What causes 16.5 Briggs and Stratton backfire a lot while in use?

There are several reasons the engine would backfire, but the three main culprits are a dirty air filter, a loose intake manifold bolt or gasket (vacuum leak), or a faulty spark plug. If the problem is not one of these three, check out a book on small engine trouble-shooting and repairs from your local library. You will find other procedures a little more complicated in the manual to help you trouble-shoot.