Because we dont have an RS125 in the US, id assume. Id also assume the poster of the video had replaced their grom engine with a finbro or larger engine, so they had one lying around.A slow death trap.
The gearing needs to be adjusted.
I am struggling to see the point when a 125cc 2-stoke go-kart racing motor can put out a usable 40bhp.
Even an off-the-shelf road bike engine is nearly double the HP of the grom.
Aprillia RS125 = 15hp before tuning