Question about a problem with my mower?

Can anyone that works on mowers tell me what to do? My 9 year old son has a tendency of laying the mower down to start it instead of leaving it upright. He says he can't start it upright and the neighbor kid told him to put the push bar on the ground, hold down the start/stop bar and pull the string. My son has done this several times and we keep having to take the mower in because it screws up the mower. It won't start and black liquid keeps spitting out what I think is the exhaust. Also the string is getting stuck half way. GRRRR!!! I am getting frustrated. I have had to take it to the repair shop everytime and they never tell me what the problem was or how to fix it at home. However when I take it in it takes them less than 20 minutes to fix. Can you tell me what to do or why it does this. HELP I need to mow the lawn TODAY!!!