment or contacts dirty; contact breaker rocker arm sticking; or pick-up brush worn or broken, or slip ring track dirty. Remedy as described.

Engine Misfires

(a) Check as in para. (b) and (c) above, to eliminate engine defects, faulty high tension cables and sparking plug.

(b) Check magneto as in para. (d) above. 

(c) If the fault persists. have the magneto examined by a Lucas Service Depot or Agent.

Coil Ignition

(a) Examine the contact breaker; if necessary, clean the contacts and adjust the gap.

(b) Check capacitor by substitution.

(c) Remove the sparking plug, rest it on the cylinder head and observe if a spark occurs at the plug points when the engine is turned. Irregular sparking may be due to dirty plugs, which may be cleaned and adjusted, or to defective high tension cables. Any cable on which the insulation shows signs of deterioration or cracking should be renewed.

(d) If sparking is regular at the plug when tested as described in (la) the trouble is probably due to engine defects and the carburetter, petrol supply, etc., must be examined.

Charging Circuit

Battery in Low State of Charge

(a) This state will be shown by poor or no light from the lamps when the engine is stationary, with a varying light intensity when the motor cycle is running.

(b) Have the condition of the battery checked and recharge it if necessary,

(c) Check wiring from battery to switch, rectifier and dynamo, tightening any loose connections or re-placing broken cables.

(d) If the cause of the trouble is still not apparent, have the equipment examined by a Lucas Service Depot or Agent.

Excess Circuit Voltage

(a) This will be indicated by burnt-out or blackened bulbs and possibly poor engine performance due to burned ignition contacts.

(b) Examine all wiring for loose or broken connections.

(c) Check the earthing ofbattery and rectifier.

(d) Examine_the battery, checking electrolyte level and removing any traces of corrosion.

(e) If the ignition is affected, clean the contact breaker contacts, or, if necessary, renew them.

(f) If the fault persists, have the equipment examined by a Lucas Service Depot or Agent.

The Battery Positive (+ve) Terminal is Earthed to the Machine. Under no circumstances must the Negative (—ve) Terminal be earthed.

Lighting Circuits

Failure of lights (machine stationary)

(a) If only one bulb fails to light, replace with new bulb.

(b) If all lamps fail to light have the condition of the battery checked, re-charging it if necessary either by a long period of daytime running or by connecting it to a suitable battery charger.

(c) Examine the wiring for a broken or loose connection and remedy. 

Lamps light, when switched on, but gradually fade - Have the condition of the battery checked, re-charging if necessary.

Brilliance varies with speed of motor cycle - Have the condition of the battery checked, recharging if necessary.

Lights flicker - Examine the wiring for loose connections, or short circuits caused by faulty cable insulation. Have the condition of the battery checked.

Headlamp illumination insufficient

(a) If the bulb is discoloured or filaments have sagged as a result of long service a new bulb of the same type should be fitted.

(b) Check the setting of the lamp.


On a number of Thruxton models, a Lucas Battery——Model PUZ7E/11 - is fitted.


During charging, water is lost by gassing and evaporation and each week the electrolyte level of each battery cell should be checked and, if necessary, topped-up.

Remove the battery lid, unscrew the filler plugs and, if necessary, add distilled water carefully to each cell to bring the elctrolyte just level with the line on the container denoting maximum filling level, or, if there is no such line, level with the separator guide.


Occasionally wipe away all dirt and moisture trom the top of the battery and ensure that the terminals are clean and tight.

Never leave the battery in a discharged condition. If the motor cycle is to be out of use for a considerable period have the battery fully charged and each fortnight give it a short freshening charge to prevent any tendency for the plates to become permanently sulphated.

Battery Earth

The equipment is designed for use with positive (+ve) earth systems. If battery connections are reversed, the equipment will be damaged.


The coil ignition system comprises an MA6 ignition coil and a 6CA contact breaker fitted in the timing cover and driven by the exhaust camshaft. The ignition coil is mounted underneath the petrol tank.

Apart from cleaning the coil in between the terminals and checking the low tension and high tension connections, the coil will not require any other attention. The capacitor is no longer part of the contact breaker but is housed separately on the frame of the machine

The best method of approach to a faulty ignition system, is to first check the low tension circuit for continuity as shown below.

