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. For the different possible setups I list those with 5500 rpm maximum (stock) and 7000 rpm maximum (modified by porting for 30mph), piston port intake and reed valve intake, and engine sizes 48cc, 55cc, and 69cc (80cc). Intake manifold bolt hole spacing is 32mm for the 48cc and some 69cc. It is 40mm for the 55cc and some 69cc Grubee engines. Below are the correct carb sizes according to this formula:
Carb diameter=K x (square root of (engine cc x (peak power rpm/1000)).
"K" for piston port engines is .9, and it's .8 for reed valved engines.
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 7000 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.
In the charts below the "Z32" refers to the long Z shaped offset intake manifold that has 32mm hole spacing (available from JNMotors, Bicycle Engines, Gas Bike). Z40 is the offset manifold with 40mm hole spacing. When it says you can use a standard intake manifold but you use the offset manifold (which is 1" longer) then reduce the listed length by 1 inch to get a total intake length (carb slide to piston) of 7 inches. When it just lists the M21 or M25 manifold then the stated hose length will provide a 7" intake length. Of course you can reduce that length if you want more emphasis on power at a higher RPM. The first two charts are for a standard piston port intake, and the last two for a reed valve intake.
5500 RPM maximum, piston port intake
7000 RPM maximum, Reed Valve Intake
5500 RPM maximum, Reed Valve Intake
Below the blue lines indicate where the intake hole is opened to by use of a rotary tool.
The Mikunis need to have the throttle cable housing shortened to be like what is shown in this drawing: