https://dallas.craigslist.org/ndf/mcy/d/denton-1984-yamaha-rz-complete-nut-and/7624254773.html
"The '79 Yamaha RD400 was supposed to be the last street 2 stroke available in the US - forever. It barely passed emissions and the way Yamaha got it thru was the use of a "power valve". This would open or close the exhaust port based on throttle position. By closing at lower RPMs, it helped to reduce the blue smoky stuff that 2 stroke lovers can never smell enough of. Yamaha carried this over to the RZ and took the 2 cylinder parallel twin back to the drawing board. With a complete redesign, liquid cooling and Yamahas Power Valve System, they were able to get it to pass emissions, barely lol. The bike was only avail in the states for 84-'85, however it was avail in Canada and overseas until the early 90s where emissions weren't as strict."
This is the first I'm hearing that the RD400F has a power-valve engine :dad: :clownshoes: And that it barely passed emissions cert.