On this 350 I’m working on with the leaky carb, the petcock is hard (really hard) to turn. I have to put a pair of channel locks or vice grips on it to get enough leverage to turn it. It seals fine, but I can’t turn it on or off without a pair of pliers.

Any way to free it up?

It wasn’t easy to turn before I put it on, but I thought gas might make it easier. I think instead of getting easier, the gas swelled the rubber in it and made it harder to turn.