I have a new Troybilt riding mower that I purchased a couple months ago, the engine has no more than 8 hours running time. It's a 344 cc, 10.5 hp, model no. 210000. When I last used the mower every once in a while the engine would miss, like the kill switch was turned on for an instant. Now there is gas on the spark plug when I take it out, but there is no spark. What should I troubleshoot for when there is no spark?