The AIS or Air Induction System on the Yamaha V Star 650 is designed to dilute the exhaust stream with fresh air to help burn unburned or incompletely burned fuel so the bike can pass emissions tests. Many cars have similar systems to get oxygen to the catalytic converters so the breakdown can happen. As far as I know the AIS has been on these bikes from the beginning and predate the later years where catalytic converters were put in the pipes and indeed other types of motorcycles had AIS or Pulse Air systems as BMW called it back into the late 70’s.
It seems to be the first thing people do with these systems when customizing their motorcycle is ripping them off and throwing them in the trash. Some of the reasons for this are simply to clean up the looks, and get rid of excess parts that could potentially fail and introduce air leaks. More importantly for people who install aftermarket pipes removing the AIS helps stop that annoying backfiring on deceleration, however, if it was tuned properly it wouldnt do that. If it was optimally tuned for your riding conditions from the factory it also wouldn’t need it, but bikes get shipped all over the world to many climates and altitudes so they have to use general settings that will work anywhere.
UPDATE: I have been reading some of the forum threads that have linked to this page and it appears to be possible to get better than stock emissions tests after the AIS is removed as long as the idle mix screws are adjusted and fine tuned. Like I said, If the bikes were better tuned it wouldnt need AIS to begin with.There is also the theory that removing the AIS causes the bike to run cooler because it is burning extra unburnt fuel and generating more heat that it otherwise wouldn’t have. I can confirm by closely monitoring my crankcase temperature gauge that my motorcycle does seem to run about 5-7C cooler with the AIS gone, on the other hand, I also had been doing lots of carb tweaks that week and cant entirely attribute it to only the AIS.
