Tech Torque: Lambretta clutch issues
The Lambretta clutch is pretty simple in its design and if set up correctly, will run for many thousands of trouble-free miles.
The Lambretta clutch is pretty simple in its design and, if set up correctly, will run for many thousands of trouble-free miles
These days when it comes to the Lambretta clutch there are a whole host of options available to choose from, being one of the most developed parts of the engine in the last decade. This is fine if you are fitting an upgraded one into a tuned engine, but there are still many Lambrettas running around on traditional four-plate clutches. Making sure it is working perfectly is an important aspect of the engine's overall reliability. If regularly serviced, it will last a much longer. The problem with the clutch is that it cannot be seen unless the side casing is removed. Due to the fact that it has a hard life from the stress it's put under, problems can remain unnoticed – that is until a part becomes excessively worn or damaged and the clutch as a whole fails.
Warning signs
The clutch comprises of several parts, which all need to be in perfect condition to allow the unit as a whole to work correctly. Wear on one part will soon spread to another, setting off a chain reaction until the whole thing fails. Therefore, if you start to have problems such as slip, don't wait too long before inspecting it to stop unnecessary damage. It is issues like the clutch slipping under acceleration, drag as you select 1st gear, or a clunky gear change that suggest something is wrong.
It may be simply the case that the plates are old or the springs have lost tension over time. If this is the cause of the problem then simply change them, but often something else is contributing. Even if all the other components are fine and the plates have simply come to the end of their life, always change the springs as well. The cost is minimal and while the whole thing is apart they may as well be changed. Remember, if you have bought the Lambretta from someone else you have no way of telling how many miles the clutch components have done. Upon stripping down the clutch, inspect each item thoroughly and if wear is present, change the part.
Crown wheel
The crown wheel, or rear sprocket as it is known, also acts as the outer housing for the clutch. There are three key areas to look for wear. The first is the edge of the skirts that the outer of the cork plates sit in. These should be perfectly flat so the steel of the plate will slide up and down it with no obstruction, otherwise they can start to wear a groove and cut quite deeply into the surface. This will mean the plate can't move and will thus cause it to remain locked in the groove. Once proper separation of the cork plates and the steels become obstructed, friction can occur with heat building up. This can lead to the clutch burning out.
Secondly, the bearing surface where either the brass bush or double roller bearing sits. Check for wear and pitting on the surface, which is easy to see. More importantly, the bush or bearing need to be inserted with the clutch spider. Only then can you see how much play there is. The spider has a hardened surface, which the bearing runs on, so wear is always on the crown wheel surface. If the bearings or bush are new, but the play between the spider and crown wheel is excessive, this suggests the latter is at fault.
The third area depends on which crown wheel you are using. Later SX and all GP models used a one-piece unit. Earlier
models used a two-piece unit, which was riveted together. Wear can occur around the rivet holes, meaning the two pieces will have movement between them. If so, replace the two-piece crown wheel for a superior one-piece unit.
Clutch spider
The main problem here is also that of grooves cutting into the leading edges. This time though, it is from the steel plates, not the corks. A similar pattern of cutting into the surface of the leading edges can take place, stopping free movement of the plates. If it is only very slight burring then the edges can be cleaned up. If too excessive, then it will need replacing. Usually, if the crown wheel has this type of wear described then the clutch spider will be the same.
The other problem is again associated with rivets, with five connecting the spider to its boss. These can elongate around the holes. Once this happens it will become loose, even if the rivets are hit over again. Because the boss is what bolts and holds the clutch in a fixed position to the gear cluster, it must remain so at all times. Once the rivets are worn the whole clutch and crown wheel can move. Even if simply a slight movement around the rivets is detected, replace the spider immediately before the problem gets worse.
Bearings and bushes
There are two options here, either a pair of needle roller bearings or a brass bush. The brass bush was originally used on the GP, but roller bearings can be fitted instead. To check for wear, presuming the crown wheel and spider bearing surfaces are good, simply move the crown wheel from side to side to see if there is any excess play.
Getting hold of good-quality needle roller bearings is no problem as they are an industry-standard size. When it comes to the brass bush option though, that is a different story. Some Far East examples leave a lot to be desired when it comes to machine tolerances. Some are so bad, play can be excessive even when new. Thankfully, some manufacturers in the UK and Italy are now producing much better quality ones with a higher degree of accuracy. Don't be tempted to buy a cheap example. There has always been a debate about which is best to use – the needle roller bearing or the brass bush. Some saying one wears better than the other over a longer duration, and which has less friction. Either can be used, as long as they are of good quality.
Clutch plates and steels
The cork friction plates need to be in perfect condition to do their job properly, making sure there is no slip at any time. Though an over-adjusted clutch can cause this problem, a lot depends on the plates themselves. This problem arises because there are many poorly made ones out there. Always try to buy the best quality you can and don't go for the cheaper ones to try and save a few pounds.
One thing to be wary of is that the plates are the same thickness. Sometimes they do vary slightly and because the gap when the clutch is depressed is small, it can mean it's not big enough. If it isn't, then the plates can rub, which will cause overheating, so always check the plates before fitting. Also, make sure they are soaked in oil before doing so.
