I have a 18.5hp briggs on a craftsman mower. Model and Type:31P777-0299E1. I have 2 problems. The mower is hard to start and surges when cold. After it warms up it idles smooth. I assume it is a air or fuel problem. I put new air and fuel filter, cleaned fuel tank, adjusted the valves, and rebulit carburetor. The surgeing and hard to start problem wasn't there before replacing filter and rebuilding carburetor. The fuel mixture screw doesn''t change the way the mower runs, (idling, or fixing surging problem). When rebuilt the carburetor I wondered if I didn't push the seat in far enough, would that cause these problems?