BEST 48cc Engine Modifications
1. Replace the stock piston/rings with a Honda Hobbit piston/rings. This raises compression because the Honda piston is 1mm higher. Use a rotary tool to grind away at the head at the outer edges if there is less than .6mm clearance between it and the piston. (Use solder from Radio Shack. Let the piston and head squash it and then take it out and measure its thickness.) If you don't have a rotary tool then buy one because you need it for the following porting changes. Also it's needed to cut the piston skirt. Don't lose the new found compression from incorrect head gasket sealing. It can't seal right if the head or the cylinder is warped. Plane them down on sandpaper on glass. Much better than the stock aluminum gasket is one you can make from gasket material from Autozone.
2. Cut the piston skirt to a length of 44.5mm (measuring from the piston top edge). Cut the exhaust skirt to 46mm long to keep it from hitting the flywheels. Make .8mm transfer ramps.
3. Replace the upper connecting rod bearing (since the stock one will wear out very quick with increased cylinder pressure) and the stock piston wrist pin with a lighter one (because the inner hole diameter is larger) to reduce engine vibration.
4. Make the following porting changes, but if you don't make the piston ramps for the transfers then make the distance from the cylinder top to the transfers 31.2mm. Use your rotary tool to raise the exhaust port 3mm so it is 25mm from the cylinder top. Make sure the bottom of the intake port is 55.5mm from cylinder top. Make sure the transfers are 32mm from cylinder top and make sure both have the same distance. Make the intake port 2mm wider on each side. These changes increase the top RPM to around 7000 without losing low RPM power.
6. Use your rotary tool to grind away at the rear of the transfer ports so that they are at least 2mm more rearward. [info]
7. Install a motorcycle ignition coil and CDI for more power and less vibration.
8. If you want to emphasize low rpm power then install a rubber hose between the carb and the intake manifold so that the carb slide is at least 6+3/4" from the piston. [info]
9. For a 6% power/speed gain replace the standard NT carburetor with a Mikuni carb. [info]
10. Here's 3 options for a better exhaust pipe:
With an expansion chamber you can have even more low RPM power with a reed valve. You can buy a Rock Solid reed valve, drill holes in your piston, and dremel a boost port in the cylinder. [info]
11. Increase the balance hole diameter in the flywheels for eliminating the horrible engine vibration
12. Use sandpaper on glass to take around 1.5mm off the stock cylinder head for more engine compression and power.