Sounds like it could still be restricting flow, thus providing enough fuel to idle but not run at higher fuel demand.
Does that make it lean or rich
Then play with carb adjustment screws
It only has an idle screw. That probably won't help.
You already determined you have a fuel flow problem. If your petcock is now fine, your carburetor is probably dirty, because if the petcock had junk in it, it let junk into the carb.