If the gap between the reader and the tach strip gets to great you can end up with a runaway engine. Just wondering looking at the picture. The reader is spaced and should be glued to the motor. The cable ties look a safety net to hold the reader to the motor rather than secure any wires. If they are really taught and they are the only thing holding the board on. It could be it’s out of adjustment. The manual has the adjustment at .030 .
Were you ever able to disable the speed control ? I know on most installations you also use the tach reader to determine the chuffs. If you run it with the speed control off and it runs fine. Take a note as to what the chuffs are doing. Not sure if this was a factory install in which case they may have used a cam on the tender and a switch to trigger them.
