These aren't so much as instructions as they are commentary on the parts of a 2-stroke motorized bike; how they should be installed, how they can be replaced, and how they can be removed. Assuming you have read the usual build instructions that come from kits and the internet, this should be more helpful.
Keep in mind that I am not a mechanical expert. If you have any questions asking about how things work, I may not be able to answer you. You will probably have more success asking the good folks at motorbicycling.com.
Fuel TankThe stock tank that comes with the kits are actually pretty good. They are referred to as either the "teardrop" tank or "peanut" tank. When I say pretty good, I mean pretty good for what they are. When properly installed, they are pretty solid and professional. It's only when they are not properly installed that they fail.
The weakest part of the fuel tank is the studs. They are usually just spot welded to the tank itself, and while that usually isn't an issue, it becomes a problem when you over-tighten them and use rubber padding underneath the tank(so it doesn't slide around). Using rubber padding is not a good idea! It creates more pressure on the studs and, with normal engine vibration, small cracks will form around the weld joints on the studs which will cause fuel to leak!
The best way to avoid problems is to not use rubber padding but instead put some double-sided tape or some kind of sticky adhesive on the bottom so the tank doesn't slip around the frame. Don't over-tighten, but tighten just enough to keep the tank firmly on the bike frame. If you can, double-nut the nuts on the studs so that they don't loosen. You can also use thread-locker instead.
If you get cracks in your fuel tank, DO NOT attempt to solder the cracks unless you have professional soldering experience. I made the mistake of attempting this and I ended up getting lots of flux inside the tank which gunked up my carb for weeks since.
A common complaint with the stock tanks is that they often come with rust and metal debris inside! Before you install your fuel tank, wash out and metal debris and clean out the rust using a rust removal formula such as CLR.
Fuel LineAbsolute junk! It will last you maybe a few weeks before hardening, turning yellow, and chronically slipping off the petcock or the carb. Instead of wasting your time with it, chuck it out and go buy some quality fuel line at an auto parts store. Just bring your existing fuel line to an auto store and show it to them so you will get the right size fuel line. Quality fuel line will last for years and will look good.
Fuel PetcockAbsolute junk! Some people have no problems with it, but after my experience with 3 of them from different manufacturers, I believe it is probably one of the worst parts to come with the kits. Throw it out! Not only is it prone to leaking, not only does it have no hard on/off positions(WTF?), but it actually impedes fuel flow! If you want to see for yourself, just open one up. The mechanism inside is preposterous, even by Chinamart standards.
The way I see it, you have the option of either buying a quality aftermarket petcock online or buy one from a hardware store; find a barbed brass nipple that will screw into your tank and will also fit the fuel line. Don't worry too much if it doesn't have metric threads, because you can usually find one that fits into the tank anyway(at least I could). Buy a fuel petcock from the store from the lawnmower section and place it inline in the fuel line. Don't worry if it is plastic since they are meant to withstand gasoline, and the one I have has survived the E85 ethanol fuel that I use.
If you want a more standard petcock, sickbikeparts.com sells a better petcock that it like the stock one but it has a nicer switch and hard on/off positions.
Fuel FilterIt's usually a simple paper filter that does it's job well, though there are better filters sold at hardware stores that have better fuel flow.
The filter is often installed in the wrong position; while either position won't effect actual performance, the best position is to have the paper cone on the inside facing upwards towards the fuel tank. That way, you can see how much dirt has collected so you can know when to replace the filter.
WiringThe wires that come out of the engine and the CDI have some cheap brass connectors so they can be connected easily(hypothetically) by hand. Cut those things off! Instead, solder the correct wires together and cover the exposed wire with shrink tube. This will create connections that are strong, durable, and pleasing to the eye. Don't bother with electrical tape! It will decay and become a mess when exposed to the elements. When you are finished enclose the wires in some flex tubing to further protect the wire, prevent them from swinging around, and make the wiring look cleaner in general. Lastly, use zip-ties to keep the wiring/flex-tube on the bike frame.
CDIIt's about as stripped-down of a CDI as you can possibly get. Never the less, it does its job.
Don't bother mounting it on the bottom bar on the frame like the instructions say to, unless its your only option. Even with rubber, it will end up slipping around from the vibration. Instead, I prefer to mount the CDI on the bottom of the top bar on the frame(the same bar the fuel tank is usually mounted on). I find that it is not only more secure but more convenient, and I don't get the CDI slipping around.
Spark Plug Wire/BootThis is the wire that connects the spark plug to the CDI. Unless it's permanently glued or welded into your CDI, throw it out! You can get a superior car plug wire from an auto parts store(like NAPA). You should do this because the stock wire starts to fall off the spark plug after a while and it also tends to fall out of the CDI. Just cut off the end of the new plug wire that has the larger connector, and screw that end into the CDI.
