Engine Tuning

Applicable to Li Series and TV175 Series 2, GT 200 - SX200 including Pacemaker. Whilst the extent of tuning is, of course, considerable, results do depend entirely on the skill, knowledge and thoroughness of the person carrying out such work. However, it is possible to obtain extra efficiency from your Lambretta and for the guidance of the Lambretta owner who feels that he has a reasonable amount of know-how and facilities available, the following information is given:

Tools

Equipment that will be required apart from normal hand tools are an electric drill, a number of rotary files, numerous hand files, a set of small swiss files, and a length of 1/4 inch diameter wood dowelling which can provide polishing spindles of varying lengths; the ends of these spindles can be slotted to take strips of emery cloth, and when used with an electric drill the polishing of components can be undertaken, and this does, of course, reduce hand work quite considerably. A few sheets of various grades of emery cloth are also required.**SJ's note - I found that wooden dowelling tends to break too easily, so I used an old cylinder stud with the threads cut off, slotted at one end. An Ld cylinder stud is about 1/4 inch diameter**
Cleanliness is an all important factor when the assembly of an engine is undertaken, and all components should be thoroughly washed in petrol and, if possible, dried off with the use of compressed air which removes any particles of metal dust that may still be lodged in some obscure corner. If such particles were allowed to come into contact with the internal working parts of the engine, irreparable damage could result.

Inlet Port

The basic requirement for a two-cycle engine to operate efficiently is for it to be able to breathe adequately and this does, of course, mean that the various ports should in all respects be polished to allow the gases to pass without undue obstruction. We will at this stage refer to the inlet port which should  be increased in circumferal measurement by removing 1/8 inch from the off side wall of the port, followed by blending and polishing. **Note that in the UK, we drive/ride on the left, so the 'off side wall' of the inlet port is the wall furthest from the carburettor. Note also that no change is made to the actual inlet timing.**

Transfer Ports

The prime reason for transfer ports is for the transferring of the compressed volume of gases in the crankcase area into the cylinder. Whilst the angle of the rear wall of the transfer port (this wall being the one nearest the inlet port) does of course play an important role in directing the gases to the correct position in the cylinder, due to the high degree of accuracy that must be maintained as regards the entry angle of the gases, and also the equipment that is required to carry out an alteration to the dimension of the port, we do not recommend any attention to the transfer ports other than the usual polishing and blending. In order to increase the speed at which the gases are transferred from the crankcase into the cylinder, the entrance to the transfer ports can be enlarged. By covering the face of the crankcase with engineers blue at the point where it mates with the cylinder base and then repositioning the cylinder an outline of the crankcase aperture will be indicated on the base face of the cylinder. **Note - I don't know if the term 'engineers blue' is used universally - it refers to a grease-like substance, which contains a dark blue marker dye.** Metal should then removed accordingly until the entrance of the transfer ports align correctly with the aperture in the crankcase, this will of course correct any misalignment or reove any obstruction to the flow of the primary gases as they pass from the crankcase into the transfer ports. The internal wall of the port, i.e. from the entrance of the port to the cutaway in the cylinder spigot, can be progressively feathered off to a knife edge.**Note - A 'knife edge' may not be the best shape here - at this point the gases are being forced round a corner, as defined by the shape of the porting in the crankcase neck - it may be better to just put a radius on this internal wall, depending on the engine bore.**

The Exhaust Port

The dimensions of the exhaust port are already quite adequate to cope with the flow of exhaust gases and therefore the only attentions that are required are the removal of any casting roughness that may be apparent and general polishing of the port area. The exhaust gasket can be dispensed with, and a gasket made out of 1/16 inch copper sheet should be fabricated, as it can be made to correspond with the exact dimensions of the port, and will, of course, give unobstructed passage of the outgoing exhaust gases. The mouth of the exhaust pipe should be treated on similar lines to the exhaust port and any obstructions such as welding deposits must be removed. When work to the various ports has been completed, any burrs that may have been caused on the edges of the ports and protrude into the cylider bore, should be removed with the use of a fine file and a carborundum stick.

Piston

With the Li and T.V. models the windows or apertures in the sides of the piston, through which the compressed primary gases pass from the crankcase into the transfer ports, should be enlarged to conform with the cutaways in the cylider spigot. The amount of material to be removed is determined by placing the piston in the cylinder at the B.D.C. position (remember ring pegs to be towards the exhaust port), and the contour of the cutaway marked on the obstructing part of the piston with  pencil; material should then be removed accordingly.

Cylinder Head

Compression is, of course, most important, and although satisfactory results can be obtained by the removal of the head gasket, it is also permissible to remove 0.025 inch of material from the face of the cylinder head. If the gasket is removed and the head machined, it is essential to check the clearance between the cylinder head and the extreme edge of the piston, i.e. not the crown, when the piston is positioned at top dead centre, thereby alowing for a certain amount of stretching of the connecting rod, at very high r.p.m. The clearance can be determined by inserting a small quantity of plasticine between the cylinder head and the piston in the area already indicated, and if the head is tightened down onto the cylinder, and then removed, it will be possible to measure the compressed plasticine and so determine the clearance between the head and the piston (the extreme edge of the piston.) **Note - I don't know if 'plasticine' means anything outside the UK - it's a plasticised modelling material.**  The clearance between the two components indicated should not be less than 0.040 inch, and gaskets  will have to be added if necessary, to give this required amount of clearance. When material has been removed from the face of the cylinder head it will be necessary to re-face the head with the use of a surface plate and a sheet of fine wet and dry emery paper in order that a gas tight joint is made when the head is finally fitted. A fine covering of liquid jointing compound can be applied to ensure a complete seal. **Note - If the head is to be machined, it is probably better to remove the total amount required from the head, and to retain the standard gasket - it will seal better in any case.**

