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With the introduction of Euro- III emission regulations for 2-wheelers in Europe, emission control for motorcycles and scooters has become a significant technological challenge. While many vehicles have shifted from 2-stroke to 4-stroke, they are mostly still open loop carbureted or in some cases fuel injected. Euro- III regulation requires emission measurement from key-on and has a high speed phase for vehicles over 150cc engine displacement. The combination of cold start fuel enrichment (with or without secondary air injection) and high exhaust flow rates is particularly challenging for controlling the emissions of THC, CO and NOx simultaneously. If the engine is tuned lean to reduce THC and CO emissions, NOx emissions will increase during high speed operation. On the other hand, if the engines are rich tuned, this increases the cold start and/or high speed acceleration mode THC and CO emissions.