Assuming the '08 is the same as my '06, that would be the vacuum line that runs to the PAV (Pulse Air Valve, if believe). The PAV is mounted on the frame at the front and just below the fuel tank (also just above the horn). The vacuum hose is supposed to run to the bottom of the PAV. If you search this board for PAV you should see some discussion on this topic. Some have disconnected the whole assembly by plugging the vacuum line, removing the PAV and plugging the port in the head where the secondary air is injected. I would first reconnect the vacuum line to see if it resolves some of your carb problems and then go from there. Good Luck!