Since there were 2 carbs having the same issue- I would check gasoline delievery to the carb. I would run the engine on LPG while the gasoline fuel line was fed into a graduated bucket to measure the amount of gasoline in a specified time & I'd check that against the spec for the gas fuel pump. Could be a faulty pump, clogged fuel filter, clogged gas tank vent or a bad cam lobe for the fuel pump- just a thought