I Put New Carb. on 15HP Kohler - Ran Great Two Yard Cuttings - Just Turnover - No Start

I put a new  carburetor on 15HP Kohler (Yardman 42" cut). The engine started right up and ran great for two yard mowings. Now the engine will not start. The engine just turns over. I get some backfire. 
 
I am not sure if the new problem is related to the carburetor or just the next problem. 
 
Thank you in advance for your help.