Can police stop you for emissions?

Emisions testing is far from trivial because it can only be done repeatedly under very specific conditions (e.g. warm bike, correct temperature etc) and even the equipment used in MOT stations is pretty unsucessful (unreliable and unrepeatable). I suspect too that unlike cars there is probably no strict legal requirement for bike emisions - unlike cars which are taxed in groups based on emisions so making drastic changes to fuelling etc would make you liable for prosecution for tax avoidence bikes' tax groups are based on size...