Learn what different scooter performance parts are for, what you need and which parts will give you the best performance increase for your money.
Performance Exhausts One of the first and most important modifications you should do.  A 2-stroke motor's exhaust helps determine your maximum rpm and your power band range.  Changing the exhaust from a stock to a performance exhaust will typically increase the horsepower by 1hp.  For the cost, this is a significant increase.  Also 2-stroke motors don't develop much torque at low rpm's.  With the addition of a new exhaust, your rpm's will increase, giving your motor more torque.  You will normally have to change your roller weights, contra spring, and clutch springs to compensate for the increase in rpm's.  Which exhaust is best for your motor is hard to say.  Different pipes are tuned for different applications.  If you are going to a big bore kit, be sure to purchase a pipe that is tuned for your size motor.  Different pipes have different rpm and power band characteristics.  You can usually go to the manufacturer's website and find these specifications.  Another good way to decide which pipe is best for you is to ask.  Ask in a forum for opinions on which pipe is best suited for your needs.
Big Bore Kits Obviously one of the biggest and most sought after performance modifications you can do to your scooter.  A big bore kit increases the size of your cylinder and in turn gives you a large increase in horsepower.  Be aware that just because you are going to a larger cylinder doesn't mean it is better.  There are several different classifications of big bore kits.  A stock 50cc scooter usually generates around 4-5hp and runs around 7000rpm's.  A mild 70cc kit will usually increase that to about 8hp and run around 8-9000rpm's.  But there are high performance big bore kits that will generate an excess of 20hp at a staggering 20,000 rpm's!!!  These are typically only for race applications and are not used for every day riders.  The engine life of these high performance kits are usually very low.  So depending what you are looking to do with your scooter, carefully choose a cylinder that best fits your needs.  Usually a middle of the road performance kit that runs at about 9-12,000rpm's and generates about 10hp is usually the best choice for most people.

When you go to a big bore kit, you will need to purchase a pipe that compliments your cylinder.  Different cylinders have different port timings.  Putting the wrong pipe on can significantly impair the performance of your new cylinder.  Try to get a pipe that is tuned for your cylinders port timing settings.  You can usually get this information from the manufacturer or by asking in a forum.

Another thing that will be needed with bigger cylinder is a bigger carburetor.  Once you increase the size of your cylinder, more air needs to get into the motor to fill the chamber.  Changing your cylinder and using a stock carb is pretty much a waste of you increased displacement.  Again choosing the right carb for your set up is crucial.

One last thing to consider when up-grading to a larger cylinder is to change your final drive gearing.  Decreasing your gear ratio will give you more top end.  While the addition of a big bore kit gives you more horsepower, it usually doesn't increase your top speed much.  You will get much better acceleration due to the increase in torque, but the rpm's of the motor will still top out near stock values.  When a lower gear ratio is used, it enables you to utilize your increase in horsepower so you can increase your top end.

Performance Variators New variators are not always required.  A common misconception is that a variator will increase your top end.  This is typically not true.  The performance variators are typically the same diameter of the stock variator, so increasing your top end is really impossible.  What a performance variator does do is level out your acceleration.  The ramp plates on performance variators are different from stock and the size of the rollers are also different.  This makes for a more smooth and constant acceleration.  But if you have good consistent acceleration with your stock variator, upgrading to a new variator is pretty much a waste of your money.  However, if you have inconsistent accelerations, jumpy starts, dead spots, or any other problems with your acceleration, a performance variator may be in your best interest.
Performance Clutch A performance clutch doesn't help with your top end or your acceleration.  A performance clutch is only used to help adjust and fine tune the engaging rpm's.  There are a few aspects that make a clutch a "Performance" clutch.  First is the number and size of the clutch pads.  Surface area of the pads make it grip better to the bell housing, making the clutch grab better.  Also there are clutches that have fine adjustment capabilities.  Meaning that instead of having to change springs when you want to adjust your engaging rpm's, you simply turn a screw and tighten the spring up.  This is the type of clutch I prefer and recommend.  It is much easier to adjust than the type you change springs with, and it has a much finer adjustment.  With the type you change springs with, you only have the settings that the different springs available give you.  With the adjustable type, you can turn the screw a whole revolution, a quarter revolution, or what ever you want!  It is very nice to finely adjust the clutch to exactly where you want it.  You also don't need to buy several different spring sets.
New Contra Springs Contra springs are only used as a tuning part.  You will only need to change the spring when you are trying to correct an acceleration/top end problem.  Read the article on the transmission to learn more about which spring you might need or if you need one.
New Roller Weights Roller weights are also used as a tuning part.  You only need to change your roller weights if you are trying to correct an acceleration/top end problem.  Read the article on the transmission to learn more about which rollers you might need.
Carbon Reeds Carbon reeds typically come in a set that contains several different thicknesses.  The idea is that a thinner reed will respond faster to different pressure changes within the crank case.  Meaning that it will open when your motor makes the slightest demand for air.  The downfall of this is that at higher RPM's, the reeds will tend to "Float" not allowing the reeds to close all the way between strokes.  Thicker reeds don't have the response of the thinner reeds, but are more reliable at higher RPM's.