Checking Low Tension Circuit

(a) Connect DC voltmeter (black lead) to CB terminal of the contact breaker and (red) lead to earth.

(b) Ensure contact points are open.

(c) Switch on ignition, voltmeter should indicate battery volts.

(d) Ignition still on, close contact points, voltmeter reading should fall to zero.

Conclusions :

No reading for test (c) may indicate faulty ignition switch, open circuit primary winding, broken lead, short circuit to earth on CB lead or faulty capacitor. Low reading indicates high resistance in the primary circuit or across ignition switch contacts. 

A reading for test (d) indicates volt drop across the contact points (dirty points.)

Failure to locate a fault in the low tension circuit indicates that the capacitor high tension circuit or sparking plug is faulty and the procedure for testing the high tension must be followed. Before commencing any of the following tests, however, the contact breaker and sparking plug must be cleaned and adjusted to eliminate this possible source of fault.

Ignition Coils

The ignition coil consists of a primary and secondary windings wound concentrically about a laminated soft iron core, the secondary winding being next to the core. The primary winding usually consists of some 300 turns of enamel covered wire and the secondary some 17,000 turns of much finer wire - also enamel covered. Each layer is paper insulated from the next in both primary and secondary windings.

To test the ignition coil on the machine, first ensure that the low tension circuit is in order as described above then disconnect the high tension lead from the sparking plug. Turn the ignition switch to the ‘on’ position and crank the engine until the contacts are closed. Flick the contact breaker lever open a number of times whilst the high tension lead from the ignition coil is held about 3/16 " away from the cylinder head. If the ignition coil is in good condition a strong spark should be obtained. If no spark occurs this indicates the ignition coil to be faulty.

Before a fault can be attributed to an ignition coil it must be ascertained that the high tension cables are not cracked or showing signs of deterioration as this may often be the cause of mis-firing, etc. It should also be checked that the ignition points are actually making good electrical contact when closed and that the moving contact is insulated from earth (ground) when open. It is advisable to remove the ignition coil and test it by the method described below.


Connect the ignition coil into the circuit and set the adjustable gap to 8 mm. With the contact breaker running at 600 r.p.m. and the coil in good condition, not more than 5% missing should occur at the spark gap over a period of fifteen seconds. The primary winding can be checked for short-circuit coils by connecting an ohmeter across the low tension terminals. The reading obtained should be within the figures quoted below (at 20°C.).


Primary Resistance




1.8 ohms

2.4 ohms



The model 6CA contact breaker incorporates two new design features. One is the provision of an eccentric screw for adjustment of spark timing. This screw, when rotated, allows the timing to be set with great accuracy while at the same time the operation is simple to carry out.

The second is the provision of a similar eccentric screw for adjustment of the contact breaker gap. Again accurate but simple setting is possible which further increases the efficiency of the system.

Lucas 6CA Contact Breaker Set
FIG. Lucas 6CA Contact Breaker Set

The bearing surface of the shaft and action plate has been treated with a special dry lubricant. Liquid lubrication (oil, etc.) must not be applied at this point, or a glutinous paste will be formed resulting in the eventual seizure of the mechanism.

Periodically the weight pivot and the cam foot pivot should be lubricated with one drop of clean engine oil. Any surplus must be wiped away to avoid contamination. As access to the pivot points entails removal of the contact breaker plate, lubrication should be carried out when the contact breaker points are serviced, i.e. every 6,000 miles, 9.600 km.

The illustration shows a twin lever arrangement, but both single and twin lever types are of similar construction. In this case a single lever type is fitted and one contact breaker is deleted. The contact breaker has an improved lightweight heel and it is used with a conventional automatic advance unit. Some units will incorporate a slot in the drive taper for location onto the driving member.

The absence of capacitors will be noted. These are provided as a separate item mounted remote from the contact breaker. 


Contact breaker gap 0.014", to 0.016" (0.35 mm to 0.4 mm)

Contact breaker spring load 20-oz.F to 27-oz.F.

(measured at contacts)

Extractor thread 5/16 UNF.


(i) Unscrew and remove the nut securing the c.b. spring to the anchorpost

(ii) Lift off the spring and heel together with the insul bush and c.b. lead termination (A).

