Lose of high end performance

This is my 1st article so I will keep it short. My DR 650 begain to lose top end power around 12,000 miles. I repeatedly took it into the shop and was told that nothing was wrong with my bike. I was not able to locate any help in the tech manual so when the warranty expired, I started with all of the adjustments, removed all mods and returned to factory stock with no improvements. To make a long story short the solution was a filter hidden under the seat and routed to the diaphram on top of the carb. This small breather filter was plugged up. A quick cleaning with carb cleaner and instant recovery of all top end power. It is now a part of my routine maintenance.