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Five systems, fuel injection, ignition modulation, intake port flow control, exhaust after-treatment, and electric propulsion systems, are discussed here in terms of engine design modifications and new systems retrofitted to the motorcycle to reduce the exhaust emission. A fuel injection system has been implemented to a 125cc air-cool four-stroke engine, and the results show that the improvement on the fuel economy, reduction of emission on HC and CO, and the fuel economy are 20%, 68%, and 10% respectively. By implementing the electrical ignition system into the target engine, the fuel economy and emission have slightly improved. The application of the inlet air flow control to a target engine also obtains 75% on CO reduction. The effectiveness of the exhaust after-treatment on a 80cc 2-stroke engine has demonstrated that the emission can be reduced up to 75%.