Steve,
I'd stop using the mesquite, and only use apple. Or drop both and switch to hickory, which I've never had problems with. Sounds to me like the bitter taste is form too much smoke or too much charcoal. If charcoal is put in too soon (it isn't totally ashed over), you will get a very thick white smoke that will give your food a bitter taste, b/c you're basically tasting the binding agents of the charcoal.
Make sure all of your charcoal is greyed over and no smoke is coming off before you put the wood on.
Also, what kind of a rub are you using? The fact that you're getting a tar-black, greasy bark intrigues me.
I'd stop using the mesquite, and only use apple. Or drop both and switch to hickory, which I've never had problems with. Sounds to me like the bitter taste is form too much smoke or too much charcoal. If charcoal is put in too soon (it isn't totally ashed over), you will get a very thick white smoke that will give your food a bitter taste, b/c you're basically tasting the binding agents of the charcoal.
Make sure all of your charcoal is greyed over and no smoke is coming off before you put the wood on.
Also, what kind of a rub are you using? The fact that you're getting a tar-black, greasy bark intrigues me.