CarburetorThe stock carburetor that comes with kits, known as the NT or NTTC carb, is actually a remarkably good carburetor and elegantly simple. It can be identified by the circular black filter box on the back with the four small downward tubes. It is inexpensive, simplistic, and easy to service. While there are some carbs that reportedly provide better performance, such as the Dellorto or RT carb, the stock carb provides good performance and is great for beginners and experts alike. If you want to keep your motorized bike simple, the NT or NTTC carb will work great. I have also found that it is incredibly durable; the plastic components can withstand ethanol fuel and the only parts prone to rusting are the bolts and the choke(both of which are easily replaceable).
The only downside to this carburetor is the filter and filter box. They stink! The foam filter inside the box won't do much to keep even large particles out of the engine. Replace it with a piece of real filter foam from a hardware store, or if you want added performance, you can instead cover the filter box with layers of pantyhose(do this with caution). The filter box can also be improved by cutting down the tubes to reduce resistance, and more holes can be drilled into the back to increase air flow.
Do not lose the idle screw! If left unattended for too long, it will fall out from vibration during engine operation. It is a special metric screw, and will be difficult to replace for some people. To prevent it from getting lost, your can tie it to the frame of your bike. It is also best to use a wingnut to keep the screw in the correct position so as to avoid tedious re-adjusting.
|Idle screw with wingnut and rubber washer adaption.|
The worst carburetor is the newer stock CNS carbs. Granted, I have never used one, but it seems like more people report problems with it than with any of the other carbs. While they can provide good performance, they are increasingly difficult to properly adjust. On many of them, the idle control is super-glued in place! However, there are also after-market performance CNS carbs that may be better. I do not know if they are the same as the stock ones(they may not have the problems I have described).
One thing that is problematic with most carburetors are air leaks where the carb meets the intake neck of the engine. This is commonly remedied using a rubber O union ring; the problem with the ring is that it relies on pressure between the neck of the carburetor and the intake neck, so if the carburetor becomes loose, the ring will lose effectiveness. Another option is to create a seal by covering the intake neck with a small layer of silicone sealant. The silicone will be more effective because it doesn't rely on pressure to create a seal, and it will help keep the carburetor in place.
Exhaust Pipe/MufflerThis is probably the most durable part of the bike. It's made of thick stainless steel, so it is difficult to accidentally break. But it is not without flaws! While there are many stock mufflers, most of them have the end cap held in with a bolt that is prone to falling out, causing the end cap to fall out during your ride! If you wish to do any modifications to your muffler, such as removing baffles or catalytic converters(highly recommended), do so as soon as possible and then have someone weld the cap on the muffler. Just a small tack will do. It will save you a lot of time and money trying to repair your end cap or replace it entirely!
If you want an end-cap that performs better and aesthetics don't matter to you as much, you can make your own replacement end-cap out of a large washer. This home-made end cap will stay on properly, and it is also much easier to remove.
When fixing the muffler to the engine, make sure to use spring washers and double-nut! Many people like using thread locker(loctite), but I have found that using quality spring washers from the hardware store works just as well and makes it easier to remove the muffler when necessary.
Some people prefer to secure the exhaust pipe to the bike frame; this is supposed to disperse engine vibration and add more support to the jug. The benefit would be less vibration because of reduced resonation from the pipe, and the pipe mounting studs are less likely to break. I have never tried this, but it seems plausible. It can't hurt to try!
Throttle ControlThe throttle is a piece of plastic, but it's not complete junk. In my experience, the stock throttle works pretty well. But alas, there is one serious flaw; the throttle is kept in place on the handlebar with a cheesy plastic peg! It does not seem like most people have had an issue with this, but two weeks after building my bike, the plastic peg broke while I was riding, and I nearly lost control and almost crashed into a fence! Even if most people don't face this issue, I recommend drilling through the base of the throttle(and the peg) and bolting the thing to the handlebar for peace of mind.
Some people have claimed that the kill switch that comes built-in to the stock throttles is faulty. I have never had this problem, but since it seems likely, make sure to test your kill switch before going on a full ride. If the kill switch fails, simply slow down and stall out the motor.
Clutch LeverThe clutch control is pretty good, but the button to keep the clutch in the disengaged position is sometimes not very effective. Otherwise it's a basic brake-lever and not much can go wrong with it.
Better clutch levers can be found online.
Clutch CableIf not properly lubricated, it can break over time. This is very bad! If your clutch cable breaks, you will not be able to disengage the clutch and the only way for you to stop is to use the kill switch and/or stall out!
If your clutch cable fails, or if you want a better one, you can get a replacement one from the Bell brake replacement kit for about $8 at Target or Wallmart. Make sure to either use bike chain lubricant on it or periodically replace it when it starts to look worn.
Clutch Cable LockThis is the little brass component that keeps the clutch cable attached to the clutch arm. If it gets lost, it is very difficult to replace! Even the most comprehensive motorized bicycle supply websites usually don't sell them separately. Hardware stores also don't usually have replacements or good alternatives.
The best replacement is a 1/16'' ID wheel collar, which are sold at most hobby stores that sell RC and model airplanes. They are cheap enough that you can buy a few for backup.
