Nothing... Which is why they are so much more expensive
i hear that bike carbs can give good performance whilst retaining some form of economy... its cheaper, and its different!
my mates got a 1300 nova SR stock car on CBR600 bike carbs and it fooking flies
apparently (here's hoping) bike carbs are more town friendly, have better economy, are much smoother, obviously half the price and.... in common with 40/45s sound the nuts and should get me to 120-130 brakes power per horse