At the risk of stating the obvious, you shouldn't need to back up the bike's battery with a van! The XT battery is small, but is well up to starting a bike in good condition. Running the starter for long periods on a bigger battery risks burning out the starter motor and giving you another headache.
The usual mantra is that if you have a spark at the right time and fuel in the right quantity, the bike must start. As you have a spark and fuel, it follows that either the spark is happening at the wrong time or the fuel is not reaching the engine in the right quantity - too much or too little. Your investigations need to start with this. (That's assuming there is no significant mechanical fault, such as a broken camchain, of course. As you can get it to run eventually, you can discount that.)
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