I have a poulan 14.5 hp 38 in cut model 271491 from sears. It won't start, it sounds like it needs gas. I have changed the spark plug, fuel & air filters. Battery and connections all good. There is no spark coming from the spark plug. So I added a little fuel to the carb and it starts to whirl like it is going to kick over (still no spark on plug) but every once in a while I get a sound like a bolt was dropped into the engine.