my stihl br600 and the kawi fx engines the valves get out of adjustment pretty quickly. it's only been about 100hrs on my stihl and they were loose. it's been 200hrs on the kawi and they are loose.
my honda trimmers and pushmower i checked after 10 years for the first time and they were still perfectly in adjustment.
i guess noone else can figure it out as good as honda when it comes to engines.
next mower i get will have a kohler engine and then that will be the end of valve adjustments.
if only honda would make ztr engines and blowers. then we'd live in a perfect world.