GENERAL INFORMATION The Lambretta Cento model was introduced in 1964, and as it's name implies, is a 100cc model. However it differs in two main respects from other models in the Lambretta range. The frame is a box structure consisting of pressed steel units welded together, and the cylinder of the engine-gear unit is almost upright (inclined forward 9 degrees) instead of horizontal as some other Lambretta models. The J 125 model was introduced in 1964 and differs from the Cento only in engine dimensions, compression ratio, carburetter jets and gear ratios. Modifications to the original design have been few. An improved crankshaft assembly was fitted to the Cento after the J125 had been introduced and there have been some minor alterations to the gear-box. The modified assemblies are now supplied when replacements are required, and there is no difference in fitting procedure. Recent models also have a slightly altered frame structure, with strengthening ribs under the footboards. The instructions given here apply to both models. All work on the machines is straightforward, and apart from the usual workshop tools and equipment a number of special Lambretta tools are required. These are listed below. The engine-gear unit must be removed from the frame for decarbonisation, or for attention to the exhaust system, or for a complete stripping fo the whole unit. The directions below assume that such a complete stripping is required. However, the flywheel magneto, and the transmission/clutch/gearbox, can all be worked on with the unit still in the frame - notes will be found in each section on any difference in procedure. Index of page contents (quick links) Decarbonisation Engine gear unit dismantling Crankshaft removal Clutch Removal Flywheel removal Removal of Gears Workshop tools ENGINE GEAR UNIT DISMANTLING Removing engine-gear unit from frame:Start by removing the side-panels, the forward location is by pegs in shouldered collars which may be loose in the frame, if so they should be removed. On the nearside, remove the earth lead from the flywheel cover, then remove the rubber cover from the junction box and take out the leads in the black sheath from the flywheel housing. If any of the terminal ends are stiff, do not pull on the cable(s), ease the terminal(s) out with fine nosed pliers. The leads and sockets are coloured for correct replacement. Finally unbolt the rear wheel and support the rear of the gearcase with the auxiliary stand. On the offside, first disconnect the rear brake cable at the pedal end, undo the pinch-bolt on the end of the inner cable, and pull the inner and outer casing free. Do not lose the ferrule in the frame forming the cable stop. Remove the rubber hose connecting air cleaner to carburettor intake. Detach the sparking plug lead, and disconnect the petrol pipe at the tap. Remove the choke by unscrewing the sleeve below the cable adjuster, and disconnect the throttle cable from it's arm on the carburettor. Secure the outer casing of the throttle wire, with tape or other means to prevent it, slipping down from the upper end. Disconnect the clutch cable at the gearbox end, pull it free from it's adjuster, and slide the trunnion back onto the cable and secure it finger tight. Now detach the gear control tie rod (flat strip) from the selector lever by removing the circlip and washer. On top of the gearbox is a bracket which forms the stops for the clutch and gear-control cables, and it carries also the pivoting lever to which the inner wires of the gear-control cables are attched. Remove the two bolts holding this bracket; one of them is the shouldered pivot bolt, it has two plain washers, one for each diameter. The complete assembly, consisting of bracket and pivoting lever and cables can now be lifted clear. (Note. If none of the cables is to be changed, this method of removal is better than removing the cables themselves and leaving the bracket on the gearbox). Remove the nut and washer holding the lower end of the rear suspension spring. Remove the bolt anchoring the lower end of the damper, and push the damper out of the way. Now place a block of wood of suitable height under the forward end of the silencer, to take the weight when the forward pivot bolt is removed. Knock up the tab washer securing the offside pivot bolt nut, undo the nut, and drive the bolt through using a soft drift. Once the bolt is out, remove the block, allowing the front end of the unit to drop. Remove the auxiliary stand at the rear, lver the rear spring free of it's mounting, and the unit is clear to be moved to the workbench. Remove the rear brake cable by undoing the stud which holds the cable to the adjusting screw; pull the cable free, watching for the lose ferrule which forms the stop. Back to Index WORKSHOP SERVICE TOOLS 70640/A Steering head spanner (flat) 70640 Steering head spanner 70641 Steering head "C" spanner 70642 Font fork spring compressor 70643 Clutch spring compressor 70644 Clutch bell holding tool 70645 Mag, side inner sleeve extractor 77184 Rear hub extractor 70648 Con rod holding tool 70650 Crankshaft bearing puller 70651 Rear hub bearing puller 70652 Main bearing punch 70653 Primary shaft bearing extractor 70654 Crankshaft drive sleeve removing and refitting tool 70659 TDC Dial gauge holding tool 70660 Engine stand 71151 Crackshaft protection bush 71376 Cable fitting tool 71377 Steering head bearing tool 58013 Flywheel holding tool 37058 Flywheel extractor Back to Index DECARBONISATION With the unit out of the frame, it is much easier to remove