My, Lawnboy Dura Force 6.5hp won't start after I have been using it & empty the grass catcher. I pull & pull, take out the spark plug replace it with a new one, most times it won't start I have to use my backup mower which is a Snapper. I have not been able to solve this problem, I have taken it to the repair shop only to continue having the same problem. Anybody have any suggestions? I,m at a total lost. I actually have 2 of these mowers & they do the same thing.