(m) Unscrew and remove the fixed contact (angle plate) securing screw (B) and lift off the fixed contact. -



When connecting the  and capacitor leads to the anchor post, ensure that the eyelet has its tag (A) inside the curve of the spring but not touching it, otherwise the lead may foul the moving contact.


1. After 500 miles (800 km)—check the contact point gap. Limits 0.014" to 0.016" (0.35 mm to 0.40 mm). Re-adjust as necessary.

2. Every 3,000 miles (4,800 km)—add two drops of clean, light engine oil to the rear end of the cam lubricating wick.

3. Every 6,000 miles (9,600 km)—check the contact point gap. Inspect condition of contact surfaces. If burned or blackened they should be cleaned with fine emery cloth or carborundum stone.

Automatic Advance Unit

The correct manner of assembly will be apparent on inspection. However, two points should be carefully noted.

(a) Each spring has a tapered loop at one end. This end should be attached to the cam pin.

(b) The cam has two weight location pins and the longest of these pins is designed to fit into the radiused range slot.


The bearing surface between the cam inner face and the sleeve of the action plate assembly is pre-lubricated at the factory and must not be oiled.


Part No. 20036 (positive earth)


The generator is a shunt-connected two-brush machine, arranged to work in conjunction with Lucas voltage regulator unit model RB 108. 

Routine Maintenance

The generator should be dismantled and serviced as required at times of major engine overhaul. Routine maintenance is not necessary although the brushgear and commutator should be inspected at two-yearly intervals.

Performance Data

Cutting-in Speed 1050-1200 rev/min @ 7 generator volts

Maximum Output 8.5. amp @ 1850-2000 rev/min @ 7. generator volts


Model RB 108 (6-volt) Serial No. 37221


All settings are accurately adjusted before control boxes leave the factory and must not be disturbed. Any subsequent attention that may be required after the period of guarantee has expired should only be carried out by a qualified automobile electrician. The control box is a sealed unit but the cover is pierced with two 1/2” dia. holes for permitting screwdriver access to the voltage regulator and cut-out relay adjusting screws. The holes are plugged with a pair of linked rubber blanks which can be withdrawn when making voltage measurements and adjustments.

The control box frame is at generator potential and so, also, are the adjusting screws: since these pass through tapped holes in the frame. It is therefore advisable before making an adjustment to select a small screwdriver having an adequately insulated blade and thus obviate short-circuiting of the generator in the event of the control box cover becoming earthed. If necessary, a piece of insulating tubing of suitable length and bore can be sleeved on to an otherwise uninsulated screwdriver blade.

Preliminary Checking of Charging Circuit

Before disturbing any electrical adjustments, examine as follows to ensure that the fault does not lie outside the control box 2:-

(i) Check the battery by substitution or with an hydrometer and a heavy discharge tester.

(ii) Check the generator by substitution, or by disconnecting the generator cables and linking large terminal to small terminal and connecting a 0-20 first-grade moving coil voltmeter between this link and earth, and then running the generator up to about 1000 r.p.m.. when a rising voltage should .be shown. If satisfactory, restore the generator connections.

(iii) Inspect the wiring of the charging circuit and carry out continuity tests.

(iv) Check earth connections, particularly of the control box.

(v) In the event of reported undercharging, ascertain that this is not due to low mileage.

Checking and Adjusting Voltage Regulator Electrical Setting

Checking and adjusting of the open-circuit voltage setting should be completed as rapidly as possible so as to avoid errors resulting from heating of the voltage regulator shunt coil.

(i) Disconnect the cable from control box terminal

Warning —Do not allow the end of the cable removed to contact any earthed parts of the machine.

(ii) Disengage the linked rubber blanks from the control box cover taking care not to mislay them.

(iii) Start the engine and drive the generator at about 3,000 r.p.m.

(iv) Using test prods, measure the voltage between the exposed head of one of the adjusting screws and a good earth. This should be between the following limits, according to the ambient temperature :—

Ambient Temperature

Open Circuit Generator Voltage

10°C. (50°f.)


20°C. (68°f.)


30%. (86°f.)


40°C. (104°f.)


