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If it has been running perfectly for six months then it definitely isn't the height of the metering lever. This is how people create multiple problems, by adjusting stuff that doesn't need adjusting. I would suggest that any adjustment of the metering lever height before finding the true cause...

When it changes all of a sudden like that, it usually means there's some crud or other blockage somewhere in the carb. I'd try a thorough disassembly and cleaning of the carb. Also check your fuel and impulse lines while you have the carb off (I do that every time regardless).