Burnt taste is sort of generic. Primer can be one cause of it. Another can be not enough fluid - burning the filler. Also, make sure your mouthpiece is correctly aligned with the atty bridge. If you look inside your atty, you should see what looks like a white rectangle at the bottom. The rectangle shape of your mouthpiece should be as aligned as possible with that rectangle; some people have reported melting plastic, and I suspect that occurs mainly when the mouthpiece is misaligned. (Check the bottom of your mouthpiece for any areas that look melted, if you want to find out if that was the cause)
I had burning issues too (and I'm not entirely sure what caused it) at first. It does improve once you get the hang of it. At worst you may want to invest in some crest mouthwash (check for
Sattec's atty cleaning thread in the "Experiments with equipment" forum for the details on that method of cleaning.)