Sketch will not start with external power source NOOB Question: when my Uno is connected to an external power source it will not start running the uploaded sketch. The Arduino web site says it's because the RX pin is not connected and then gets random data.

The site suggests grounding the RX pin using a resistor (granted it suggests a 10K and I used a smaller one ) or connecting it to the TX pin.  

From Why doesn't my sketch start when I power up or reset the Arduino board?


  Most likely because you are sending serial data to the board when it
  firsts turns on. During the first few seconds, the bootloader (a
  program pre-burned onto the chip on the board) listens for the
  computer to send it a new sketch to be uploaded to the board. After a
  few seconds without communication, the bootloader will time out and
  start the sketch that's already on the board. If you continue to send
  data to the bootloader, it will never time out and your sketch will
  never start. You'll either need to find a way to stop serial data from
  arriving for the first few seconds when the board powers (e.g. by
  enabling the chip that sends the data from within your setup()
  function) or burn your sketch onto the board with an external
  programmer, replacing the bootloader.


From Why doesn't my sketch start when I'm powering the board with an external power supply? (Arduino Diecimila or earlier)


  Because the RX pin is unconnected, the bootloader on the board may be
  seeing garbage data coming in, meaning that it never times out and
  starts your sketch. Try tying the RX pin to ground with a 10K resistor
  (or connecting RX directly to the TX pin).


Both failed for me. 

What should I do?
The problem was, as Gerben said, the battery was too weak to run the engine.