you didnt tell how long is it doing so.
On a hunch do you have children at home? my son was a crawler and i had a CD 100 at that time, one day i started it, it gave very strange sound upon checking i found that some one had stuffed styrofoam (white material used for packing) in the silencer (muffler) turned out that my son was playing with that material (foam) a day before. I took out all of that with scissors.
If that smell has been since first day it might be something else, if it started coming after some time then you should check it silencer with a torch. some people also install mud flaps in front of engine when riding these flaps can touch engine head or silencer and give the smell. In my opinion its engine do not get so hot to burn a pipe only engine head or silencer gets that hot check both of the areas and
KINDLY DO SHARE THE RESULT.