I suggest you begin by establishing whether this is a case of hunting, or something else. For reference, and you may already know this, surging is a technical term that refers to a specific effect of lean mixture. Hunting is a term applicable only to governed engines, and can either be due to lean mixture or a malfunction in the governor - usually, friction between two parts rubbing together.
Your engine was made on 9 June, 2000. Here is the Illustrated Parts List:
http://bsintek.basco.com/BriggsDocumentDisplay/18jsBLXBnfBhU7y.pdfHere is the Operator's Manual:
http://bsintek.basco.com/BriggsDocumentDisplay/flmvtLX-nfBhU7y.pdfFrom its date of manufacture, and from the presence of Item 211 on Page 5 of the IPL, it seems your engine has governed idle: there is no idle speed adjustment screw, and the engine is under governor control even at idle.
I suggest you begin by checking whether the problem is in fact lean mixture. You can do this, both at idle and at higher speeds, by letting the engine run until it warms up, then at each speed, try moving the choke lever slightly. If you find that you can make the engine run steadily at any speed by finding the right choke setting, but its speed cycles continuously when the choke is wide open, the problem is lean mixture.
Your carburetor is an emissions type, with no provision for mixture adjustment. Lean mixture could be caused by a blocked vent in the fuel filler cap, an air leak at the carburetor mounting flange (where it is bolted to the intake port of the engine) or by a carburetor malfunction. When you get to the point where you know whether the problem is lean mixture, and whether the filler cap vent is blocked or there is a gasket leak at the mounting flange, please report back and we can talk about next steps.
