Yep, that might be it, take the little box off just behind the carburetor, inside you will see a set of springs and a "Lifter". there needs to be around .010 of clearence between the bottom where the spring mounts and the top of the lifter when the cam is is at the low side. If you have never done this it's kinda hard to explain. Why not go to the internet, there are some good tutorials there.