CG:
I am a beginner, and I know a few things abount engines. This thread started about " can I run an engine without a prop?. It progressed into 4 cycle engines and lawnmowers, I know, but the fact is still there. Do not run a 2 stroke or 4 stroke model engines without a prop, if you do run it, you will be paying for repairs. Rotary engines neeed the counter balance.
I will maintain, it is a beginner question, and it got more involved, but the question was answered in the first 3 or 4 posts.