I have a Yard Machine / MTD 6.0 HP, 21-inch walk behind. Model number is 11A-439G129. The only way I can get the mower to start is to remove the spark plug, and put raw gas into the cylinder. Priming doesn't help. After putting gas in the cylinder, it will start with a regular pull, and the mower runs fine. Once the engine is hot, I'm able to start it with the regular pull. I have removed / inspected / cleaned the primer bulb and the channel from the bulb to the carb intake, and both are working. I'm assuming this isn't a fuel supply issue, or the mower wouldn't run, or at least it wouldn't run well. Any guidance?