Q. Which big bore kit should I get?
( Minarelli / 1E40QMB / Jog )
The first things you need to figure out are what your budget is and what you hope to accomplish with the big bore kit and possible accompanying parts. Let's start with your goals for a new engine combo. Are you looking for all out power at any cost? Do you have a specific speed or acceleration goal so you can keep up with traffic or whatever the reason may be? Are you just looking for something better than what you have without spending a lot of money? Would you like to do a lot all at once or as you go?
In general, the higher the performance and RPM level, the more you'll sacrifice in other areas.
Prices tend to increase as cylinder kits are rated for more power and higher engine speeds. For example a street/sport type of big bore kit with porting and construction more similar to a stock cylinder costs much less than a racing kit made of different materials and with more advanced porting. Some of the price increase is probably simply because people are willing to pay it. An example that comes to mind would be Polini Sport/Contesta and Polini Corsa kits. Essentially the same kits, but one has the ports positioned about 1mm higher in the bore. I have seen the Corsa kits sell for as much as $50-75 more than the Sport kits from the same sellers.
High output/RPM setups require additional parts in order to perform as they should, while a basic big bore kit may work alright with what you have and some tuning supplies. If you wish to use any cylinder kit that makes it's power at much above 9,000-9,500RPM, you'll need to also use an exhaust that is designed to work in the same RPM range as the cylinder kit. Otherwise you will never realize the potential of the new cylinder. Most stock exhausts work well enough up to around 8,500RPM, and may allow a mild sport or street setup to rev out another 500-1,000RPM still making some power. A tuned exhaust intended for that RPM range would make more power, but the stock exhaust will often perform acceptably. If you were to use a stock exhaust with a racing cylinder that makes power from say 10,000-13,000RPM, then you would likely find that the cylinder was too aggressive to work in the stock exhaust's RPM range and the stock exhaust is too mild to work in the cylinder's intended RPM range. In that case you may be left with a setup that's much slower than a stock engine and unable to be properly tuned.
If you plan to use any engine combo that will rev above 10,000RPM regularly, you also need to consider upgrading to an aftermarket crankshaft. Stock cranks are known to fail prematurely at elevated RPM. As revs increase farther you may even need to replace the stock flywheel and stator with a lightweight flywheel or an inner rotor ignition kit.
Carburetor and CVT requirements also increase as you move toward the racing style of kits. Stock carburetors often work alright (when tuned) with mild combinations. With more power and more RPM larger and larger carburetors must be used. At a certain point this may require using a larger intake and you will want to replace your stock reed block as well to make the most of the setup. The stock CVT is also not designed to work at high RPM and power levels. You can get by with a stock (tuned) transmission with a street/sport build, but beyond there you will probably find it necessary to upgrade your CVT and clutch. In addition to the need for more parts, RPM-loving engines are usually more difficult to get in tune so expect to spend a lot of time dialing them in.
On top of all of the labor and expenses already mentioned, as power and RPM levels rise engine reliability/longevity tends to fall. This is not to say that you can't go faster and have a reliable scooter, but when you start pushing beyond street/sport builds you should expect them to require more maintenance and to experience more parts failures than you would with milder setups. This is partly from the greater stress on all components from increased RPM, cylinder pressures, and heat, but also from component designs used for racing purposes. Most high output kits were never intended for day to day street duty, but rather purpose built for competition. If you choose to go with a race-oriented kit and ride it often, even if you buy all of the necessary components, be prepared for high maintenance costs.
Taking all of the above into consideration, most riders are quite pleased with an upgrade from a stock cylinder to a 46-48mm bore street/sport cylinder kit that peaks below 10,000RPM. Most work with a stock exhaust, carburetor, and CVT, although the carb and CVT will require some tuning. They can provide a substantial increase in torque and horsepower while remaining reliable. If more power or speed is desired, a tuned exhaust or CVT upgrades can be added later. Many of these kits are quite responsive to porting and tuning as well. Know your goals, know your budget, and do your research before purchasing a cylinder kit. If you aren't sure if you'll be able to accomplish what you want with kits you're considering, ask for help on the forum.
Remember to leave room in your budget with any kit to purchase tuning supplies, installation supplies, and any tools needed to complete the big bore kit installation.
Big Bore Kit Installation Tutorial
Minarelli / 1E40QMB Wrist Pin Sizes
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