Small Engines (Lawn Mowers, etc.): lawnmower runs but choppy, carburator, varnish

Question
Hi, I have a 3.5 MTD lawnmower. I replaced the spark plug, the oil, the air filter and bought new gasoline. it starts, but runs choppy, and doesnt idle very well.  what next?  

Answer
 Hello Donna:

 It Sounds like the Carburator needs to be Cleaned and Overhauled. Gas over a Period of Time Forms A Varnish Inside the Carburator. This Varnish Builds up and Causes the Jets Not to Perform Properly. Hope this Helps. Let me know what Happens, Please.

 Good Luck

 Respectfully

 John