What causes a mower engine Briggs Stratton 25hp to surge with gas and flood out after starting normally?

The engine is receiving more gas than it can consume. Sometimes the carburetor needle valve will vibrate to the open position or the float valve will stick open. Also, a fouled spark plug will not provide sufficient spark to ignite the fuel. Check out a lawn mowers and small engines repair manual from your public library for troubleshooting and repair procedures.