My Briggs & Stratton won't run unless the choke is open completely. It'll sputter, cough and die when run without the choke open. Cleaned out the air filter and fuel filter and topped the tank off with fresh gas but none of those things fixed the problem. Any thoughts as to what might be wrong and how to fix the problem? The engine is a 17hp Briggs & Stratton on a Craftsman riding mower.