Give it a quick look and you might mistake this for a Yamaha TZ250. The truth is you’d only be half right. This did indeed start life as a 1991 Yamaha TZ250, but owner Brian Herzfeldt replaced the two-stroke 250 with a 660 four-stroke Single with five-valves. Power is claimed to be in the 70 hp range, with curb weight of 289 lbs! With a power-to-weight ratio like that, Herzfeldt affectionately calls this bike the “Yamamonster.” Combined with the excellent handling the TZs were known for, this thing rips. In fact, you might have noticed the number 1 plate the Yamamonster carries. Herzfeldt currently campaigns this bike in the AHRMA Sound of Singles 1 class, which is open to Singles of unlimited displacement, where it has won the national championship the past three years.