Performance! How do I get more?
Carb- The bigger the carb (meaning bore and jets sizes) the more fuel you can introduce to the cylinder. A venturi bore speeds up the airflow across the main jet and pulls more fuel into a somewhat compressed mixture. (There is a limit on how large to go though)
Reeds- Stiffer reeds close tighter and faster sealing your precious air/fuel mixture into your case. This aids in the pre compression stage of your mixture. Stiffer reeds also don't flutter at higher rpm, which creates turbulence before the charge reaches the combustion chamber.
Ported head- (this also includes the porting of the case at the transfer port bottom) this allows more fuel/air to enter the combustion chamber via lesser resistance and larger cross sectional area. (Up to a limit, this gets WAY more complicated) This includes matching the ports between the case and the head and smoothing any obstacles to the mixtures flow.
Indexed/matched piston- Indexing the piston (the way I do it) effectively increases the transfer port size at the top. It also advances the intake timing, allowing more fresh charge into the combustion chamber. An angled “index” will guide the charge up and to the rear of the chamber which will swirl and create a vacuum at the front of the chamber; this helps scavenging and creates more space for the return pulse charge. This makes your power band peak stronger and allows your expansion chamber to be more effective. Matching the piston is cleaning up the underside to maximize flow and to enlarge the transfer port at the bottom, this is mostly effective on the Cag piston port style motors.
Tuned pipe- A properly tuned pipe will create a return pulse at precisely the right time to pack the left over fresh charge back into the combustion chamber. At your power band peak this essentially acts like a turbo.
Performance clutch- I'm mainly talking about the clutch engagement time here. If a clutch is set to engage at later rpm, you can build more horsepower before it engages. This will give you substantially more acceleration. The motor will also rev faster at the lower rpm due to the lack of a load on the motor. This also acts as if you have a lighter piston/crank, until engagement that is.
Sprockets- Bigger rear sprocket or smaller front pinion = more acceleration, Smaller rear sprocket or bigger front pinion = less acceleration, more top-end Pinion to Sprocket ration ~= 1:10, that means 1 pinion tooth ~= to 10 Sprocket teeth. If you're looking for a good all around gear ratio to ride with everyday, and you want some good clutch pad life, stick with your first thought…7/72 or even 7/74. You'll have much better acceleration, but not quite the top speed as a 7/66 or a 7/68 would have. Also, try a 6-tooth pinion. It�s easier for your engine/clutch (as far as these centrifugal clutches go) to push a 6t pinion than a 7. Try a 6/68 if you want.
What should I mod first?
#. Air Filter induction kit / exhaust can upgrade #. Re-gap spark plug spacing to .025 Inches. You may also want to pick up the NGK BM7A (Use the spark plug disc you get at any auto parts counter for like 99 cents) #. Fuel filter and Higher Quality fuel line #. Carb porting and polishing #. Primarily an engine teardown, modification and reassembly. It includes bumping the compression, indexing the piston and other porting. Depending upon the builder and methodology, it can be done to yield more bottom end, midrange or top end power.
How do I maintain and keep my bike running smooth?
The spark plug must be clean, and must be replaced promptly when signs of wear begin to show. The condition of the spark plug can tell you a lot about how your motor is running. The spark plug should be a light brown or golden color.
The air filter is used to clean the air going into the gas motor. Periodic cleaning of this filter is recommended to protect your motor against the damaging effects of dirt and abrasive particulates. Your air filter can be located on the back and it has 2 screws.
The chain on your gas-powered scooters should be lubricated before every ride. The chain should also be checked for tightness on a regular basis. If the chain is too loose a popping sound can be heard and the gas-powered scooter will jerk under acceleration, or the chain will just jump off altogether. If the chain is too tight it will be noisy and bind, this can be felt by pushing the scooter with the motor off.
The carburetor has many adjustments on it and it is recommended that experienced persons make these adjustments only, however we will point out the major points. The carburetor is responsible for pumping the fuel and mixing it with the air at a precise measurement. The carburetor does this with very small diaphragms, orifices and ports, which are easily clogged by contaminants. This is why is extremely important that you use only clean new fuel mixed with 2-cycle oil. KEEP YOUR CARB FREE OF DEBRIS!!!
Taken from: http://www.pocketbikeplanet.com/showthread.php?t=156