Could be several things that could be the problem. A carb problem could cause the symptoms that you describe. Partially plugged main jet will cause running problems like you are talking about. Weak fire, IE color and intensity are not reliable indicators, as long as you have spark. Have seen spark plugs cause problems, but I don't know if you tried changing plugs. Possible air leak between the carb and the engine, intake manifold gaskets. maybe a head gasket problem.