When properly prepared and worn-in these scooters are cost-effective, relatively simple, and relatively cheap and easy to maintain, and even fairly reliable with proper maintenance. Unfortunately they are often left inoperable by owners that don't know how to maintain them, poor dealer set-up, and a few cheaply made parts.
There are many places online which can be of lots of help when trying to work on or understand these scooters, but it's fairly hard to know where to look for help, doubly so if you don't understand the way it works. To that end, I'm going to explain from start to finish how the drive train on a 50cc GY6 scooter works, part by part. Most of this info can be applied to other motorcycles, scooters, and cars, and since these scooters are designed to be so simple they're a great place to learn from as well. So without further Adieu, lets look at the air box!
Step 1: The Air Box
1: Allow air into the engine
2: Clean said air of sand, dust, bugs, and other things that would do bad things to the rest of the motor
3: Make sure the *right* amount of air makes it to the carburetor
To do this, air comes in the main body of the box through a vent that faces the front of the scooter, usually with a small hose on it. It then filters the air with an oil-soaked foam air filter. The oil and generally porous nature of the filter allow air to flow through but catch the vast majority of trash in the air. The air filter box will also usually have a small hose feeding into it from the front of the motor for gases building pressure in the motor to be burned off rather than just allowing them to vent into the air as an emission control mechanism.
Step 2: The Carb (here Be Dragons)
The GY-6 carb has an electric choke as well, which is what the wire on it is attached to. This generally reduces the amount of air going into the motor some until it warms up, making it easier for the motor to start up when it's cold. Some people have reported this choke to be problematic, though it hasn't been in my experience with about 6 different scooters. When trying to troubleshoot it, make sure it's insides are thoroughly cleaned and none of the carb's small brass orifices are obstructed.
In the worst case scenario, or sometimes just for sanity sake, if you know a motor will otherwise run, but the carb isn't running well, it's usually pretty cheap to replace the entire carb on a GY-6 engine for about $35 shipped online. Definitely a viable option.
Step 3: Intake Manifold
Below the intake manifold is the heat separator. It's essentially a piece of plastic that keeps the engine's heat from making the carb too hot to behave properly and is just between the intake manifold and "top" of the motor.
Step 4: Valves!
1: Let gas and air in the motor
2: Squeeze it
3: BLOW IT UP!
4: Get rid of the blown up gas
Valves are the key to making sure these things happen in the right time and in the right order. Do it wrong and the motor can either catch fire, knock holes in places that shouldn't have holes, bend the valves, or just not work at all. Luckily this isn't something you need to mess with hardly ever.
The engine's spinning is tied precisely to the time the valves opena and close, and they're essentially gates to either let air and fuel in, or let burned gas out. When it's done at the right time. More on how that timing is done later, but that's enough for now.
Step 5: SUCK! (1st Cycle)
Step 6: SQUISH! (compression Stroke)
Step 7: BANG!!! (ignition and Power Stroke)
If you've missed out on getting the amount of air or fuel right, having the mixture squished tight enough, or haven't given it a spark at the right time, your motor will not run.
Step 8: BLOW!
Now we're just about done mucking around inside the motor's power-making-areas, but explosions tend to be kinda loud, so we should probably do something about that so the neighbors don't throw things....
Step 9: Shhhhh.......
Note that as with much of the more cheaply-made GY-6 scooters, chrome and paint on exhaust parts will rust fairly quickly if not cleaned and/or painted better than it comes regularly since it's close to the ground, gets very wet when it rains, and gets hot. People often frequently see that exhaust studs, bolts, and nuts will sometimes back out and fall off, Usually without much warning, so I'd highly suggest putting some lock-tite on all exhaust parts and keeping an eye on the header nuts (the part where the exhaust mounts to the motor), and the muffler mounting bolt(s) so your muffler doesn't fall clean off of the scooter. Aside from that it's a fairly maintenance free part.
Step 10: Meanwhile, Back at the Crankshaft....
1: The crankshaft is counter-weighted to compensate for the weight of the piston
2: The crankshaft also spins around and around to allow the up and down motion of the piston to be translated into rotational energy, which can be fed into the transmission.
The below GIF shows how a four cylinder engine's crank works, it works the same with three fewer pistons as well. The second image is an actual GY6 crank shaft. See how the top of the real crank has a large cast iron chunk? That's there to offset the weight of the piston flailing back and forth.
Step 11: Gotta Keep It Rollin' (The Rotor and Stator)
Rotor: The rotor serves 3 purposes, first as a weight, which gives the crank shaft enough momentum to push through the parts of the 4 stroke cycle where it's not making power (see step BANG!), to help cover and safeguard the stator's delicate wire coils, and to induce power into those coils with magnets lining it's inside. The rotor of pretty much any motorcycle or scooter engine will look like a hefty steel bundt cake pan. The inner wall of the rotor is lined with the magnet that induces the flux that collects in the coils of the stator that makes the power that charges the battery and runs the scooters electrical system. And the green grass grows all around all around.
