You guys may want to check the clamp between the carb and cylinder. I had my engine apart this winter and was disturbed to see fine dust residue on the inside of rubber adaptor. It was like the clamp wasn't tight enough or maybe there was a distortion in the rubber so that it didn't seal completely.

 

Are you talking about the whole lower section of the airbox? I saw an airbox at a used place and they wanted something like $60.... That'd be cool to get a new one for $12. Me and a buddy Jerry-rigged it so I could put some more laps in at the track - it was flopping around and only hanging on by the carb clamp.

It seems that a lot of the parts (including factory plastics) for the 400EX are very cheap from Honda, I wonder if that's because they build them here in the states?

 

The part I got for $12 is the plastic rectangle box. I had to put the tube from the carb to the box on to the new box, but that was it, came with clips and all! It is awesome that the 400ex is made in the US. No tariffs, taxes, or shipping from Japan. Much cheaper. That is why the rims are so much (Made in Japan)

 

I ordered my airbox for $12 yesterday... Thanks for the tip, after seeing that used one for $60, I figured I wouldn't even bother asking the dealer what they wanted.

Also I can confirm that new airbox lids are $5, for those of you wanting to experiment and keep a stock one (for wet days) as well....