Small Engines (Lawn Mowers, etc.): 18 HP Briggs Engine, twin cylinder engine, hp briggs

Question
I have lawnmower with a 18 HP briggs twin cylinder engine (40A707).  When i try and start the mower it starts very rough and only runs for a couple of seconds.  When i take off the cover for the air filter i see gas spraying out of the carburator.  I took the plugs out and cleaned them it started a little better but  won't stay running.  I but fresh gas in the tank.  i have new plugs i will be putting in and a new battery.  Also when it doesn't start it only cranks over a half a turn and stops.  Someone told me it might be a stuck intake valve. Is this true, and if it is what can i do to unstick it? thanks for your help.

Answer
 Hello Phil:

 The Backspray is normal if the Engine is trying to Start and a Small Amount can be Seen even while the Engine is Running if you Look Closely enough. The Carb needs to be Cleaned and Overhauled. Also if you are going to Invest in a Battery; Consider Replacing the Battery Cables with Larger Diameter Cables. The Larger Diameter Cable Allows the AMP Flow from the Battery to the Starter to be Greater and The Starter will give Better Service and Less Problems. Im not thinking a Stuck Valve. No mention of Backfiring. Hope this Helps.

 Good Luck

 Respectfully

 John