I purchased this unit late last season, and used it a few times no problem. I went to mow my grass this year and it won't start. The first time I tried, on about the fourth pull it started for about 3 seconds and shut off. Since then I can pull all day on it and it won't do anything. I checked the boot on the spark plug (the actual spark plug isn't standard size and I don't have the tools to remove it), replaced the oil, it has new gas in it, cleaned the air filter and made sure the switch for the gas system was on. This unit has a choke on it but no primer bulb. I'm at a loss as what to do next. Any suggestions?