Bangkok Tuk-Tuks probably will look like very dirty vehicles in an already very polluted city. But all 3-wheeled tuk-tuk's currently on the road in Bangkok are fueled by LPG and emissions levels of an LPG vehicle, compared to even the best fuel-injection based petrol engine, is on the lowest levels of exhaust emission...
Lets say that seeing smoke is by definition not always dangerous exhaust gas...
How long did the whole transaction of getting license plates for the motorbike take?
Yet the same Thai authorities allow pollution-belching tuk-tuks, taxis, government buses, private vehicles and old motorbikes to perambulate Thai roads as free a wood larks!
Go figure. This posting was original posted as comment for article Getting a 1992 Honda CBR600F through Thai Road safety and Emission Testing