Does the stumbling clean up once you get ABOVE a certain RPM? Or does it remain in a rough running condition in all RPMs above the point at which it stumbles.
Answers to those questions would help answer some of the others you have asked.
Petcock? Doubt it, but fix the leak for your own safety.
Fuel pump? Maybe, but it would depend on your answers to the questions I just posted and even then would require some more testing.
Metering jet? Carbking made an excellent point earlier about measuring the jets rather than simply reading the stamped number. And I have my doubts that is your issue, regardless.