Now having said that, most people with a typical application will not notice a difference in the different thicknesses.  Only extreme race set ups will a performance difference be noticeable.  What does this mean for you?  It means if you purchase them for your scoot, don't expect to see any performance increase from them.

The one main reason why people use carbon reeds is for engine preservation.  Reed valves have a nasty tendency to break.  After all, they are simply really thin flaps that are bent open and slammed shut thousands of times over and over again.  The stock reeds are made of steel.  If the steel reeds break, chunks of steel are dropped down into your crank case causing catastrophic damage to your motors innards.  Carbon reeds are much safer because if they break and fall into your crank case, the resin that makes up the carbon reeds simply melts and burns off.  This saves your motor from serious work if a reed fails.

New Carburetor A new carburetor is usually needed when you upgrade to a big bore kit.  The size of the carburetor is determined by the size of the orifice where the air comes in. a 17mm carburetor has a 17mm diameter opening.   A 17mm vs. a 20mm carb is really only going to differ in the diameter of this orifice...  The main jet will be a bit larger to compensate for the larger amount of air coming in.  A bigger carburetor isn't always better.  You only need a carburetor that is big enough to air to flow freely and fill the cylinder.  If you go with a carb that is too large, you will have trouble getting your air/fuel mixture correct.
New Final Gearing The purpose changing your final drive gears is to give you more top end while at the same time decreasing your rpm's. The peak horsepower of you engine is typically generated at around 7000-8000 rpm's stock.  Stock, if your running say 40mph at 8000rpm, from then on you are increasing your RPM's and decreasing your horsepower while trying to go faster. The gearing just uses the full potential of your variator, and makes it so when you are at say 8000rpm your peak horsepower, you will now be going 50mph instead of 40mph.

However, stock 50cc cylinders really don't generate enough horsepower to push the scooter through the air any faster than it can with the current gearing installed.  So going with different gearing on a stock cylinder is usually a waste.  However, a gear kit with the addition of  big bore kit is usually a big benefit.

Kevlar Belt Kevlar belts are pretty much the same as the stock belt.  The only difference is that they are stronger and wear a little better than stock.  If you are going to a higher performance scooter, it is usually advised to get the stronger belt.  This belt will do nothing for performance.  It is simply stronger to help cope with the increased torque of a modified motor.  My advice is don't worry about replacing it until you have a problem with your stock belt.
Clutch Springs A tuning part that helps determine at what rpm your rear clutch engages.  Remember that a 2-stroke motor doesn't develop any torque at low rpm's.  So if your clutch is engaging early and trying to push your scooter at say 2000 RPM's, then you probably need to go to a heavier clutch spring.  Going to a heavier spring will allow your motor to spin up to higher rpm's before the clutch engages and pushes you along.   This in turn will give you better acceleration because you are moving while in your power band range.
Rev Limiters
Performance shock There are several different levels of performance shocks.  There are adjustable, gas shocks, internal spring and gas shocks, the list goes on and on.  Basically the only reason for a new shock is if yours has gone bad, or you simply don't like the ride you are getting from your stock shock.  With performance shocks, you pretty much get what you pay for.  If you buy a $50 shock, expect it to be pretty much the same as your stock shock.  If you pay $300 for your shock, expect to get a really nice ride, and have plenty of adjustments to it also.
Torque Driver A torque driver is the front half of your rear pulley of your transmission.  This half of your pulley has angled grooves cut into it to help lower your transmission's ratio when the scooter's rear wheel comes under torque.  Meaning that it is designed to "Down shift" your scooter when you start up hills or come under a heavier load like going into a heavy wind.  The performance drivers have a steeper angle on the pulley to help "Down shift" faster when the rear wheel is put under heavier loads.
Performance Crank A performance crank in most cases are not required.  Many will say that a stock crank will not handle a 70cc kit.  In most cases this usually isn't true.  The only time you really "Need" a performance crank is when you are running a pretty extreme set up.  What I mean is this, a stock 50cc cylinder usually runs around 7-8000rpm's and makes about 5hp.  A standard 70cc kit will usually run around 8-10,000rpm's and make about 10hp.  Your stock crank can usually deal with this increase in rev's and the higher torque.  Now if you purchase a higher performance cylinder that is rated to run 12,000rpm's or higher and generates say 15hp or better, then yes, you should plan on purchasing a new crank with your cylinder.  The stock crank simply won't hold up to this type of application.

Also if your stock crank ever does bite the dust, or your bearings go, it is a good time to upgrade to a performance crank even if you aren't running an extreme set up. A stock crank is going to run about the same price as the performance crank, so you might as well go with the better crank.

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