I notice my sportster seems to lag when I give it a lot of throttle from a dead stop. It's fine once I get going, but it seems like it barely want to go if I give it to much throttle from a dead stop. After about 10 mph it gets up quick. The engine is fine. Is this just how they take off or is it time for a new prop? I also had 650 pounds of people in it and a full tank of gas.