Why upgrade? Because the standard carb doesn't function well and often is too little for the engine which limits peak power. If you want an engine that starts easily, idles smoothly, accelerates crisply, and has normal peak power then you have to upgrade. There is no way around it. The only problem is that the inner diameter of the stock intake and of the offset intakes is only 16mm. Of the possible 16 different setups charted below there are 7 that need 18mm or more to match the needed 18mm or 21mm carburetor. For those ones there needs to be a modified intake manifold. Here are the needed carb sizes according to my carb size calculator.
Can't I upgrade with a CNS carb or Speed carb? Yes but they are the same size as stock, 14mm, and are cheaply made, hard to jet correctly, and often develop problems.
What about the Dellorto SHA carbs? They are OK if you modify their fuel column but otherwise they aren't recommended because of their lack of adjustability. (read more) The 16mm or 18mm Mikuni is my carb of choice.
How do I change the engine port timing to allow it to rev up to 8500 RPM? Just remove 1.6mm from the piston intake skirt and use your rotary tool and cutting disc to raise the exhaust port by 1mm increments till you achieve the speed you want. Use a small file to round off the newly formed lip where you removed metal. That is to prevent the piston rings from snagging on that lip.
Below the blue lines indicate where the intake hole is opened to by use of a rotary tool.
Jet sizes listed above are mostly educated guesses based on my experience with the 18mm Mikuni on my reed valved 60cc engine. The $85 16mm Mikuni comes from Treatland with a #70 main jet and #25 idle jet. Click here for a $13 air filter that fits it. The $70 18mm Mikuni comes from Treatland with a #70 main jet and #30 idle jet. Click here for a $23 air filter that fits it. You can get from Treatland also the needed main jets and idle jets. The $65 Dellorto 21mm comes from Treatland with unknown jet sizes since it is a clone. You should also order #90 + #94 main jets for $2.75 each and the #45 + #48 idle jets for $2.75 each. You can also order a $15 black foam air filter for it from Treatland.
The Mikunis need to have the throttle cable housing shortened to be like what is shown in this drawing: