I have an ATA-125D and the motor runs well and it actually sounds really good. However, I have an ATA-150D and the motor is really really loud and has a loud and high pitched whine. I don't know about gear(s) and transmissions, but it sounds and feels like it's stuck in 1st gear. At full speed it feels like it's going to explode. Maybe there is only one gear, I dunno. I just can't get over how much better the smaller 125 runs than my 150. Thanks for any advice or comments.