I'm always looking to work on something that's new to me. Since they have become popular the high revving 4x's (I'll use this instead of typing out stroke each time) have been known to be expensive to service and keep in good running order. This might have been true with the engines of the first few years of 4 strokes but after 2005 I think the main Japanese mfgs have most of the bugs worked out. KTM had a VERY dependable engine that was good ever since it was introduced in about the year 2000. This engine was called the RFS type engine and it's probably one of the most famous engine family for off road bikes. But that's another story.. and it has to do with why I bought my KTM400 dual sport bike.
Just a little 4 stroke history. From old heavy 4 strokes to new modern and high revving ones. Yamaha took off out of the gate with the first four stroke big bore diameter / super short piston and piston stroke engine. From the wiki:
For many years, the motocross world almost exclusively used two-stroke engines. AMA racing classes had two classes: 125 cc and 250 cc two-strokes, with no provision for four strokes. Most riders considered four-stroke engine technology to be antiquated and uncompetitive.Additionally, most four strokes produced very little power (the 1996 Honda XR400 made 32 bhp (24 kW), compared to the 40 bhp (30 kW) produced by most 250 two-strokes of the time).
In 1996, the AMA changed racing rules to allow 450 cc four strokes to compete in the same racing class as 250 cc two-strokes.Yamaha engineer Yoshiharu Nakayama first came up with the idea of creating the first competitive four-stroke race motocross bike.The Yamaha YZ400F was developed to fit into this category. It solved the power dilemma by borrowing superbike technology and giving the YZ a five-valve head, liquid cooling and a 12.5-1 compression ratio.