Mounting HardwareReplace all the mounting hardware! The studs, bolts, and nuts, all of it! They are almost always made of low-grade Chinese steel, and whether you want to believe it or not, you WILL save your money and time by purchasing quality mounting hardware. Stock studs and bolts end up failing after a while, and the threads are easily stripped! If you don't heed my advice and instead use rubber padding on your mounting, you can count on your stock studs busting from the transfer of vibration. If rubber padding to reduce vibration is a must, go with high-grade hardware.
The studs and nuts are all metric, so unless you want to re-tap the motor for non-metric threads, you should stick with metric hardware.
Sickbikeparts.com sells better quality mounting hardware that is the right size so you don't have to worry about measuring and jerryrigging new hardware.
Chain Tensioner/Idle PulleyAlmost universally hated, the stock chain tensioner is easily one of the worst parts to come with engine kits. While I personally think that having a tensioner makes life easier, the stock one is merely held on to the frame with a clamp; this poses many problems such as loosening(thereby derailing the chain and causing the chain to bunch up in the drive gear cover) and even falling into the spokes!
The simplest remedy is to use rubber padding to keep it in place and use spring washers and double-nuts to prevent loosening. This does not guarantee the absence of tensioner failure, but I have gotten away with this setup for a long time without any issue. More importantly, it is best to prevent your rear sprocket from wobbling too much, as this can actually pull the tensioner into the spokes.
To make a better chain tensioner, you can take the existing one and add a spring between it and the bike frame. That will help to keep it in place and prevent it from going into your spokes.
MagnetoIf possible, remove the white wire entirely. Using it will do nothing but reduce power to your engine(slower bike!).
For a better way to use your engine to generate electricity for lights, you can purchase "generators" which are like a smaller magneto that fits next to your existing magneto.
For added protection for moisture and water, put some silicone sealant on the hole where the magneto wires go through.
Spark PlugMost of the stock spark plugs are a no-name "LD" spark plug. I have never had an issue with mine, but some have claimed to get bad stock plugs. If you need a replacement, the NGK B6HS will work perfectly. In fact, I would change to that spark plug anyway, just for the reliability.
Many have claimed superior engine performance with the use of Iridium NGK spark plugs, such as the NGK BPR6HIX or the BPR7HIX.
Engine HeadCome as either a straight head or a slant. What that means is that a straight head has the spark plug pointing straight up, and a "slant" has the spark plug positioned in a slanted position. The only advantage to the slant is that it adds a little bit of space to fit the engine in a small bike frame. And it can make the plug closer to the plug wire if the CDI is far away. Otherwise, there is no performance advantage and any claims of better performance with the straight head vs slant have so far been unfounded.
Chain GuardI couldn't get mine to fit on my bike properly, so I left it off. The main reason for it is to prevent oil/grease from flying up from the chain on to the rider during operation. I use thick moly grease on my drive chain, and since leaving off the chain guard, I have never noticed a problem with getting grease on my clothes.
GasketsMost of the gaskets for your motor can be replaced with gasket material from any auto supply store, such as Autozone. However, it will mean having to cut out the gaskets yourself. In the future, I plan on providing some printable templates, but until then, you will have to replicate your existing used gaskets. I have found that coating them with cooking oil, and them pressing them on the gasket material leaves a good imprint to go by. You can also try tracing them with a fine-tip marker. Cut out your gaskets using a razor blade or an X-acto knife.
BEWARE: Your base/cylinder/jug gasket MUST be the correct thickness, or the timing of your engine will be severely effected! When replacing this gasket, your safest bet is to buy an OEM replacement. Otherwise, measure the width of your existing gasket, and try to replicate that width in your new gasket.
GearsThe gears on the typical china girl engine are kept on their shafts with a tapered friction fit. In other words, they are squeezed on to their shafts with a tight fit, and that's how they stay on. Each of them has a small slot for a woodruff key, which keeps the gears from slipping on their shafts and they are meant to sheer when there is too much pressure to prevent damage to the gear teeth.
Because the gears are squeezed on, they cannot be unscrewed; a gear puller tool must be used, and they are usually included with each kit.
The gear puller works by screwing it into a gear, and then the smaller bolt piece screws through the center of the puller; turn it hard, and it will push the gear right off. This puller tool can be used on all of the gears on the engine, including the small drive chain sprocket.
Be careful of stripping the threads on this tool! The gear puller tool is cheap and screwing it in at an angle can damage the threads and make it inoperative! When screwing it in to a gear, be sure to put a dab of grease or oil on the threads to provide some lubrication, and make sure you are screwing the tool in straight. If the tool looks like it is going in at an angle, don't force it! Unscrew it gently and try again.
When removing gears, you MUST use the puller! I have tried removing gears on my motor in the past(because my puller was stripped) using screwdrivers, pry bars, etc, with no success and I have even damaged my crank case with it. If you don't have a puller, or if your current puller is stripped, you are better off buying a new puller off the internet. There are no shortcuts for this! Many vendors sell them, and if you plan on taking your engine apart or adding a centrifugal clutch, you will need the puller. I purchased mine from boygofast on eBay, and it arrived in the mail in just 2 days!