Machine has only done 9000 miles. Machine starts most times will rev up normal when left to tick over or when machine reaches higher temperature will cut out then start up and fires down exaust continualy on slow over advanced tickover. Which seems to point to timing being out left to call will then start.And same again. Carb looks in good order good fuel, there is still a spark when motor cuts out. Any help would be gratefully excepted. Fred. ps i am a motor mechnic and doing my head in.