As turned out from the factory these give really high performance after careful running in, but it is to assist riders who want to do a little better that these instructions are issued.

There is no short cut to obtaining superlative performance. It is invariably the result of a lot of painstaking work and careful attention to detail with little account of the time taken.

For the standard Viper and Venom models racing carburetters, and racing hand controlled magnetos are obtainable and can be supplied fitted before delivery or bought and fitted later. A Tachometer (Revolution Meter) is also available, but it should be noted that it can not be driven off the automatic timing unit, and a hand controlled magneto and special driving gear are necessary. The full list can be obtained from the Service Department on request.

It is however possible to improve performance or to restore the power of a machine that is “ off colour " by careful work alone. This will be dealt with first.

Bear in mind that work expended only upon the engine will be wasted if power is lost in the transmission to the rear wheel. It is essential to start by checking over the entire machine, paying special attention to the elimination of unnecessary frictional losses.

Check both wheels for freedom, including verification that the brake shoes clear the drums when the pedal and lever are released. Test by applying and releasing several times in case there is sluggish working of cam spindles, etc.

Driving chains must be in good condition properly lubricated, correctly adjusted and in alignment. To prevent any deflection of the primary chain line under very heavy loading, the Venom, Clubman and Scrambler models have the driving side rear engine plate reinforced. This plate can be supplied for fitting to Vipers or earlier models that did not have it.

The gearbox will absorb some power, but the loss can be reduced by the use of the correct grade of oil and by not over-filling.

Tyre pressures must be as recommended by the makers, otherwise power will be lost, not to mention the detrimental effect on steering, road holding and the tyres themselves.

The clutch adjustment is particularly important. The slightest amount of slip when running on full throttle will reduce the maximum speed several miles per hour. The slip may be so slight as to not be noticeable in normal conditions. Always allow sufficient freedom in the thrust bearing for the full spring pressure to be exerted on the friction linings.

Do this by carrying out the following “ Drill " in the. strict order given:-

(1) Fully slacken off the mid-way clutch cable adjuster, and slip the cable nipple out of the handlebar lever.

(2) Open throttle and air controls fully. With gears in neutral depress the kick start against compression. If the clutch slips omit operation (3) and carry on with operation (4).

(3) With the clutch adjusting tool engaged in the spring holder, pull the rear wheel backward a little at a time, checking for clutch slip after each movement, being sure to remove'the peg from engagement with the spring holder before checking. As soon as. any slip can be detected proceed with operation (4). Only the slightest perceptible slip is needed.

(4) Refit the cable nipple to the lever. Readjust the mid-way cable adjuster to take up all play in the cable. Take up play and no more. Do not force the adjuster. Tighten lock-nut. Refit the adjuster tool to engage the spring holder and pull the wheel forward a little at a time until the cable pulls freely for 1/8-in. to 7/16-in. from the casing without operating the clutch mechanism. (See page 29 for further details.)

Do not overfill the chain cover. For actual racing conditions the primary chain case drain plug can be left out, if a separate continuous drip feed to the inner side of the chain just ahead of the clutch sprocket is provided.

The best method is to provide a separate small auxiliary tank leading the oil by means of a double outlet to the ends of the rollers and controlling the flow by a needle valve or a carburetter jet of from 65 to 70 c.c. (6 to 8 drops per minute from each pipe).

Work on the engine must all be directed to the object of reducing frictional losses, starting with the flywheel assembly, check it for freedom in the main bearings when the crank case is hot. Correct the shimming if necessary noting that there is a slight preload (.004-in.) with the crankcase cold. If a new big end or new big end parts such as rollers are fitted, check the bearing with the crank pin fitted into one flywheel so that the cage is accessible. With the connecting rod assembly on the crank pin it should be possible to turn the connecting rod round whilst holding the cage and rollers stationary. There will be slight perceptible up and down play in the bearing when dry.

Check connecting rod alignment as although distortion is unlikely the slightest misalignment will cause heavy drag and loss of power. Piston Rings must bear evenly on the working faces, and also all round the lands. Factory replacements are correctly gapped. If necessary rings can be lapped into the grooves with metal polish. Be careful to clear it all away before final assembly. Clean out the metal polish with the rings in the grooves to avoid the risk of giving them a permanent ‘ set ' during removal or replacement. For prolonged racing with the Venom model it is advisable to use Clubman oil control ring, part number SL3/68. On inspection the piston crown after running should present a dry appearance, and no traces of burnt oil. Oil passing the rings or down the valve guides contaminates the fuel mixture and severely reduces power. 