the carburettor. Slacken the pinch-bolt clamping the instrument on to the induction stub, the "wriggle" it free. Remove the sparking plug and the cowling cowl, which is held by three screws - the cowl makes a convenient receptacle for nuts bolts etc, failing a box or boxes. Now remove the exhaust system - two nuts secure the flange to the cylinder, the silencer is supported by a bracket to the front of the engine unit, and the tail-pipe has a bracket secured by one of the gearcase nuts, towards the back. Remove the cylinder head after removing the four nuts and washers which hold it, and lift off the gasket carefully. This is a soft alloy gasket and it may be used again if in good condition. Lift off the barrel, taking care to lift it squarely so as not to distort the four long studs, and then remove the gasket below. Remove the piston rings, and store them carefully, noting the sequence of removal, so that if they are used again, they go back into the grooves in which they were fitted. Now remove the gudgeon pin circlips, using circlip pliers, and press out the gudgeon pin, marking one end so that it can be replaced correctly. Lift out the caged needle rollers from the small end. Remove carbon from piston-crown, ring grooves, cylinder head and exhaust port. Clean everything thoroughly, especially the joint faces of the cylinder head, cyliner (top and bottom), crankcase and exhaust pipe flange. Take care during this part of the work not to scratch or damage any of the parts or joint faces. Back to Index

REMOVAL OF CLUTCH With unit in frame: If access to the clutch and/or gears only is required with the unit in the frame, proceed as follows - the procedure is necessary because the driving chain can only be removed together with both sprockets. First, drain the oil from the gear case. Remove the 12 nuts and washers holding the gear cover - one of them holds a clip for the rear brake cable. Remove also the stud holding the silencer to the forward bracket, which will allow the exhaust pipe bracket at the back of the cover to be levered free of it's stud. Move the clutch lever gently to free the cover and lift it clear, keeping a watch for the clutch pushrod (about 1 inch long) which may drop out as the cover is removed. Remove the chain tensioner plates, held by two screws with washers. Use tool 70643 to compress the clutch springs whilst the retaining circlip is removed. Lift off the clutch plates, keeping them together and in order; there are 3 plain plates, 3 friction plates, and the pressure plate. Take out the 5 springs. Use tool 70644 to hold the clutch bell from rotating, whilst the stud securing the engine-shaft shock absorber assembly is removed. The assembly comprises a spring collar, the spring, a sliding dog, the sprocket, and a splined sleeve. Remove the collar, the spring and the sliding dog. Still holding the clutch with tool 70644, knock back the tab washer and undo the clutch retaining nut (r.h. thread). The clutch bell and the body (which incorporates the sprocket) together with the chain and the engine sprocket may now be lifted clear. If access to the gears is required, proceed as under "removal of gears". Removal of clutch, with unit out of frame: The following procedure assumes that the crankshaft has already been removed as under "removal of crankshaft". Use tool 70643 to compress the clutch springs whilst the retaining circlip is removed. Lift off the clutch plates, keeping them together and in order; there are 3 plain plates, 3 friction plates, and the pressure plate. Lift out the 5 springs. Use tool 70644 to hold the clutch bell and body, knock back the tab washer, and undo the retaining nut (r.h. thread) Lift off the clutch bell, followed by the body, which will bring along with it the chain. The clutch runs on a needle roller bearing, which may be removed, and behind it are two washers, the larger nearest to the housing, the other smaller. Back to Index

REMOVAL OF FLYWHEEL Remove the five screws holding the flywheel cover (one secures an earth lead). Then remove the circlip which holds the conical dust cover onto the flywheel, and remove the dust cover, turning anti-clockwise to release it. Use tool 70648 to hold the crankshaft whilst the flywheel nut is removed - if the unit is still in the frame, use tool 58013 on the flywheel itself. The nut is 17mm and has a left hand thread. Now screw cap 71151 onto the shaft to protect the threads, whilst the flywheel is pulled off using tool 37058. The flywheel is located by a key which should be prised out of the keyway on the shaft. The leads which were disconnected from the junction block (see under removal of unit) are sheathed and pass through a flange comprising an upper plate, a grommet, an inner plate and a gasket. Undo the two screws holding this assembly and ease the parts off along the sheath. Bring the leads clear of the hole in the casting. Mark the stator plate and it's housing so that it may be replaced in the correct position. Now undo the three large Phillips screws, with washers, and remove the stator with leads attached. Back to Index REMOVAL OF CRANKSHAFT Behind the stator is a bearing housing secured by three nuts, which should be removed. Use two of the stator screws in the holes provided, screwing them in will push the housing out. There is a gasket behind it. Now drain the oil from the gear-case, if this has not already been done. Lay the unit on the flywheeel housing and remove the 12 nuts holding the cover, with their washers (one securing the exhaust post will already be