ok, i've been having trouble with my motor. i got it about a week and a half ago, i've been through a total of about 5 tanks (my tank is a half gallon size). and i've maybe been topping at 17MPH, and then it slowly loses power until it maybe maxes 10 mph without me pedaling. i've been thinking that its just this infamous break-in period i've read so much about. but recently i found that when i close the petcock almost all the way it picks up speed like crazy, but my bike vibrates very violently, and it is really loud, it feels like its going to explode, so i let up a little. is that normal? please help.