The symptoms indicate that the engine has had an overheat, unless there's been foreign object ingestion. A teardown will reveal the difficulty.

When you change fuels, you must completely reset the carb. You richen the high-speed needle and then get it readjusted to just rich of peak. Many people say that they rarely, if ever, adjust their engines once they get a good setting. This is a prescription for short engine life, since the engine will change settings frm day-to-day as the weather changes, let alone as you change fuels.