The prime position on the petcock bypasses the diaphram function and allows fuel to flow by gravity. The engine does not have to be running for fuel to flow on this setting. It is used when the carb float bowls have been emptied for whatever reason and it is for initially filling the float bowls. The reserve and run position both require engine vacuum to activate the diaphram which opens a valve in the petcock which then allows fuel to flow. Consequently, the engine must be running for fuel to flow in either the run or reserve position. You said when you get the bike started, it dies as if it is running out of gas. It follows then that it could be running out of gas. If the petcock is faulty, it will not flow gas or enough gas. Sucking on the vacuum line checks to see if the petcock does indeed allow fuel to flow in a good stream. If it does flow, then the problem is not with the petcock. The next step would be to drain the gas from the float bowls into a small container, comparing the amount that comes out of each float bowl. The amounts must be the same and should be aprox 2 ounces per float bowl. If there is any difference between them then float heights must be set for the correct amount of fuel to flow. If floats are correct, then your problem is with idle jet and passageway blockage. We're trying to isolate why you have a problem.
Earl
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