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4Strokes.com Honda Technical: XR400R Airbox Mod By ThumperTDC
With the airbox in the sub-frame, I took a pencil and ran it along the inside edge of the sub-frame to roughly mark the new opening. I then removed the airbox from the sub-frame and used a straight edge to make a nice line to cut. With the straight lines drawn in, I used a drill to drill out the corner so they would be rounded when I cut out the top (any size bit should work, just depends on how round you want the corners). With a Dremel tool and a cutting disc I went to town cutting on the line I had drawn in. Once cut out, I used a file to smooth up the edges. Finally I cleaned out the plastic from inside the airbox and put everything back together.
With this mod, I did not make any jetting adjustments, due to my bike was running slightly on the rich side at the time of this mod.
What I noticed right off the bat was a much crisper throttle response, the engine seemed to rev faster than before. All in all there was a noticeable "seat of the pants" improvement, best of all it made my jetting perfect.
If you ride in a wetter climate or cross rivers, opening up your airbox like this will make it easier for water to enter you engine.
The main reason why I tried this mod was that all the 2 strokes I have seen have an open airbox like this and seemed to do well where I ride. If you have done all the free mods and are looking for a little more, this would be one I would recommend. With proper jetting you will notice more power and a big grin hiding behind your helmet.
Credits: Submitted by ThumperTDC to our "Best Low-Buck Mod" Contest, and edited by 4Strokes.com.
Addition to Mod
If you have modified your airbox and have a gauze type air filter, you may have noticed how much junk gets caught in the air filter. I get pieces of dirt, small rocks and some weeds in my air filter. So I made a pre-filter by using some open cell foam, the type used for small air conditioner filters. It can be purchased from most any hardware store. I cut two pieces to fit over the opening of my modified airbox. Then I drilled some holes in the top of the airbox to secure the foam with plastic push-in type fasteners (used for car door panels and such). I just push the plastic fasteners through the foam and into the drilled holes.
Credits: Submitted by Royo to our "Best Low-Buck Mod" Contest, and edited by 4Strokes.com.
Disclaimer: Views or procedures expressed herein do not necessarily reflect those of 4Strokes.com. As with any modification, there could be adverse effects which could cause damage or harm to you or your motorcycle. 4Strokes.com assumes no liability how this information is used.
4Strokes.com Technical: Honda Related or Non-Model Specific