In order to keep the valves within their tolerances as long as possible, always use the correct grade of engine oil.
Over-revving your bike too much will also accelerate the wear due to the valve springs resonating. At very high frequencies, they can start to ‘bounce’, which causes them to open the valves without the cam’s timed assistance.
Using the wrong fuel can also have an impact – for most road riders, typical premium unleaded is fine, but some people believe that the higher the octane, the more power their engine will make. This becomes an issue when octane booster additives are put into the tank along with very high-octane fuels (often used in racing or at track days). This combination can run extremely hot, pushing an engine beyond its capabilities.
When valve clearances do need adjusting, they’re almost always too tight – this is because the valves start to bury themselves into the seats in the head, so the valve’s stem rises, getting closer to the cam. If they go too far, the bike will become harder to start, will backfire on the over-run, and lose power.
Over-tight valve clearances will lead to increased engine wear and damage; the cams will suffer more wear as the followers are in contact with them for longer, the valve seats will become damaged, and the potentially increased running temperature could cause problems in other parts of the engine.
Some wear is normal on the valve – the light polishing seen on a healthy set will likely have happened in the first few minutes of the bike being run, often when it was spun up on the rolling road as it came off the production line.
Checking the valve clearances on your bike is very important, and in most cases is simply a case of setting the engine to a specific crank position, then using a feeler gauge to check the gap between the cam lobe when it’s off its tallest point, and the follower, which is what pushes the valve open; measurements are usually started at TDC (Top Dead Centre), which is where the piston in a chosen cylinder is at its highest point of the compression stroke. It’s an easy check, but getting to the cams in the first place is a very, very big job in most cases, which is why it’s one of the most expensive service bills on a bike. In the case of many motorcycles, any bodywork has to be stripped before the tank comes off, then the airbox taken out, the radiator swung away and the cylinder head covers removed. This is the most labour intensive service and often the service most often ignored due to larger servicing costs, this is not optional and should be adhered to dependent on your motorcycle model. We often find most bikes are sold when these services are due, service schedules are bodged or assumed and the new owner is left feeling ripped off thinking they had a good deal. Please always check for FULL SERVICE HISTORY and obtain copies of the invoices so you can see what was done on each service, do not assume a fully stamped book means the correct servicing has been done, an annual service every year doesn't mean that the dreaded valve clearance check has been performed does it?
Often, valve clearances on modern quality motorcycles that have been ridden on the road are within tolerance at the first inspection – the manufacturer will give a minimum and maximum gap. By the second inspection they’re more likely to need adjusting – usually with varying thicknesses of small shims that sit under the follower in a ‘bucket’. To do this, the mechanic will note down all the clearances (these are recorded on the service sheet – along with every other maintenance record – from the measurable components), then remove the cams to get to the shims. Each one must be kept in order, so the technician can measure the shim’s thickness with a Vernier calliper, then work out what size shim he or she needs to replace it with to get the correct gap. New shims are measured before fitting into the relevant followers, the cams refitted and the gaps checked again.
Some bikes – a good example being the Honda C90 – use a screw-type adjuster in the tappets that follow the cam. This is a lot simpler, and certainly on a machine like this – which has very easy access to the engine – the cost impact should be minimal.
Whatever system is employed for setting the valve clearances, the measurements have to be taken with the engine absolutely stone cold. Many customers bring their bikes to the workshop on a trailer, as otherwise the mechanic would have had to have waited a very long time to make sure the motor had no heat in it and the customer for that matter!
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