Add to the above post that the air filter and carb are located in an idiot position that just begs for dirt , nud , water and crap to get into the engine ...
Also ask yourself WHY the 4 stroke MX bikes such as YZF's should need any more regular maintenance than a high performance road bike engine such as a YZF 1000 Yamaha (which they are based off) and you'll start to see things a little differently ...
Horizontal engines were originally designed in the late 60's to be run on leaded fuel ... the lead lubricated the valves and seats and virtually eliminated wear ... These days pollution laws have forced oil companies to remove all traces of lead from the fuel and as a result valves and seats don't last anywhere NEAR as long as they used to ...
To anyone who THINKS their engine shouldn't need regular valve clearance checks ... It'd be a good idea to use a Valve Saver additive in the fuel ... AND don't go revving your engine hard or let it get too hot ... And that means run your air/fuel mixtures on the rich side to keep the valves cooler and accept sluggish performance ...
Most cars and Jap road registerable bikes have high quality valves and seats to minimise the wear ... but it's still a lot quicker than it would be with leaded fuel ... Most cars had self adjusting hydraulic lifters which totally eliminated the need for valve clearance adjustments ... Manufacturers HAD to go hydraulic to get people to buy their cars ...