The biggest mistakes novices make are choosing a cam that is way too big and a carburetor that is way too big. It is much better to have a cam and a carb a little on the small side than to have one too big. It is no fun fighting an ill-performing engine that does not live up to expectations because of mismatched parts. It is also a waste of a lot of money. You have to think of an engine as the sum of its parts, not just one part. Every choice you make will affect every other choice you make. Do your research and take the advice of experienced people and you won't go wrong.