You can't feed the impeller too much water! Open her up all the way. Make sure the cup does not "walk" off the intake BEFORE you fire up the motor. As soon as she starts, jump out of the boat and make sure the cup is still in place. It should take no more than 15 or 20 seconds before the water starts to flow from the exhaust.
Keep an eye on the water flow! Also, be sure the telescoping support is perpendicular to the hull, not the ground.