Irish,
Couple of things here........ keep in mind that a lot of smokes only require about three hours of smoke as after that time the meat doesn't absorb much after that. As far as adjusting the vent, that may slow down the burn a little, but it will also allow the smoke to stay in the chamber a bit longer and you may get a smokier result, so try it. I always burn both ends as it gives me the amount of smoke I like and if you want a bit longer burn fill in the center row. If that still is not enough, simply reload it or get a larger AMNS from Todd.
Happy smokin'