Assuming as you said, the float is OK, it's most likely percolation from manifold heat. Since the intake & exhaust manifolds are intimately connected it's easy for the heat of the exhaust manifold to bleed up to the carb and "boil" some of the gas out of the bowl, into the throat. It's then like trying to start a hot engine with the choke on. My 38 has a similar problem. A spacer between the carb & intake can help. I've read of some having success putting a heat shield under the carb but I've not done that. I just live with it. I make sure when the engine is hot that I'm going to be at my destination long enough for the engine to cool back down before starting again.

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