If it is running on the governor and it really is surging - that is cyclically changing speed between consistent high and low values, each few seconds - it is almost certainly just lean mixture. Try adjusting the main mixture screw. With the engine running (and surging), note where the mixture screw slot is, then turn it anti-clockwise half a turn. If that makes the problem go away, you just need to go through the B&S adjustment procedure by finding the rich and lean limits then putting it halfway between.
If it isn't mixture, it is probably a governor malfunction but I've never personally seen one that was anything but mixture. I yield to Bruce's much greater experience in this stuff.
