Playing overtones and not the correct notes on baritone saxophone?

For one of my big bands my teacher asked me to play bari for the concert. I normally play alto. The bari is not a great horn and the reed is chipped a little. When I play, I get overtones and the octave key seems to not work. I have tried changing everything in my embesure, which seemed to help a little bit, but I am still getting overtones a lot. What I'm asking is what is the correct embesure to play on bari and why am I playing wrong?