Kolher 26 hp sears mower has rpm surges--will not idle smoothly, is this a dirty carb or could there be something else?

You may have a governor linkage problem... Air cooled engines usually use simple linkage from the governor to the throttle. As the engine operates, the linkage vibrates and "eggs out" the round holes the link rods ride in... springs do the same... follow the linkage back, piece by piece, from the throttle and check each piece. Some governors use cooling air volume to measure engine speed. There may be a vane in the cooling air stream that should be inspected... good luck...