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The case is becoming pressurized from combustion gasses blowing past the rings.
Do a compression check to confirm. It is likely low. Make sure the valves are not too tight (staying open) and recheck. If still low add a little oil to the cylinder and recheck. If it is the rings, and I'm pretty sure that's what it is, the compression should momentartily increase.
If this confirms my diagnosis your engine needs a top end rebuild.
I had the same bike and it needed a top end at less than 2,000 miles.
To remove the head and cylinder you have to remove the engine first. Noit a fun job, but do-able.
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Happy to serve.
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