An unsteady reading may be due to the voltage regulator contacts requiring cleaning, in which event, remove the cover and clean the contacts, preferably using silicon carbide paper, followed by methylated spirits (de-natured alcohol). If the reading is steady but occurs outside the appropriate limits, the voltage regulator must be readjusted. In this event, proceed as in (v) below, otherwise, stop the engine, restore the original connections and re-fit the rubber blanks. .

Note- When viewed from the domed embossed end of the cover with rubber blanks uppermost, the left-hand hole gives access to the voltage regulator adjusting screw and the right—hand hole to the cut-out relay adjusting screw.

(v) Clip one of the voltmeter leads (of appropriate polarity) to a good earthing point.

(vi) Using a test prod, contact the other voltmeter lead against the exposed n ‘ head of the cut-out relay adjusting screw.

(vii) Tum the voltage regulator adjusting screw (clockwise to raise the setting or anti-clockwise to lower it) until the correct open-circuit is obtained.

(viii) Check the setting by stopping the engine and then again raising the generator speed to 3,000 r.p.m.

(ix) Stop the engine, restore the original connections and re-fit the rubber blanks.


The Model 45SA Ignition switch incorporates a "barrel" type lock. This type_ of _lock uses an individual “Yale” type key and renders the ignition circuit inoperative when the switch is turned off and the key removed. It is advisable for the owner to note the number stamped on the key to ensure a correct replacement in the event of the key being lost.

Three Lucar connectors are incorporated in the switch and these should be checked from time to time to ensure good electrical contact. The switch body can be released from the tool box panel by removing the large nut retaining the switch in the panel and the switch pushed out. The battery leads should be removed before attempting to remove the switch to avoid a short circuit.

The lock is retained in the body of the switch by a spring loaded plunger. This can be depressed with a pointed instrument through a small hole in the side of the switch body and the lock assembly withdrawn after the lock and switch have been detached from the machine.


The 6H Horn is pre-set to give the best performance and, in general no further adjustment is necessary. If the horn becomes uncertain in its action, giving only a choking sound, or does not vibrate, it does not follow that the horn has broken down- the trouble may be due to a discharged battery , a loose connection, or short-circuit in the wiring of the horn. In particular, ascertain that the horn push bracket is in good electrical contact with the handlebars.

It is also possible that the performance of a horn may be upset by its mounting becoming loose.


The following adjustment will not alter the note of the horn. It will take up any wear of the moving parts, which, if not corrected, may result in roughness and loss of performance.

Operate the horn push and slowly turn the adjustment screw (located at the back of the horn body) until the horn just fails to sound. Release the horn push and turn the adjustment screw clockwise, one notch at a time, until the original performance of the horn is restored. This usually entails a clockwise motion of one quarter to three-quarters of a turn.

On no account must the centre core and locking nut be disturbed. If the original performance cannot be restored by adjustment, do not attempt to dismantle the horn, but return it to a Lucas Service Depot for examination.



The SS700P Headlamp is of the sealed beam unit type and access is gained to the bulb holder by withdrawing the rim and beam unit assembly. Slacken the screw at the top of the headlamp and prise off the rim and beam unit assembly.

The bulb can be removed by first pressing the cylindrical cap inwards and turning it anti-clockwise. The cap can then be withdrawn and the bulb is free to be removed.

When fitting a new bulb, note that it locates by means of a cutaway and projection arrangement. Also note that the cap can only be re-placed one way, the tabs being staggered to prevent incorrect re-assembly. Check the replacement bulb voltage and wattage specification and type before fitting. Focussing with this type of beam is unnecessary and there is no provision for such.

Beam Adjustments

The beam must in all cases be adjusted as specified by local lighting regulations. In the United Kingdom the Transport Lighting Regulations reads as follows :—

A lighting system must be arranged so that it can give a light which is incapable of dazzling any person standing on the same horizontal plane as the vehicle at a greater distance than twenty five feet from the lamp, whose eye level is not less than three feet six inches above that plane. 

The headlamp must therefore be set so that the main beam is directed straight ahead and parallel with the road when the motorcycle is fully loaded. To achieve this, place the machine on a level road pointing towards a wall at a distance of 25 feet away, with a rider and passenger on the machine, slacken the two pivot bolts at either side of the iieadlamp and tilt the headlamp until the beam is focussed at approximately two feet six inches from the base of the wall. Do not forget that the headlamp should be on “full beam" lighting during this operation.

