| by the sounds of your discription this new carby is wanting either more fuel and or less air intake. normally when an engine only wants to run with the choke on it means its got enough air intake or it wants more fuel. i would firstly drain the fuel bowl to see how much fuel is in it, this will indicate fuel to the carby then remove the fuel bowl to check the float is working up / down [ do not bend the float to seat fitting ] if the float is moving freely then you will need to check the main jet for correct size plus you need to make sure there are no air leaks from the carby to manifold and manifold to reed block. check to make sure the manifold is not cracked aswell. | 
