I use this mower for small jobs and right now I have a problem. I pull on the rope and turns it on, but then it dies. I've replaced the air filter, spark plug and the gasket that goes from the filter housing to the carb. I've also removed the muffler to see if there was an obstruction, but the same thing keeps happening. I pull on the rope and it fires up. after about 3 seconds it dies. The only way that it keeps running is.....if I keep priming the bulb fast after it fires up. Any ideas and thank you for your help.