Well, it seems it is not only to the R2104 anymore.....
I saw this this morning comming by, 2 offroad engines with a same issue.



This is what will happen if you go too quick, it is showed on the exhaust side. So your sight looks so far OK.

The brown dirt looks like burned castor oil but the pic is not sharp enoug to zoom in much closer. If it is already drying up (getting hard) I would advise to ckean the inside before the next start.