Larry,

Your explanation of the effects of a dirty injector do not conform with the
symtoms that I am experiencing.  The motor starts up fine and runs well until
its hot.  Then the problems starts with black smoke, mis-firing, and no power. 
Then I turn if off, and then back on again and it runs fine for a short while
and then the problems starts up again.  Would a dirty injector contribute to
such behavior?

Charles