When you replaced the petcock did you also clean the gas tank??I have a 2002 400 with a new carb and a new petcock. starts fine and idles great. It won't take throttle to rev after start unless it idles a while. Carb screw is out 2 1/4 like manual says. Acts like it's not getting a sufficient supply of gas. Fuel line was replaced by a guy that worked on it. Don't get it.
Is the new carb OEM? If not, that's likely the problem because it does sound like you still have a carb problem and probably the jet needle.
Please search this forum. There are many threads about the same problem you are having.
Hope this helps you.