Jack,

You are correct.  The Rotax is a total-loss ignition system.  If the voltage at any point gets much below 11.5v you will get insufficient power through the coil and intermittent spark as a result.  It should feel as if the motor is effectively, sputtering and shutting off, because that’s exactly what is occurring.  A fully-charged battery in most cases has the ability to run all day.  HOWEVER, if you are running the older Lead-Acid Sealed battery, if you take it below about 80% charge at any point you really end up damaging the cells long-term ability to ‘recharge’.  The Li-Ion battery is much more tolerant of this due to it’s internal makeup, but I still don’t recommend it.  The best bet is to have your battery on charge ANY time you aren’t on track, including sitting in the trailer, or at home in your shop or garage.