Which way is it pulling? If it's pulling to the left, that could be engine torque. You might need to add some right thrust. If it's pulling right, it may have too much right thrust. Usually the thrust issue comes more prevelant as you begin to accelerate on the take off roll. Try easing into the acceleration, instead of just gunning it from the start--this allows you to make small corrections as it builds speed, instead of having to correct for it all at once.