Hi,
first make sure your blades are some what tight. Not too tight, but tight enough so they don't move around too much. Then take each blade and move it forward so they create about a 45 degree angle between them and the head. Now carefully hold the tail and power up slowly, when you reach a pretty high rpm let off of the throttle. Now try to fly it. I found that when the blades move back to create a negative angle the HB tends to bobble around like its on a teeter totter. Hope this helps