What Sandbag said. There's a tiny bit of dirt floating around at/in the jet. You get the initial bit of gas through it, the dirt gets sucked up, clogs the jet, the motor starts to die, the governor pulls on the auto choke lever, gets a bit of gas that way, makes a puff of blue rich smoke, the dirt drops a bit and gets more gas and the process goes over and over.
I call it the auto choke lever, probably called something else though.