Every compass I have tried adding seems to create a timing problem when added to my code.  In order to run under Teensy I need to change usage from the "Wire" library to the "i2c_t3" library.  I wanted to use the HMC6343 and library.  The HMC6343 needs a millisecond to respond to commands.  That was done in the HMC6343 library with a delay(1).  That was changed to a loop with micros() to wait.  The i2c_t3 library has many microdelay() statements that were also changed.  No joy!

Any suggestions appreciated.