There should be ample clearance between the flywheels and crankcase, and there is nothing to be gained by polishing the internal parts. The flywheels of engines previous to Viper No. VR.1144 and Venom No. VM.1114 were larger in diameter with less side clearance from the crankcase. Flywheels from such engines can be modified by us and re-balanced. Otherwise do not modify the flywheels or alter the balance in any way.

The inlet and exhaust ports are ground to a smooth finish after the valve seats have been fitted. Correct shape is more important than a high degree of surface finish. In dealing with the valve seats the inlet can be narrowed and the inner edge blended into the port without reducing the diameter of the base of the seat. This work must be carefully done. Do not reduce the width of the exhaust seating however. Nimonic ' 80 ' exhaust valves are now standard equipment on Viper and Venom engines, but can be obtained for earlier models. When grinding in the valves aim at a smooth matt finish on the seats. Do not attempt to polish them. 

Valve springs should be replaced if they have had considerable service. If new springs or any new valve parts such as valves, collars, etc., are fitted or if the valve seats have been trued up or new ones fitted, the installed length of the springs must be checked. The measurement between the top face of the valve spring bottom fixing collar and the underside of the loop of the spring where it rests beneath the top collar must not be greater than .562-in., and should lie between .542-in. and .562-in. Required correction can be made by fitting shims or washers between the bottom spring mounting and the cylinder head over the valve guides. A reduction in spring poundage either through weakened springs or incorrect installation will allow valve bounce to set in at engine speeds very much lower than the potential maximum, and will of course reduce power output considerably, apart from the risk of bent valves. 

Compression plates are fitted in production to get the required ratios for premium grade fuels of approximately 80/100 Octane rating. The standard compression space volumes are:

Viper 'Clubman '—42 c.c. to 43 c.c. Viper—47 to 48 c.c.

Venom 'Clubman ‘-64 to 65 c.c. Venom—68 to 69 c.c.

Compression plates are obtainable in 2 thicknesses, .010-in. and .031-in.

The differences in volume made are : 

.010-in. Plate on Viper =1.04 c.c.

.031-in. Plate on Viper =3.24 c.c.

.010-in. Plate on Venom =1.48 c.c.

.031-in. Plate on Venom =4.6 c.c.

The compression ratio can be raised slightly if desired, but if the bore is worn the small ridge must be removed from the top edge. For prolonged racing the standard split skirt piston can be replaced with the full skirt Clubman type which having greater clearance will reduce drag. Note, however, that there will be an increase in mechanical noise.

Valve timing is checked with clearances differing from those used for running-—See page 26. It is not possible to check timing accurately with both pushrods in place. Remove the inlet push rod whilst checking the exhaust timing and vice-versa. Greater accuracy of reading is obtainable by using .052-in. exhaust clearance, and .053-in. inlet clearance for checking, when the following diagram is obtained :

Inlet opens 45 degrees B.T.D.C.

Inlet closes 55 degrees A.B.D.C.

Exhaust opens 65 degrees B.B.D.C.

Exhaust closes 35 degrees A.T.D.C.

Do not forget to reset to running‘ clearances after checking. These can be increased to .006-in. inlet and .008-in. exhaust for prolonged speed work.

Check and set contact breaker point gap before timing the ignition. When checking see that all backlash is taken out of the gears. Correct setting is 38 degrees before T.D.C. fully advanced.

Although some owners carry out extensive work on reciprocating parts to reduce weight we are unable to recommend it unless the operator has had a very great deal of experience. It is all too easy to remove material from places where a change in shape or section will make all the difference between the continued reliability of the component and its fracture under load.

As silencers are usually obligatory for Clubman type events a special silencer and complementary exhaust pipe are obtainable for them. The pipe is bent, and silencer fixing modified to give more ground clearance. Apart from this they do not differ from the standard parts. They can not be fitted in conjunction with the standard footrests and pedals.

