My walk behind starting giving me a problem today. Was mowing a lawn and it ran out of gas. Filled it up and finished that lawn with no problems. On the next lawn it starts right up but sputters and stalls. I choke it, and it starts right up and runs smooth......until I push the choke back in, then it sputters and stalls. Repeated this several times, but always sputters and stalls when I push the choke back in. What's happening??? Thanks in advance!