I had a similar prob on my new to me DRZ400E. I suspected the carb, but couldn't sort it. Sent the carb away for expert cleaning and checking out. Was a little better, but not good enough. I went on to do the valve check and noticed the inlet cam timing was one tooth out! No more red header or back firing but still not 100% happy yet with the jetting/setup. The FCR is a real pig to get spot on.