Wear can occur on the tangs where they sit in the crown wheel. Even if this is only just beginning to occur, change them regardless.
The steel plates don't look important, just circular pieces of metal. What is important though, is that they are perfectly flat. If slightly warped, they will possibly rub the cork plate when the clutch is depressed, causing heat build-up, which will eventually lead to the start of them burning out. A simple test is to hold all the steels together and hold up to the light along their surface. There should be no light present between them. Straightening them is not possible so they must be replaced. Also, the edges where they locate into the spider may be worn or
burred, preventing separation. Again, if so, change them.
The top or chamfered steel can vary in its thickness and just how much depends on the gap between the plates when the clutch is depressed. The top plate has to be thick enough to prevent the top clutch plate protruding, but thin enough to allow sufficient free play when depressed. With the clutch fitted and a compressor wound all the way, in there should be around 2-3mm free play between plates, and certainly no more (and no less the 1.5mm on 5/6-plate conversions). If the gap is too big then a thicker plate is required, and if the gap is too small then a thinner one needs to be put in place.
The problem is that because all the components that make up the clutch are widely manufactured and swapped around, there is no knowing what the actual plate gap will be until it's all fitted together. Because they are not expensive it is best to buy several varying in thickness. That way when it comes to fitting the clutch you have the correct one rather than the wrong one. If correct play cannot be found with adjustment of the top plate, varying thicknesses of inner steel can be used, but this is not really a problem for 4-plate clutches, but more an issue with 5/6-plate conversions.
Springs
Describing clutch springs is almost impossible as there are so many different ones available, all offering to do the job correctly. Over time old springs will lose their tension. Also, even if you have only slightly upped the power of your engine by fitting an expansion for instance, then the springs should be slightly stronger to cope with the power. If your clutch has done several thousand miles and you are changing the plates, then it is better to change the springs at the same time. If your engine is standard, there is no harm in buying slightly stronger ones. If it has been tuned, significantly raising the power, then the whole clutch needs upgrading to a higher spec.
Simply fitting stronger springs in a standard clutch will not be enough to cope with the extra power. See the Scootering magazine ‘Bang for Buck' series by Darrell Taylor for in-depth info of what bhp/ft-lb each clutch combination can cope with.
Always check the springs for their height and that they are all the same. Line them up on a flat surface and look with your eye. If they are all the same height then fine, but if one is slightly different to the rest, change it. If you don't, then as the clutch is depressed it may not be level, causing heat buildup. If a clutch has burnt out, change the springs, even if they look okay. Often weakened springs are the main culprit for clutch slip starting to occur.
Clutch bell
You may not think the clutch bell can have issues. The five legs that come down to the bottom thrust plate can get ‘stepped' on the edge close to the plate. This is from the bottom ‘free moving' steel and only happens when the spider and crown wheel have become badly stepped. The other problem area is with the thrust plate. One type uses a cap and was used up until the production of the SX series. The other is a flat plate, which comes in to contact with a brass bush and was used on all GP models.
The plate can become loose, more so on the SX style version because it is riveted on. If it is loose, like any of the other riveted clutch items it must be replaced. If the thrust cap needs replacing, make sure the fit isn't tight as it needs to be able to spin freely. Some aftermarket ones don't have any free play and once the clutch has been depressed a few times it can weld itself solid. This will then start to wear into the thrust arm on the side casing and suddenly you will notice the clutch lever go slack due to there being so much wear on the inner arm. Always check the cap spins freely if changing it.
Outside influences
Lack of service and abuse of the clutch will always cause wear. However, there are two outside factors which can cause problems, the most common being over-adjustment of the cable itself. There should always be some movement of the lever as the thrust arm moves across. Depression of the clutch shouldn't begin to happen until the lever is pulled in by around 2cm. Make sure you
adjust accordingly, especially if you have fitted a new inner cable.
More annoyingly, and harder to spot, is if the kick-start shaft is rubbing on the outer steel. This will occur between the chamfered edge of the steel and the kick-start shaft body that sits against the side casing, and in some cases can rub quite badly.
What it’s doing is compressing the clutch when you are turning the engine over with the kick-start pedal. This gives the same result as if you are holding the clutch lever halfway in and kicking it over, making the plates slip. Over time it will cause unnecessary wear, but it can be prevented by making sure that the shim that sits underneath the circlip holding the shaft in the casing is correct.
If it is too thin, this will allow it to move inwards when kicking over and rub in the clutch. The gap between the two is very small, which is why using the correct shim is vital.
Sometimes a replacement kick-start shaft can have a thicker casting around the highlighted area. If rubbing is occurring, you can grind the outer edge of the shaft to give slight clearance. It only needs 1mm of space to prevent this from happening.
If it still rubs, it is possible to have the side case machined where the shaft face sits, but this must only be done by the same amount of 1mm. Any more and the plunger teeth may not engage in the 4th gear. Remember, if the side case is machined then the shaft will need an even thicker shim to compensate and allow for enough clearance.