ok.. It's possible that the more difficult terrain heats the engine more than usual. Hot weather adds to this heat. And long periods of idle with no air flow over the engine heats it more .. and it all adds up.
Perhaps idle adjustment was set under milder conditions.. if idle is adjusted on a cool engine it likely won't idle when hot. Try getting the thing as hot as practical before setting idle speed (and idle mixture?).
i really know nothing about the Solex .. i guess it has a "Solex" carb. Depending on the carb it may be possible to adjust more than idle speed to prevent it from dying.