I didn't know if I was getting into a pilot jet issue because of the dying or not. Hope not!
I did turn up the idle a bit and it does fine. Actually, I only notice it when the bike is cold and hasn't fully warmed up yet. [/quote]
You have gotten into a pilot jet issue. When you put on the new muffler you CHANGED the air/fuel ratio throughout the entire rpm range, including the pilot circuit. Do the basic carb tuning routine. For the low range, where this particular problem is, do the following:
-Start and warm up the engine for several minutes.
-Turn up the idle speed several hundred rpm.
-Adjust the pilot adjusting screw so that you get the highest rpm possible.(if it is working correctly when you to go too far in either direction the engine should slow, or even die in the extreme)
-Adjust the idle rpm back down to the desired speed
At this point, your pilot circuit (low range) should be tuned correctly so that your engine is getting the proper fuel/air ratio, will idle and run well, and will not die when you roll off the throttle. If it does not, then there is another problem somewhere in the carb.