Well,
You never did give all the details of the engine. What is it? A craftsman 2 stroke or 4 stroke? Give us some pictures of it mounted. Are you using the original gas tank or did you install a different tank? Try running it without the fuel cap.
It doesn't help to give us bits and pieces of information. If you had stated earlier that it ran before you mounted the engine, you would have saved some time. In the future, give as much information about your problem as possible and be specific.
Good Luck,
Chris
AKA: BigBlue