Hey,
The 400's have a flat spot right off of idle, then the fuel surges, and you get a little hit. Check out bajadesigns.com, they have an XR400 section on their menus. It tells about the dual tapered needle, slide, and jetting, as well as the airbox mods. Supposedly, it fixes the flat spot problem. The needle and slide are relatively cheap also, so it's worth a try either way in my opinion. On the other hand, you can spend ~$400 for a Mikuni Pumper carb which also, supposedly, works wonders for the 400's. In the near future I will be trying the Pumper carb on my 400. I've got an Xr's Only pipe and jetting and I'm not displeased, even at low speed, low RPM riding. However, I think I understand what your friend is complaining about. Search around these forums for XR400 mods, Mikuni Pumper carbs, and check out bajadesigs.com. Good luck.