By: George Robinson
Since first writing this column almost five years ago it has sometimes been necessary to go over the same subjects again and indeed to update as the MAX has evolved. There have been reports of poor performance recently and, as usual, the engine gets the blame. In most cases the cause of an unexplained drop in performance is an external malady. The most common is the fuel supply system that starts at the petrol station and ends in combustion. The fundamental element of an engine running properly is the quality of the bang on combustion. I have been caught out by poor fuel on more than one occasion and have sometimes not realised the problem until sometime later. Always try to buy top brand petrol from a filling station with a high turnover. The fuel is likely to conform to the manufacturer’s blueprint and not be a mix of different brands.
Unfortunately, there is no way to be sure that the fuel you buy is good but it is well worth testing the petrol you intend to use for racing. I well remember when unleaded was introduced on the continent, it could seize the best 100cc engines for no apparent reason. Having bought your fuel and mixed it with your choice of best quality synthetic premix oil, it’s worth giving the fuel supply line a health check. Is the pick-up inside the tank in good condition? Because it’s out of sight it’s often out of mind. The tube should be long enough for the brass weight to sweep the bottom of the tank, the tube does shrink and harden with age. Rotax have introduced an in-line filter that is strongly recommended, we have always used the bronze sintered type filter that replaces the bob weight in the tank. Both filters are not necessary but one or the other has to be a good idea.
The Rotax filter has a paper element and will therefore lose efficiency with age and use and should be replaced every 20 hours. It’s always worth renewing the fuel line when it starts to discolour, it does deteriorate and can lead to air leaks at the joints, it’s also handy to be able to see what’s going on inside. If the fuel runs back from the pump towards the tank this is a sure sign that there is a problem in the pump. Neither rebuild kits nor indeed pumps are too expensive. The ideal situation is probably to have a spare pump ready to fit and then you can rebuild
the faulty one at your leisure. These pumps are more than capable of delivering enough fuel providing that they are in good condition. If their efficiency is impaired they lose a lot of performance and can give very strange symptoms. So often the pump is forgotten as a possible source of trouble. The fuel may well run back from the carburettor towards the pump, or appear to do so. What is actually happening is that the fuel is draining into the float chamber that is less than full with its needle valve open. If the float chamber is full then the tube to the pump will remain flooded.
This is why it is always a good idea to blow the fuel through before trying to start the engine. The engine will start more easily if there is plenty of fuel on demand, this will also eliminate the long periods of cranking required to pump fuel through on pulse at cranking speed. The carburettor is another area for attention. If the fuel filter has been used then dirt should not be a problem, however it is the first thing to check. Just below the brass fuel inlet tube there is a 12mm aluminium nut and behind this is a small plastic filter. It must be spotlessly clean and in good condition. If there is any sign of dirt here it is advisable to strip the carburettor down and give it a thorough clean, the problem being that this small filter does a good job of catching the grot and stops it from reaching the needle valve until you remove it from the housing.
Then some of the dirt is brushed off the filter and is left in the fuel way ready to be washed straight into the needle jet as soon as you blow the fuel through. If the internals of the carburettor are a bit too daunting then it’s a relatively inexpensive overhaul at the local service agent. Now that carbs have to be in completely standard form for MSA racing it may not be a bad idea to have your agent check it in any case just to be sure it conforms. Having worked through the fuel system and providing you are not a mile out on the jetting then you should be able to eliminate this area from your enquiries. The electrical system is also equally ignored as a source of trouble. To trace the electrical system start at the battery. The battery as supplied is very reliable and is even better since the introduction of the new foam pad and top-securing strap. The old plastic cover did a good job but was sometimes awkward to fit and prone to cracking if not treated with respect.
The wiring loom is very simple, the black wire is the negative or earth side and runs straight to the engine and should be secured to the engine not the coil. The second thin black wires provide a neutral to the ignition pick up and the coil, these should also be secured to the engine. The thick black wire is for high tension and is required for spark and starter motor. The thin wire is low tension, needed to complete the circuit to run the ignition pack and crank sensor. Dead simple really! The red wire from the battery is for live feed and only needs to be thick to run the First in the World for 44 years high tension starter motor, the rest of the ignition runs off the thin wire to the on/off switch.
This switch incorporates an automatic fuse, that’s why it may seem expensive for what it is. It is most important that this original switch is retained, the fuse is there to protect the rest of the ignition system, it is also illegal to substitute another part as it would not be as supplied by Rotax and thereby outside the MSA regulations. The loom must be kept in good condition and checked for damage and cleanliness on a regular basis. The coil or ignition pack to give it its correct title is usually very reliable, the most common cause for damage is the incorrect fitting of earth wires. If these black wires are fixed to the coil side of the rubber buffers and there is any problem with the short earth strap the coil will be destroyed. Fix them as previously stated to the engine side and you don’t even need an earth strap. The starter motor needs to be properly looked after as detailed in last month’s column but since the repair kit has been available there have been a lot less failures. Prevention is always much better than a cure.
On Senior engines the power valve must be regularly checked and cleaned. The amount of carbon build up will vary according to the oil that you use but serious competitors will strip and clean their power valves after each meeting. The symptoms of a sticky power valve are lacklustre performance and power that comes in with a bang. An inoperative valve completely kills top end and the engine will not rev, very easy to diagnose.
The exhaust should be trouble free providing it is not ignored, the wadding should be changed every ten hours and it is probably worth replacing the baffle tube at the same time. I prefer to install the new tube with bolts as opposed to rivets, only because it is difficult to get rivets as good as the originals and these repairs have been known to fail. The nut and bolt option can look very neat and is completely safe if done properly. The problem with the new style CIK bodywork goes on, the general guidelines were well covered in last month’s TKM News column but the problem for MAX is that the CIK have decreed that the pods may not be cut unless to accommodate a JICA type starter.
This leaves the MAX with a problem on some karts with some types of bodywork, It is best to check with the chassis supplier before buying the new type pods. In some cases it is certainly possible to mount the engine slightly closer to the seat or introduce spacers between the engine and mount in order to raise the engine just enough for the radiator to clear the pod. The Clay Pigeon warm-up race meeting at the end of February was a fantastic success, all three MAX classes were very well supported. Some good clean racing was had by all, the atmosphere was great and it was dry as well, a cool breeze the only thing to spoil a great weekend, I found a good spot out of the wind by standing behind Ian Rennison!