Sounds like a needle and seat issue. That would be the first suspect when changes in grade affect performance. Turn the gas off and pull the fuel line, pull the bowl off and see how full it is. It should be 2/3 or less.. If it is too full of fuel the carb bowl does not vent properly and can suck up extra fuel. If the bowl is nearly empty you will have to adjust the float to keep more gas in the bowl.. If you have never changed the needle and seat it is an inexpensive preventative maintenance issue if you have the carb off anyway.
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