Carburetor jet o-ring! Had a similar problem with my Briggs engine and this fixed my problem. Ethanol in the fuel takes a toll on the rubber components in the fuel system. The o-ring will shrink allowing fuel to get past the jet and the increase in fuel will make the engine surge. I replaced the o-rings along with doing a carb rebuild kit and did the governor adjustments and problem solved. If this doesn't fix your issue, your governor gear may be on its way out. I also had to replace one of these on the same engine 20 hours after my carb rebuild.