My Ryobi starts OK but dies after about 3 seconds. If I press the X3 button by the carb every 2 seconds I can keep it going but I can't cut grass at the same time. Question 1. What does the X3 button do? I assumed it primed the carb from the petrol pipe like most mowers but it seems to just suck air through the carb. Question 2. What's the proper fix?