A 2006 Technical Review of progress made by manufacturers to meet 2004 and 2008 on-road motorcycle emission requirements can be found at the following link:

Background

Why ARB revisited the Regulations in 1998

Despite the substantial progress that had occured in the effort in reducing air pollution in California, the State still had the worst levels of "smog" in the nation. The California Air Resources Board (ARB), in coordination with the U.S. EPA and other local and State agencies, was implementing programs required under the 1994 State Implementation Plan (SIP) for reducing air pollution from a wide variety of mobile, stationary, and area-wide sources of air pollution. Along with other motor vehicles, ARB identified on-road motorcycles as a potential source of air pollution which could be reduced cost-effectively. Reducing air pollution from on-road motorcycles was possible because the existing regulation for these vehicles reflect technologies which were 15 to 20 years old, while adapting some pollution control technologies developed for automobiles to motorcycles had the potential for reducing emissions from motorcycles significantly.

See the following links for more details:

• Changes to Regulations for On-Road Motorcycles - An ARB Fact Sheet     

• Why the ARB revisited the Motorcycle Regulation? - An ARB Fact Sheet

• Formal Regulatory Documents

• October 7, 1998 Workshop Notice to Discuss Proposed Changes to the On-Road Motorcycle Regulation

• July 1, 1998 Workshop Notice to Discuss Proposed Changes to the On-Road Motorcycle Regulation

To get more information, please contact Sharon Lemieux, Manager, Emission Research Section, at (626) 575-7067, or via e-mail at sclemieu@arb.ca.gov.