A Surging engine can be a single problem or a combination thereof. A plugged up carb can be the major problem but also could be the worn throttle shaft in the carb which is leaking air causing lean mix. Surging can also be a byproduct of leaking intake and exhaust valves or a leaking intake tube( tube from carb to the engine, or the oring at the carb and tube) An engine that surges while idling but runs fine under load or high RPM has plugged up idle air mixture ports. The list goes on. The fuel mixture could be off too. If yours has a mixture screw located on the carb, start by turning it in all the way, then back it out 2 1/2 turns.(don't mistake the tiny idle screw on the throttle plate for the mixture screw) Start the engine. Now with the engine warm and running at about half speed turn the mixture screw out until the engine begins to sputter,(take note of the amount of turns to this point) then turn it in until the engine begins to die.(take note of the amount of turns to this point) Turn the screw back out halfway between the 2 points noted above. Your carb is tuned as well as can be. If anything in this has tweaked you, go looking for it. Otherwise, with the prices of engines these days, you can buy a new one with a Briggs 2 year warranty for about the same price as having yours repaired in most cases, but not all.
Scott Deaver: B&S Authorized Master Service Technician, June 2005
