When you took the old petcock off did you find a lot of trash in the tank?
Look at those fuel hoses, looking for a tight bend that has gone kinked on you and is restricting your fuel flow.
See if your gas cap has been holding a vacuum instead of letting air into the tank to replace the gasoline used (pop the cap and only put it on loosely).
Pull the hose from the carb nipple, turn the gas on and off with the petcock and look for a clean full flow of fuel from the hose.
We had one dude that didn't take off the clear plastic "shipping protector" tubing sleeves from over the fragile screen sections on his new petcock, he thought they belonged on there and he bolted it up that way .....
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If none of these, take your carb off and clean it and report what you find inside.