I second the air infusion theory of the fuel lines. If they are brown, they're old and probably need replacement. Each has an O-ring on each end that can fail. Some failures leak fuel, others admit air which causes stumbling or hard starts. I replaced the fuel lines in mine and still had a massive air leak that could be traced back to the prefilter, which had a bad O-ring. The new clear fuel lines made it easy to view the air leak and follow it to its source.