i agree its a possible carb fault, maybe its icing up, had that once and i needed an additive to fix it, this modern fuel is bad for that, tried different carb and that was no good, why it iced up is still a mystery , it was impossible to use in cold weather . eventually i found a replacement for the heat block between carb and head that had a heater element in it and that fixed it. came off a car but i cant remember which one.
turned that on via a switch and a relay before starting and kept it on till the motor was good an hot then it was ok.