The compression vent hole is a tiny hole located above the exhaust port. Less than 1mm in diameter. It just bleeds off some compression at kick starting speeds.
210 psi compression is fairly high for a 300. It would certainly provide lots of resistance for the kicker and estart. Many GG engines come with too wide a squish measurement. Maybe yours is a rare one with tight squish and high compression.
My 250 tests at 200 psi. It kicks over fine (and cranks OK on the estart - engine warm). The 250 piston has less surface area than a 300 piston.
Maybe some 300 owners can chime in on compression readings. And yes there is variation between models of testers. Mine has a fitting that screws into the plug hole with a 6" hose to the gauge. Compression readings are only valid for a specific engine as a way of monitoring the health over a period of time.
If most 300 owners said their machines all came in at 180 psi, then maybe high compression is your problem. If fits the scenario.
Google the method for taking a squish measurement with solder. Try it and let us know what you measurement you get.
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2011 EC250E
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