free, and one at the forward end secures the silencer bracket). Note there is a clip for the rear brake cable under one of the studs. Move the clutch lever gently to free the cover and lift it clear, keeping a watch for the clutch pushrod (about one inch long) which may drop out as the cover is removed. Remove the chain tensioner plates, which are held by two screws with washers. Still holding the crankshaft with tool 70648, undo the bolt holding the engine-shaft shock absorber. This comprises a spring collar, spring, sliding dog, the sprocket and a spined sleeve with a dished oil thrower last of all. Remove tool 70648, put the connecting rod at the bottom of it's stroke, and the crankshaft may be removed from the flywheel - it may need to be eased out by tapping from the offside, using a soft drift. If the crankshaft bearings are to be renewed, proceed to dismantle as follows. Flywheel side: the bearing housing, already removed contains two oil seals, and the bearing and outer track. Lever out the inner oil seals and remove the ring of rollers. The outer track will remain in the housing - the latter should be heated to remove the track, behind which is a shim. The inner track is on the flywheel-side of the crankshaft. It may be withdrawn by using tool 70645, and a new one replaced with tool 70652. If there is a thin shim behind this track, it must be replaced with a new one - this is important, because the shim will be distorted, possibly broken during removal. Offside: the bearing is retained by a circular housing secured by three countersunk screws, which are punched over - a good fitting screw driver is necessary for removal of these screws. Within the housing is an oil seal, and a Hallite washer. Lever out the seal and washer, and replace with new parts. Drive out the bearings from the crankcase - it is unlikely the housing will need heating for this. Back to Index REMOVAL OF GEARS The rear hub must be removed first. Knock back the tab washer, undo the nut and use tool 77184 to withdraw the hub. Note there is a split cone which may stick in the hub or be left on the shaft. On the offside, the gearbox endplate is secured by four nuts and washers, and will come free by gently levering when the nuts are removed. With the plate is a ball bearing and a needle roller bearing and behind these are two washers, a small one surrounded by a large one. The latter sits in a recess in the face of the first gear wheel and controls endfloat on the layshaft. This washer can be supplied in various thicknesses, to give the required clearance when assembling. Remove all the gears together, if necessary they can be tied together to keep them in place. On the mainshaft, there is a shim between the large gearwheel and the roller bearing. The latter may come away with the gears, leaving the outer track in the casing; it can be withdrawn if necessary for renewal, and a further shim will be found behind it. Tap the layshaft out from the brake side; on the smaller diameter is a rubber seal which abuts against the bearing, a dished washer and a spacer. On the larger splined diameter of the layshaft is a cursor sleeve with grooved ring, in which a loose pad, on the end of the selector arm, seats. Watch for this loose pad as the layshaft comes out. With the layshaft out, the cursor sleeve can be slid off the shaft - if this si done, two balls and a spring will be released from their housing, in the shaft. The layshaft bearing is held in the gear-casing by a plate retained by four nuts; a thinner plate acts as tab washers for the nuts. An oil seal is fitted in the bearing, and may be renewed as a seperate part. With the hub off, check that the brake cam spindle is free, as it gets very dry. If stiff, it may be removed by first taking off the brake shoes, which are held on the pivot by a circlip and a washer. Then remove the circlip which holds the cam lever on to the splines, mark the cam lever so that it can be replaced in the same relative position, and knock the cam spindle out gently, if it is stiff. Clean thoroughly before smearing with grease and re-assembling. Whilst the gear cover is off, examine the kickstarter mechanism. Uncertain operation arises from damage to the ratchet teeth on the faces of the piston and first gear. There are two possible causes for this damage - the first, and most common, is that the kickstart pedal is allowed to work loose on its splines, because the clamping bolt is not tight, and this allows the whole ks. shaft to move inwards, so that the teeth foul each other. The second cause is burring or distortion of the pin which limits the trvel of the ratchet piston. New parts are usually required to rectify this damage, though sometimes the teeth on the ratchet piston can be cleaned up by stoning, to work satisfactorily. The kickstarter mechanism is easily dismantled. Grip the hub of the pedal in a vice, between soft clamps, and with the case uppermost. Swing the case round against the spring, and hold in this position whilst the two studs securing the cam plate are removed, and the cam plate is lifted away. Let the case swing back under the action of the spring, remove form the vice, undo the pedal clamping stud and remove the pedal. The shaft may now be removed from inside the case, and the ratchet piston will come out of its housing after the pin has been unscrewed - take care that the piston does not fly out under pressure from its spring. Re-assembling with the required new parts is straightforward, the reverse of the above dismantling procedure. Back to Index