I agree with Paul on the header pipe gasket issue, though I don't see how that would affect running. Sometimes simply tightening the nuts will fix that issue. If not, a new gasket will.
As much as I hate to say it, I'm leaning toward an idle circuit issue. That would be the idle jet itself (you wouldn't believe how many 'ok' idle jets wind up being clogged. And, not to put to fine a point on it, the same goes for 'good' diaphragms. These things can be beastly.) and/or the idle circuit passages themselves. There are three small holes at the leading edge of the butterfly plate. These all need to be clear. The problem is that the idle jet hole is less than .015" and the passages in the carb body are not a lot larger.
The saying "... when you have eliminated the impossible, whatever remains, however improbable, must be the truth...". Check the idle jet again, making sure that you can see daylight. Better if you can pass a small wire through.
The passageways in the carb body can be confirmed to be clear by spraying a fluid through them and witnessing that fluid coming out of the three holes.
The idle circuit has a strong influence up to about mid throttle and a bad idle circuit won't let the engine make enough power to pull the hat off yer head.
It's good that you could fill a bowl with gas in a minute. That indicates that the petcock is fine.
By the by, do you have Vaseline, Chapstick, a Thermarest patch kit or any similar sorts of things? Those can be useful to limp a bad diaphragm home. Vaseline or Chapstick can help seal the edge of the diaphragm and hold it in place while the cover is put back. Applied to both sides of the edge that could help if the cap is warped and also ensure that the edge doesn't get pinched.