Check your compression and spark. Mine did that and when I checked for spark I found that I had a bad coil. After getting it firing on all of the cylinders and tuning the idle mixture and idle speed it keeps it's prime now. I thought that mine had a weak fuel pump for awhile but it wasn't running right so it wasn't sucking the fuel properly.At slow speed, loose prime and the motor dies. I reprime by squeezing the bulb which feels to be full of air.