I have been reading about this symptom and some other things I had read I need to check are for burnt valves or gunk in the manifold. I just had my valves adjusted last Friday and according to the shop, all of my valves were on the loose side, none were tight. It was them that pointed out that my carb was not functioning correctly. I have never personally seen a burnt valve, but I would assume if the valves were getting adjusted, the adjuster would be able to tell if the valve was burnt or not. Just looking at them, they all look normal. Can a person visually check a valve and see if it is burnt?