You might try running some seafoam through the tank first before opening the carb. It works well at clearing restricted carb circuits. See that your choke is opening fully in the full throttle/no choke postion, opening fully in the idle position, and closing fully in the choke position. I did not see an adjustment for that in the manual I linked to but I only looked briefly. My 12.5 came out of adjustment and acted similar to what you describe with puffs of smoke at start and sputtering at full throttle, difficult to start while choked. Dig deeper into the manual to see if there is a choke adjustment procedure listed. I noticed a speed setting adjustment and that may automatically set the choke. Slapper will likely know or RestroRob if he tunes in. Other causes could be blown head gasket or loose spark plug. Personally, I would not use starting fluid in a small engine. No offense Herler, just a matter of opinion. Good advise on checking the plug wires and caps. Also see if there is an in tank screen ahead of the fuel filter and that it is not restricted with debris from the bottom of tank. Good Luck!