Removing and Re-Fitting the Headlamp

Disconnect the leads from the battery terminals then slacken the light unit securing screw at the top of the headlamp. Prise the top of the light unit free.

Detach the pilot bulbholder from the light unit and disconnect the main bulbholder leads at the snap connector. Disconnect the four spade terminals from the lighting switch and the terminals from the ammeter. The lead for the warning light should be parted at the snap connector and then the harness complete can be withdrawn with the grommet from the back of the headlamp shell. Finally remove the pivot bolts to release the shell and collect the spacers.

Re-fitting is the reversal of the above instruction but reference should be made to the appropriate Wiring diagram on page 109 and 110. Finally, set the headlamp main beam as instructed previously

Do not tighten the headlamp pivot bolts over the torque setting of 10LB/FT (1.4 kgm) -- 5/16 Whit. S. Med. B.S.84.


Access to the bulbs in the Model 564 and 679 tail and stop lam unit is achieved by unscrewing ihe two slotted screws that secure the lens (see illustration). The bulb is of the double filament offset pin type and when a replacement is carried out, ensure that the bulb is fitted correctly.

Check that the two supply leads are connected correctly and check the earth (ground) lead to the bulb holder is in satisfactory condition.

When re-fitting the lens, do not overtighten the fixing screws or the lens may fracture as a result.

Lucas Model 564 Stop-Tail Lamp


A green warning lamp has been incorporated in the electrical system mounted in the headlamp shell.


Model 57SA Lighting Switch is fitted to all machines with the exception of the following engine numbers :-

VMT775C to VMT814C

VM6488C/89C, VM6491C/92C/93C


These machines are fitted with an 88SA type switch (for wiring diagram apply Works). For all other application refer to wiring diagram page 110.


Although every precaution is taken to eliminate all possible causes of trouble, failure may occasionally develop through lack of attention to the equipment, or damage to the wiring. The following information sets out the recommended procedure for a systematic examination to locate and remedy the causes of some of the more probable faults. The sources of many troubles are by no means obvious and in some cases a considerable amount of deduction from the symptoms is needed before the cause of the trouble is disclosed.

When checking the continuity of circuits, a flashlamp battery and bulb should be used. On no account must the end of a live cable be flicked to earth against the motor cycle frame. If a separate motor cycle battery is used, a low wattage test lamp must be included in the circuit. If, after carrying out the examination, the cause of the trouble is not found, the owner is advised to get in touch with the nearest Lucas Service Depot or Agent.

Engine will not start

(a) Check battery and associated wiring.

(la) Remove the H.T. cable from the sparking plug terminal and hold the cable end about Y’ away from some metal part of the engine while the latter is slowly turned over. If sparks jump the gap regularly the ignition equipment is functioning correctly. Check for engine defects or examine sparking plug.

(c) Check for a fault in the L.T. wiring, i.e. from battery to switch, coil and contact breaker; If the wiring proves to be in order, examine the contact breaker; if necessary clean the contacts and adjust the gap setting. Check capacitor by substitution.

(d) If, after carrying out these checks, the ignition system is still inoperative have it examined by a Lucas Service Depot or Agent.

Engine Misfires

(a) Examine the contact breaker; if necessary, clean the contacts and adjust the gap.

(b) Check capacitor by substitution.

(c) Remove the sparking plug, rest it on the cylinder head and observe if a spark occurs at the plug points when the engine is turned. Irregular sparking may be due to dirty plugs. which may be cleaned and adjusted. or to defective high tension cables. Any cable on which the insulation shows signs of deterioration or cracking should be renewed.

(d) If sparking is regular at the plug when tested as described in (b), the trouble is probably due to engine defects and the carburetter, petrol supply, etc., must be examined.


Failure of Lights (machine stationary)

(a) If only one bulb fails to light, replace with new bulb. 

(b) If all lamps fail to light have the condition of the battery checked, re-charging it if necessary either by a long period of daytime running or by connecting to a suitable battery charger.

(c) Examine the wiring for a broken or loose connection and remedy.