What was the color of your spark plug before you put the UNI filter on? I like the color of light tan/brownish and both the Batavus and Avanti seem to like that color also because they have logged many miles on just one spark plug.
Now might be the time to start increasing your jet size to see what your little Bat likes. My Bat is a heat demon since I shaved the head, modified the carb. and installed the UNI filter. I've rode the Bat long and hard so unless you're running lean there's nothing to worry about. Look on the bright side. Winters coming and you just increased the BTU's on your leg warmer.