Remove the air cleaner and look into the carb inlet. When the sputter starts, I'm going to guess you'll see a little bit of liquid gasoline dribbling from the venturi. (the restriction halfway down the carb body). That indicates a leaky float valve, and/or fuel pressure that's too high. It's like a toilet tank float valve that fails to shut off and allows water to leak into the bowl constantly. The cause could also be a carburetor float level that's set a little too high, or a float with a pinhole in it that allows it to partially fill with gasoline.
Jerry
