Small Engines (Lawn Mowers, etc.): help, carburator, push mower

Question
i have a 20 inch push mower with a 3.5 b and s engine when you start it runs fine for a second then starts to speed up and slow down and so on do you have any suggestons have tried about everything i know  need answer a.s.a.p thanks

Answer
 Hello MArtin:

 From your Description, I Suggest the Carburator be Cleaned and Rebuilt. Varnish that the Gas Forms over Time Clogs the Ports and Passages Inside the Carburator and Restricts the Proper Gas Flow to the Main Jet. This gives the Engine the Surging from Fast to Slow. Hope this Helps. Let me know what Happens, Please. Thanks.

 Good Luck

 Respectfully

 John