the motor is lean!!!!! I bet the motor is new. You have to run os motor's real rich untill they break in. And then you have to take your time leaning it out. Dont know why that is but that's what i have found. If you break the motor in like the book say's ( 1min rich and then 1 min leaned out, back and forth for the first whole tank ) They break in fast. When you get about 2 gallons on the motor you can lean it out and get the power without the knocking. If you dont take the time to break it in, it will take twice as long to get rid of the knocking.

hope this help's

BTW put a 11x4 on it to break it in and then put a 11.5x4 or 12x4 on it after a gallon.