The vent hoses are to maintain the same air pressure in the float bowls as the outside air pressure to provide consistant fuel flow through the carb. They are routed up to the handlebars to get them out form under the hood where air pressures are greater. If they are left under the hood they could push extra fuel through the carbs giving you a rich condition.
These vents seem to be an item that is commonly overlooked.
Hope this helps.
'92 Indy 650
'95 Indy XLT 600, plastic skis, skid plate, cobra windshield, XTRA 10, full Fox package, 1" riser