I was reading all the post above, and what the hell is everybody talking about?
Pure tech: the air filter makes sure that dust or other particles doesn’t get into the system. The combination of air and gasoline must be as pure possible to be sprayed in the cylinder. If dust or particles gets into the cylinder the moving piston can cause damage or if its as big as a stone a total jam.
Now: for the Husq: the air filter is made out of a paper harmonica. This paper harmonica is present under the seat and extra covered with a plastic shield to guarantee the paper filter gets not damaged. In this position its unlikely that a hard object like a stone gets into or trough the harmonica (even without plastic cover) because the seat and the bike itself blocks the way. If you can guarantee that the paper filter stays in place and the connection with the carburetor is 100% sealed than you wont get problems (but the paper must stay undamaged).
The reason everybody want to remove the cover is because they want the bike have more power… without the plastic cover its normal that the carburetor gets a bit more air sucked in because the whole surface from the air filter is free. It’s not so that a huge amount of air sucked in, the more power the bike gets. At some point to many air will do the opposite and the bike wile ride worse. It’s the search for the ideal combination that is important. For you guys that wants the bike get more power, change the stock paper air filter by a racing air filter that lets the air much more (but controlled and more ideal) in the carburetor:
As I quote:
The flow of this DNA filter is 40.94 % more than the Husqvarna stock foam filter!
DNA replacement air filters are designed to increase horsepower and acceleration while providing excellent filtration. These filters are washable and reusable and are custom designed to fit into your existing factory air box. Their drop-in design means adding performance is simple.They offer bigger air-flow, with almost 99% filtering efficiency. It’s ideal for Husqvarna 401 .
With this air filter you can place the plastic cover back as the air go-trough is huge (40% more than stock)!
Please note: For the 390cc (375cc) recalibration (gasoline spray that is) is recommended for optimal performance. (but for you non tech guys: not obligated)
For the Vitpilen 701 you can find the race filters also:
I hope this helped you guys understand the process of an air filter. (100% seal + no paper damage = OK)
ps: excuse my English 🙂