first a little history:
Facing tightening emissions regulations, in 1997, the A.M.A. increased the allowable displacement capacity for four-stroke engines, in an effort to encourage manufacturers to develop environmentally friendlier four-stroke machines.[3] In 2006, the 250 cc division was renamed the MX Class, with an engine formula allowing for 150–250 cc two-stroke or 250–450 cc four-stroke machines.[4] The 125 cc class was renamed the MX Lites Class, allowing 0–125 cc two-stroke or 150–250 cc four-stroke engines. In 2009, the MX class was renamed the 450 Class and the MX Lites class was renamed the 250 Class, to reflect the fact that all the competing manufacturers had adopted four-stroke machinery.
in 1998 Yamaha brought to the SX/MX world the first true competition worthy 4stroke racebike that could not only run with the 250 2strokes, but beat them. in 1998 Doug Henry won the AMA 250 MX Championship on the YZ400f...the 1st 4stroke to do this and used the 5valve patented technology off of the fzr750 streetbikes.
it would take the other brands several years to even produce the same bike.
in 2001 Yamaha stunned the world again. Doug Dubach had been testing the yz250f in public by putting yz400f stickers on it. nothing says secret quite like doing in the open public!
using the R1 street bikes 1 cylinder, the YZ250F totally took over the AMA 125 SX competition and Ernesto Fonseca won that title. the first 4stroke to win that class title. the Yamaha YZ250F was the only 4stroke in "racing" form from any brand until the 2004 year. totally alone for 3 full years.
while cannondale (defunct) may have started the reverse motor, for 2010 Yamaha introduced the YZ450F with the intake in the front, exhaust out the back, rearward slanted motor. yes, husaberg had also done this, but show me one that won something with it.
http://www.yamahamotorsports.com/sport/products/modelgallerylib/209/1/0/gallery.aspx
now in 2014 Yamaha did the same with the YZ250F...rearward slanted motor, intake in front, exhaust out the back.
http://www.yamahamotorsports.com/sport/products/modelgallerylib/12/1/0/gallery.aspx
when the other brands finally caught up to the revolution that Yamaha started, all factory teams where on 4 strokes and have been since.
not a single 2stroke has won an AMA MX/SX event since 2006. (maybe even 1+ years earlier than that i cant prove the exact date).
when you completely change the face of an entire sport, you are
clearly the leader and innovator!
steve