Stator: As I said, the stator's job is to suck power via magnetic flux from the rotor's magnet and provide it to the scooter's electrical system. To do this it uses at least one but usually several coils of wire which pick up that charge and pass it forward as and AC voltage usually somewhere between 12.2 and 16v depending on engine RPM. The stator also has a sensor (the small black plastic bit in the picture) which tells the CDI when to set off the spark in the spark plug.
There are performance kits that use a lighter rotor and which flip the positions of the rotor and stator, but the power gained for the amount of smooth idling lost is nowhere near worth it IMHO. If you're interested in more on this look up Inner Rotor Kits or IRKs.
Step 12: Valves Valves Valves... (not the Videogame Developer)
The valve clearance is the amount of space between the rocker arm (which pushes the valve open) and the top of the valve itself when the valve should be closed. With normal bigger-brand scooters and motorcycles, it's considered standard practice to set this height as part of pre-sale setup or pre-delivery inspection (PDI). Unfortunately for the GY-6's reputation this is far less commonly done and is usually left to the owner to ensure is done correctly. When you order a crate scooter online or from a local dealer who simply ordered a shipping container of scooters, pulled them out of the crate, and started selling them you're relying on the factory valve setting to be correct.
While normally it's good enough to run fine out of the crate, and usually close enough to do so for a few hundred to a couple thousand miles, as the motor wears in the exhaust (and sometimes the intake) valve will tighten up, robbing the motor of compression and eventually leaving it too open to run at all. So how do you fix this? With the lid off of a pudding cup of course!
Actually a proper feeler gauge between .05mm (about .002") and .1mm (.004") is the preferred way of setting valve height, but in a pinch an aluminum pudding lid smoothed flat works fine too. The important thing is to set it to where the rockers aren't touching the valves when the motor is at top dead center (TDC). The following link has a great photo set an directions on how to do this:
Adjusting the valves on a GY-6 should be something you're familiar with and take the time to do at least every thousand miles or so, or better yet when you change the oil if you're really vigilant on maintenance. It'll save you some fuel economy and power and save you the headache of troubleshooting other problems when they come up which present similarly to out of spec valves.
Step 13: Tran-su-miss-yen
Once your motor is running, you have to deliver it's power to the rear wheel when you want it, and in ratio you want it. That's where the GY-6's Continually Variable Transmission (CVT) comes in. It takes the power from the motor and runs a drive belt between two pulleys. When you're Idling the front one is small and the rear one is big. As you speed up the motor a centrifugal clutch kicks in, making it connect the rear pulley to the wheel and away you go! Speed up some more and the front pulley gets bigger while the rear pulley gets smaller. This allows the power made by the engine to stay at a low ratio (think low gear in a car) at lower speeds where more torque is needed and a higher ratio (think 4th gear in a car) to allow the scooter to go higher speeds.
The neat trick though is this is all done automatically by weights in the clutch's housing, so all the user has to do is twist the throttle and go. Need more torque going up a hill? It'll adjust down some. Running on a flat road and trying to go as quick as you can? It'll adjust accordingly. Here;s a handy little applet that explains it better than I can describe it:
Again there are performance parts out there to change the weights, springs, drive belt and other clutch parts. If your motor is ever running fine, but it just isn't moving at all, double check that the belt hasn't broken. A worn belt will also change shape over time and not be able to get into the highest drive ratios, which means you'll lose top speed.
Also of note: there's a small gear box between the wheel and the CVT which needs to have it's gear oil changed from time to time as well. I'd suggest changing it with 90 weight gear oil every oil change or two though it's supposed to be safe for much longer. If you never do it'll eventually lock up the gears and then you're in trouble.
Step 14: Burning Rubber! (okay Maybe Not....)
Another thing too: the stock batteries on GY6 scooters tend to be pretty darn weak, so if you can kick start it fine but the it's not electric starting go to a local battery shop (not an online dealer selling the original Chinese batteries) and ask for a 7ABS size battery. Most stores won't have a cross reference for the stock batteries let alone the million different aliases the GY-6 family of scooters runs under. Any decent shop should be able to cross reference by the Yuasa model number YTX7A-BS though.
Like I said in the beginning, most of the information in this Instructable can be used on other scooters, motorcycles, and other 4 stroke engines aside from the specific quirks of these scooters in particular. Feel free to ask if you have any questions or look online for the various GY6 forums out there as they are a font of knowledge and far better than any Chinglish manual that may (or more likely may not) have come with your scooter. Have fun and be safe out there!