Small Engines (Lawn Mowers, etc.): Craftman 17.5 starting problem, carburator, lawnmower

Question
Just brought a used Craftman I/C 17.5 hp OHV ridding lawnmower with no history. The unit will not start, only time is when the air clean is off and I cover up the intake with my hand then it runs but I must keep letting air in the hole just a little to keep it going.
Is there something missing (part) or an adjustment that I don’t know about?  

Answer
 Hello Thai:

 The First thing to do is Clean and Overhaul the Carburator. From your Description, the Carburator is Dirty and Has Varnish Built up inside of it. Hope this Helps. Let me know what Happens, Please.

 Good Luck

 Respectfully

 John