test run rotax 503 without prop?

You need some sort of flywheel/load, I would be afraid the back lash without a flywheel, or load could damage engine and gear components...
 
You don't really have any throttle control without a load. It will overspeed very fast and easy. It will idle, but will over rev with just a little throttle.
No way to tune without a load.
 
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