Libavcodec and CBR problem

Hey guys!

   I'm trying to do a little stream application using libavcodec. It 
works fine using MPEG4, except the bitrate control is not accurate.

   I mean, it has some influency over the resulting bandwith, but when 
fast motion happens, my bitrate value appears to be ignored. and it 
overflow my network bandwith - ( i'm doing stream over udp, so i start 
to loose packets )


   Do i have to calculate  context->qmin and context->qmax  for each 
bitrate option?
   Do i have to use ff_rate_estimate_qscale ( ratecontrol.c )  on my 
own code?

   Can CBR be really accurate? , ex: to really enforce bitrate values?


Thanks!