He may have told us the problem.
Said petcock is dripping.
If the Petcock leak is coming from the vent hole, the leak is an indication that the diaphragm may have a hole in it.
If the diaphragm has a hole, it may not be allowing the valve to open completely.
It can not be a problem of air being included with the gas flow. Because the gas fills the carb bowls, which vent off any air that might be mixed with fuel.
Your most probable problems are; bend in the hose (that closes when gas line gets hot)
old gas hose (collapsing when hot)
Gas tank vent plugged (not operating correctly)
improper vacuum line hook up to petcock (must be 1 carb (#2) to petcock)
too small or improper installed as filter
petcock malfunction (torn diaphragm)
I may have missed some? (others can add if I did)?
Ride safe, Ted
I was thinking along these lines today while hiking. I think Ted my be on the right track. The diaphragm has a hole in it allowing leakage through to the external weep hole. What if the diaphragm is also leaking fuel INTERNALLY, so that vacuum is pulling it into the carb throat through the vacuum nipple, making it way too
rich, rather than too lean, and not being metered properly. When PRIME is selected, fuel flows through the petcock without going through the vacuum circuit, so the engine runs normally. I am not at home so I don't have my old petcock or photos to refer to. Just some thoughts.
I hate to say it has been so long since I got rid of my OEM petcock and vacuum tubes, I can't remember which carbs put vacuum to the petcock.