What performance has told you is all correct. If ,you do not have a gas leake where air is being drawn in. From what you said I believe there is a leak somewhere. Put your finger over the opening of the cup when on post flow and see if you can hear a leak from somewhere else. turn the gas pressure up quit high when you do this. If you still can not hear a leak you will need to systematically check every joint from the O ring on the torch all the way back until you find either a bad O ring or a loose joint, or a bad crimp on a hose. There is a chance it could be contaminated gas also. To check that you will need to change bottles. It would not be the first time some 90/10 or 75/25 was put in a bottle marked pure argon.
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