The carb settings have a lot to do with vibration at low speeds. Also the idle speed itself. To slow a idle sometimes makes it worse. The more time you get on a engine and it gets broke in real good it will also help the smooth running aspect! You can make a hand throttle with a Popsicle stick drilled for a small bolt. Bolt it to a "L" shaped piece of metal so it pivots at bolt. Have the bolt kinda snug but so you can pivot the stick. Attach the needed linkage to carb throttle lever. Now you got a hand throttle! Good Luck Capt,n