Small chance but maybe the tiny compression vent hole in the cylinder is plugged up. The hole does not exist in a 250, but the 300 needs it.
You can remove the pipe and look in the exhaust port to see if the piston is scored. It should look good with only 5 hrs on the machine. What does the spark plug look like?
It sounds like you have binding somewhere. Crank stuffer bolt partially backed out? Excess bearing preload?
For a problem like this, a dealer would have to strip the engine for a thorough inspection. Take the bike to him or maybe remove the engine and ship it? Any dealer would want the whole machine to check over.
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2011 EC250E
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