Exhaust. It is important that the exhaust sytem be intact and correct for the engine. From the manifold to the outlet the system must be leak free but remain flexable. If parts of the exhaust were rigidly mounted to the chassis it would vibrate itself to pieces hence the use of springs and rubber mounts. All attachment points need to be inspected to ensure they are intack and serviceable. Pay attention to ball joints and flex tubes. Make sure that the muffler outlet is centered in the hole as not to rub or melt the belly pan. Ensure the muffler and/or expansion chamber has not become a home for mice. Any resistence to air flow or change in volume inside the exhaust will affect performance. Remove the exhaust and shake it. Nothing should rattle inside. Hold one end up and smack the side with your hand. Do you see acorns and seat foam falling out? Stick a garden hose in one end and give it an enema. After it flows clear switch the hose to the other end and repeat. It is amazing how much crap can be in a pipe. When you reinstall the parts make sure that nothing touches the system that can burn or melt. Wiring harness, tool bag, spare belt ect. Any heat sheilds that were originaly on the machine were there for a reason and should be replaced if they are missing. Any leaks repaired either by welding, muffler bandage or replacement. A leaking exhaust will make a filty mess under your hood.
Fuel and carbs. The biggest enemy here is dirt. Dirt can also mean rust. Vintage metal fuel tanks rust on the side. Ever wonder what happened to the fuel gauge on the cap when you take it out of the tank for the first time? It may be laying in the bottom of the tank along with a few decades worth of pickup lines and filters. A new inline fuel filter is a must with any sled project. Cost is $3.00 and you better by two right away as the first one can plug up fast. Complete replacement of all fuel lines is a must. And don't forget about the piece from the crankcase to the fuel pump. Do not use black gas line like a car engine uses. Mixed fuel has a nasty habit of disolving the rubber hose from the inside. Use quality line made for sleds. The 30 cent a foot vinyl stuff from the hardware store is not the way to go. Quality line will cost a buck a foot and is a bargin at that price. Carbs and fuel pumps need to be completely disasembled and thoughly cleaned, Soaking overnight is the way to go. There are tiny little passages that have to be open thoughout the pieces of a carb. Any moving parts need to move freely. Any rubber or plastic parts need to carefuly inspected. If in dought replacement is the way to go. Complete overhual kits are available and cheap at $25 for carbs and less than $10 for pumps. Check that the correct neddles and jets are in your carbs. It does not matter what brand or configuration of components you have they all work the same if they are in serviceable condition. A mylar gasket that is stiff or a seat that is worn will be a headache when you try to make it run later. Most carb kits come with instructions and most important, a diagram as to how it all goes back together. Some diaphram carbs can have over 50 parts and some are really tiny. A clean well lit work area and your wife's muffin pan will aid in a good rebuild.
Electrical. Wiring harnesses should be inspected from the headlight to the tailight and everything in between. Connectors pulled apart with the pins and sockets cleaned. Rodents love to chew on wires so look for abrasions or cuts that can short out to the chassis. Any connections to the framework, tunnel and engine should be removed and cleaned with a wire brush to ensure continuity. Any heat sinks on voltage regulators need a clean contact with the chassis to disipate heat and mainatin a good ground. Any crimp terminals should be cleaned. If you see anything in the wiring that looks like it is not original pay close attention to what was changed and why. Too many previous owners thought they knew more than the factory engineers that designed the snowmobile and made changes, many times for the worse. Keep in mind that the electrial system on a sled is actually two sperate systems. Ignition and lighting. The only thing they share in common is chassis ground and the flywheel.
The lighting system starts out as AC or alternating current at the startor. It remains AC unless the sled has electric start, in that case at some point DC or direct current is required to charge the battery, engage the selinoid and crank the starter. Some models actually used DC to run the lights. Some really old engines had such weak lighting coils that they did not use voltage regulators. Modern sleds with high output coils use a electronic voltage regultor but most vintage sleds use the same basic regulartor and new ones only cost $20. Very few sleds used fuses or curcuit brekers so a short curcuit can take out a coil requiring replacement. Some older models actually used two lighting coils such as early Yamahas where one coil was for the brake light only and the second coil was only used when the key switch was in the RUN/Lights position. Some sleds did not use a light switch, The lights are one whenever the engine is running. This makes for a simpler system and is a saftey feature. Pay close attention to the light bulb sockets. A shorted wire in the socket will dim out all the lights on the sled.
The ignition system can be points or CDI. CDI stands for Capacitive Discharge Ignition and is the more reliable system as the only moving part is the flywheel. But when it does fail it costs a lot more to fix. Components can include the CDI box, trigger coils , pulse coils, exciter coils, pickups or any combination of these. The remaining componets are shared with the points system and include external or primary coil(s), spark plug wires(high tension wires), plug cap and the spark plugs themselves. Spark plugs need to be of the proper heat range for the engine design and fuel used and the proper length. There are no universal spark plugs for engines. Some CDI systems use simotainious fire where both sides fire at the same time. They have only one external coil and fire twice in one revolution. With some CDi systems the timing is not adjustable mechanically. There are two types of systems out there: open and closed. This refers to the way it stops the engine. In an open system the contacts on the key/kill switches are open to run. On a closed system they have to make a conection in order to run. With the open system you can unhook the connector at the engine the engine will run.
The simpler points system consists of points, condensors and coils all mounted on the stator plate. The remaining external componets are shared with the CDI type ignitions. All points sytems are open type systems. When you stop the engine with the key or kill switch you are making a connection between the points and hence no spark. There are several stator designs out there and on some if you have a failure on one side it will affect the other. On the rest you can still have spark on one side. Point gap has to be checked carefully. Timing is adjustable by small movement on the stator plate in the slotted holes. All components of a points system can be checked with an OHM meter. All key/kill/saftey switches can also be checked. CDI Boxes can not be tested this way. Whatever type of system is on your sled a wiring diagram is a great help in troubleshooting. When testing you are looking for opens or shorts. A short is zero ohms. An open is infinate ohms. All coils have some resistance. It may be as low as 0.5 ohms or as high as 3000 ohms, but that is not the same as zero or open. Key switches can have as few as two connections or as many as seven. The most commen is five. Two are for ignition and are NO (normally open). Two are for lighting and are NC(normally closed). The fifth one is for the electric start and there may not be a wire on this terminal if the sleds was not built with ES.
So what did I miss?
Oh yeah, the engine.
