If it's surging, it's possible it has a vacuum leak. The fastest way to check that is check for a lean mixture by removing the air cleaner, then cover the air horn with a rag while it's idling. Try to cut off as much air as you can before it dies cause it should die. If it actually runs better cutting off the air just a little bit and refuses to die when all the air is shut off, then it must be getting air from somewhere else and that indicates a vacuum leak somewhere. First thing is grab the carb with both hands while it's idling and try to shake it to check if it is tight. If the idle rpm changes any at all while shaking the carb, the bolts are loose or the base gasket is leaking. Now check for a stuck open EGR by purposely reaching in or under the EGR and pushing it open with your finger. The engine should stumble or stall, if not the EGR is stuck open or the gasket is leaking. Now if cutting off the air in carb did cause it to run better, take a can of starting fluid and spray around the carb base gasket, all along the intake manifold gasket on both sides and any and all vacuum hoses, lines and connections listening for the RPM to change indicating a vacuum leak.

Next if it's not running lean, make sure it's not running too rich and flooding out. Look down the carb air horn with a flashlight and mirror, (so it don't backfire in your face) and look for gas dripping on the throttle plates. If so, then first of all check and make sure the fuel pressure isn't too high, then if it's flooding it could be a dirty needle and seat of the float, a hole in the float or a float that needs adjusted.