23 HP vanguard Conquest engine surging / hunting

That sounds very much like either (a) poor fuel or (b) too lean a fuel-air mixture. Try dumping the fuel out, refilling the tank with fresh fuel, and putting a SMALL amount of carburetor cleaner (the liquid, NOT the spray) into the fuel tank. Don't put in very much, a full bottle is good for about ten gallons... so put only about half an ounce of it (a couple of tablespoons) in the tank.

If it runs fine immediately, the fuel was bad. If not, then run it for fifteen to thirty minutes with no load, just idling; with luck, the carburetor cleaner will clean up the carburetor's internal fuel passages.