I have been asked to cover the exhaust system this month, so here goes. The Rotax exhaust in itself is a very reliable item, nonetheless it must be well treated and have some regular maintenance. First of all it is important to check over the outside of the whole pipe to ensure there are no cracks starting to appear. This should not happen at all if the exhaust is properly mounted. The most common places for a crack are around the mounting brackets and from the small welds that attach the parallel silencer to the main expansion chamber body. It is now permitted within the regulations to weld up any cracks, however it is not permitted to modify the system in any way. If the crack was so serious that it required a repair that a scrutineer might question, it is probably not worth doing. As I said, it is very unusual to find any cracks in any case but it is worth checking first before you start overhauling the rest of the pipe. The best way to start is by giving the whole thing a really good clean.
Check the flange joint where the exhaust joins the engine manifold. The manifold is always perfectly round while the female end, a pressed steel element of the exhaust system, is not always as concentric as it could be and therefore can leak. It is often possible to gently reshape this with careful use of pliers or a ball pane hammer. Remember though it is illegal to modify the exhaust in any way. I usually prove the state of the sealing area with engineers’ blue. As long as you have a reasonable width of contact area, the seal will be fine. Next remove the three rivets that hold the silencer baffle in place. It is usually easy to remove the baffle tube but not so easy to extract the exhaust wadding itself. I use a long spring puller that goes right up to the end of the wadding and usually brings the whole lot out in one go. However you remove it, you must be sure to extract all the old wadding. The recommended service life of this wadding is ten hours, so it is easy to overlook this simple operation and suffer a loss in performance as a result. If the wadding starts to burn away then it will leave a witness mark as a ‘hot spot’ on the outside of the beginning of the parallel silencer tube. For this reason it is important to keep the exhaust clean and well painted. The hot spot will burn through the paint and be instantly recognisable. If this happens a new wadding and baffle tube must be fitted at once. It is much easier to replace the three retaining rivets with bolts. I use M4 x 10 button heads with a Nyloc nut. It will never let you down and is easy to undo and replace the next time. Another, often overlooked element of the exhaust is the mounting. As chassis manufacturers have latched onto the lucrative MAX market, they have gradually improved their supplied exhaust mounts. Everyone seems to have come up with a different solution but most are effective enough. However the problem begins with the attachment of the pipe to the chassis.
While a lot of chassis manufacturers supply their karts in MAX or 100cc specification, they very seldom supply the suitable rubber mounts to affix the exhaust system. These mounts need to be stiff enough to hold the exhaust firmly so that it does not come unseated at the joint to the manifold. However the exhaust must not be so solid that it becomes another torsion bar across the back of the kart. A good seal at the manifold is vital. If the seal is compromised while the engine is running it can cause loss of exhaust back pressure and badly affect the operation of the power valve. The type of silicone sealant must be the highest temperature variety available. It is equally important not to apply too much silicone, as it is compressed into the joint it can squeeze into the exhaust partially restricting it. New, or as good as new, springs are essential. They are very cheap and cannot work properly if weak or stretched. Using a proper spring puller will save your fingers and minimise the risk of a damaged spring due to overstretching. Finally a good coat of high temperature exhaust paint will spruce the pipe up nicely, with the very real benefit that you will see immediately if the baffle wool has started to burn away. Please remember that black is the only colour allowed within the regulations so do not be tempted to give it a tasty coat of red or yellow! I know a couple of people have been complaining about poor starting and occasional misfires over the winter. In both cases that I have seen the wiring loom and ignition coil looked as if they had spent the past few years in a farm yard. It is so important to keep all the electrical components in good, clean condition.
The multi-plug on the bottom of the coil has a water tight seal around it. This can become dislodged when disconnecting and as a result damp can enter the connector area. This causes havoc with the ignition and in the worst cases can cause the ignition to fail. All the earth wires are black and should be connected to the engine side of the coil. If this is done the coil cannot be damaged by a fault in the negative or earthed side of the circuit. The earth strap becomes almost superficial, in fact the engine will run perfectly well without it. The rubber cotton reels that insulate the coil from the engine are there to minimise vibration and must always be used. If not, the vibration will damage the coil in a very short time. By the same token the coil must not be able to touch any part of the chassis, the classic culprit is the right hand extra seat stay. If the body of the coil bashes against the stay for any length of time it will almost certainly be damaged.