Crankshaft

Added primary compression can be obtained by removing the crankshaft and filling the balancing holes in the webs of the crankshaft with a light material such as aluminium magnesium or cork. As it is easier from the owner's point of view to use cork (a bottle cork is quite suitable), a piece of cork slightly larger in diameter than the holes in the webs of the crankshaft should be obtained. The holes should then be cleaned thoroughly and coated with a layer of Araldite, **Two-part epoxy resin adhesive** , the cork can then be forced into the holes, and allowed to set. The surplus amount of cork that will of course protrude on either side of the web can then be removed. The small holes in the end of the crankpin can be filled with Araldite, and on reassembly the gasket which is normally positioned between the magneto side bearing and the crankcase can be dispensed with, as the exclusion of the gasket allows the housing to enter further into the crankcase and does of course, assist towards increasing the overall primary compression. A liquid jointing compound can be used, and a gas tight joint will still be maintained. **Note - The use of cork is only a temporary measure, and my experience of epoxy resin adhesive is that it eventually swells after being exposed to the gas/oil mixture in the crankcase. Although cork has been used in racing engines, it usually breaks up after a period of time in road machines. Filling the ends of the crankpin has been claimed to be a factor leading to overheating of the big-end bearing. In any case, the increase in primary compression would be negligible.**

Crankcase

Upon removal of the crankshaft access will be gained to the crankcase area, the area in which the primary compression gases are compressed by the downward stroke of the piston before they are forced up the transfer ports. As previously stated, when giving attention to the transfer ports gases will pass more quickly over a polished surface and consequently the entire crankcase area should be highly polished and any casting protrusion which obstructs the flow of gases into the transfer ports should either be removed or radiused so as not to create restrictive turbulence.

Ignition Timing

Various tests have been carried out which have indicated that when engine revolutions exceed a certain figure there is a tendency for the ignition timing to advance; therefore if as is normal practice when carrying out tuning modifications to an engine the ignition is set to the maximum advance position, the ignition becomes over-advanced and a loss of engine revolutions occurs. **This sounds to me as if the heel of the contact breaker is not properly following the cam profile at high engine speeds, and it needs a stronger return spring!** The correct setting for the ignition timing after the modifications as described in this article have been carried out, is for the contact breaker points to break 2.5mm B.T.D.C., this measurement to be made with the use of the dial gauge positioned across the top of the cylinder (Service Tool Nos. 48060-57988). Alternatively, if such equipment is not available a "degree wheel" can be used and with a pointer which can be attached to a position on the engine, the contact breaker points can be set to break 22 degrees B.T.D.C.  The means of determining the exact moment of breakage is with the use of an electrical test meter, or a simplified method which is quite satisfactory is by inserting a bulb in circuit with the low tension feed wire, i.e. the green wire coming from the magneto, this should be connected to a 6V 25W bulb; a wire should then be taken from the earth connection of the bulb to the positive terminal of a 6V battery, by running a wire from the negative terminal of the battery to a point on the engine, thus a circuit is completed and the bulb will be illuminated. It therefore follows that with the contact points closed the bulb will remain bright, but directly the points break the bulb will dim. This enables the points to be set accurately. Before carrying out the timing procedure indicated the stator assembly which carries the lighting and L.T. ignition coils should be secured with the securing bolts positioned in the centre of the elongated adjustment slots.

Carburettor

Whilst larger-bore carburettors are available and will give good results, this article has been written with overhaul costs in mind, and in view of the many problems which can become apparent with the fitment of increased diameter carburettors we recommend that, unless the machine is being used for out-and-out sprinting, the original carburettor be retained, and if larger-size main jets are fitted, then satisfactory results will be obtained. The actual size of the main jet can only be determined by the trial-and-error method, starting with a jet size 10 above the original (i.e. STD 100 + 10 = 110). On Slimstyle models it is advantageous to remove the horizontal central cast bridge from the air scoop under the driver's seat, thus allowing a greater volume of air to enter. The procedure which should then be adopted for determining the correct main jet is as given above.

Gears and Clutch

The choice of gear ratios does depend on the application to which the machine is to be put. As there are five sets of gears, all with different ratios that are obtainable we feel sure that you will be able to make a choice that will suit your own particular requirements. We would point out, however, that although complete sets of gears can be fitted, it is not possible for individual gears from different sets to be interchanged, and if attempted, damage will result. Due to the increased power that will be available as a result of the attention the engine has received, clutch springs, Part Nos. 19620015 must be fitted to eliminate the possibility of clutch slip, which not only causes premature wear of the driven plates, but also allows power to be lost throughout the entire speed range. **If I remember correctly, these are the clutch springs originally fitted to the TV200/GT200.**

Sparking Plug

A sparking plug such as a Champion N.4 or an equivalent with a heat range of 235 (Bosch Scale) is quite suitable.

Petrol Oil Mixture

The lubricant must be a mineral oil of the type specified. Whilst every effort has been made in this article to improve the performance of your machine, Lambretta Concessionaires cannot be held responsible for any failure which may occur in altering a machine from standard specification. Any tuning that is carried out to a Lambretta scooter is done entirely at the owner's risk.