Do not in any circumstances alter the silencer by modifying the baffle or shroud tubes. The design is carefully worked out to give the best power output consistent with reasonable silencing. From experience we know that any alterations always reduce the power and speed---we have never known the contrary to result.

For races where open exhausts can be used the choice lies between a constant diameter pipe or a megaphone. The former is better for standing start events, and sprints, but the megaphone will give a better top end power output at the expense of the output at the lower end.

The length of pipe is important and the measurements quoted in the appended tables of carburetter settings are all as measured down the centre line of the pipe. Changes in the exhaust system make resetting of the carburetter essential.

Whilst settings are quoted these must be considered as a guide only. Final setting must be done to suit the course, the altitude, and the prevailing weather conditions. Always err on the slightly rich side, even at the expense of ultimate maximum speed, to avoid risk of overheating, particularly in lengthy races where heat ‘ builds up ' and may not become excessive until the major part of the distance has been covered.

Sparking plug recommendations appear on page 72.

PERIODICAL MAINTENANCE

The crankcase and gearbox fairing must be removed to top up the battery, or re-adjust the dynamo belt, clutch, or primary chain. It is fixed by large-headed screws. These can be turned by using a coin in the slots.

A new machine should be checked over after 500 miles use by the dealer from whom it was purchased. See preceding section dealing with the “First 500 mile Free Service Check," (Page 20 Owners Handbook).

Subsequent maintenance work is detailed below and the recommendations are based on an assumed weekly mileage of five hundred. It is, however, impossible to apply rigid limits to maintenance and the intervals quoted are not necessarily the minimum. In some conditions of use more frequent attention may be desirable or even essential.

As a general thing the rule should be a little and often. The effects of neglect are usually expensive in the long run.

The oils and greases suitable for the machine are specified on page 6. 

Every 500 miles. Check control cables for freedom and condition, and oil the exposed sections. Oil pivot bolts and nipples. Remove, free off, lubricate and replace any found to be tight or deteriorated.

Check levers and pivots for security of fixings and pivot bolts, etc.

Check level of oil in oil tank and gearbox and top up if needed.

Oil brake cam felt and brake rod trunnions.

Oil stand pivots and folding kickstart footpiece pivot.

Oil chains.

Check tyre pressures.

Every 1,000 miles. (Additional to above). Remove battery vent plugs check filling and top up with distilled water until acid level is 1/8-in. above separators.

For Batteries. See page 83.

Test battery terminals for security and clean and grease if needed, being sure to remove any corrosion that may have formed.

Check clutch adjustment and re~set if needed.

Grease trunnion shaft bearing nipples—one each side.

Check primary chain and re-adjust if needed.

Top up primary chain case if needed.

Examine rear chain. If dirty, remove, clean, re-lubricate and re-fit.

Every 2,000 miles. (Additional to above). Drain engine oil tank, re-fit drain plug. Do not disturb any oil pipes. Remove, clean and re-fit crankcase suction filter (page 21). Re-fill oil tank.

Check dynamo belt adjustment and re-adjust if needed. To adjust, slacken the dynamo clamp bolt (illustration page 22) and rotate the dynamo in the strap until the belt is reasonably tight. Do not overtighten and do not move the dynamo endwise and put the pulleys out of line. Tighten the clamp bolt when the adjustment has been completed.

Grease speedometer reduction gearbox on rear hub.

Grease rear brake pedal bearing. Oil on Clubman models.

Every 3,000 miles. Smear contact breaker cam ring inside and out with Mobilgrease No. 2. Apply a spot of clean engine oil to the tip of the pivot post. No oil must be allowed on or near the contacts.

Every 5,000 miles. (Additional to above). Drain gearbox, re-fit drain plug and re-fill.

Check, clean and re-adjust contact breaker.

Remove magneto high tension pick-up (see page 86). Inspect brush, clean brush, brush holder and slip ring. Re-place.

Remove, clean, and re-adjust sparking plug. Do not take apart.

Every 10,000 miles. (Additional to above). Remove fabric oil filter element. Fit new element. Do not attempt to clean the dirty element.

Drain and re-fill front fork struts. (This is optional).

Check condition of commutator and brushes. Clean commutator if blackened, re-place brushes if necessary. (This work is best entrusted to an Electrical Service Agent).

Remove speedometer flexible drive, pull out inner cable, coat with grease and re-fit.