Do you have a choke cable hooked up and functioning properly? It sounds like you have a carb that has / had an automatic choke setup on it. Did you install the correct base gasket for the carb that has the notch to align with the vacuum port? If you can post a picture of your carb on your engine we can maybe identify things better for you. Have you done any adjustments on the carb, checked the ignition system, checked the timing?