You can loosen the injection pump, turn it slightly clockwise, and retighten. Advances the timing and white smoke should clear right up. Not a rare problem with 5003 series tractors. Happened on about 20-30% of the 5003's we sold.
Mark the pump and housing before loosening any bolts and you'll have a "reset" in case you turn it too far.
will this same thing work on the 5103's????