What honda says:
Honda Activa 125 BS6 is powered by a PGM-FI HET (Honda Eco Technology) engine having Honda eSP (Enhanced Smart Power) technology
These are the major changes I see:
1) Engine internal components have been re-designed and lightweight parts have been used along with low-friction parts. Now the bore x stroke ratio has changed from 52.4 mm x 57.9 mm to 50 mm x 63.1 mm. With the bore reduced and the stroke increased significantly, the idea is to increase fuel efficiency at low rpm and city driving traffic conditions and Honda does claim that the fuel efficiency on the Activa 125 BS6 goes up by 13 per cent.
2) The power output drops marginally from 8.52 bhp to 8.18 bhp at 6,500 rpm and even the peak torque output sees a slight decrease from 10.54 Nm to 10.3 Nm at 5,000 rpm ,but this torque is available at lower rpm. The overall capacity is not very different, at 124cc, but the motor now has a much longer stroke, and of course, it is now fuel-injected.
3) Now there is a very much improvement in engine refinement, both in terms of sound and feel. Honda’s new silent start system replaces the conventional starter motor with an ‘AC generator’ starter, and what this means is that there is no more gear meshing sound to deal with. It’s feels good on starting and now I like the process of thumbing the starter button and having the engine fire up smoothly and quietly. So its clear that the scooter uses an Alternating Current Generator (ACG) that facilitates a noiseless start.