Ahhh, I probably know your problem, delt with it twice myself when switching to reserve a little too late. Assuming that you've got the problem I had the little ball which allows fuel past when vacuum is applied (from the vacuum line from the front cylinder) is sticking. You can feel the little ball when sucking or blowing on the vacum line intake with the petcock removed.
When running the bike has enough vacuum to move the little sticky ball and diaphragm in the petcock but when the engine is turning slowly from the started there isn't enough vacuum to open the flow of gas. After a while of waiting a bit of fuel slips past the petcock, enough to start the engine creating more vacuum allowing even more fuel to flow.
Until I shot carb cleaner past the detached petcock cleaning the little gummy ball (cleaning gas film off it) I found that by detaching the vacuum line from the front carb and sucking on it to get fuel to flow got me running much faster than sitting around for 45 minutes to let the fuel seep by it).
With the petcock removed and sucking and blowing by mouth I could feel the little ball unstick and move inside the petcock. Even after some carb cleaner I never got the ball to move super freely but it now works much better with little problem after running out of fuel and needing reserve fuel.
Bottom line, first assure that there is no vacuum leak in the line going to the petcock. Then remove the petcock and blow carb cleaner through the vacuum inlet. Rebuilding the petcock doesn't hurt either. Like I had you probably have a “sticky” one way ball valve in the petcock and a petcock vacuum diaphragm that lets a little fuel leak trough.
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Last edited by Spencer1; 06-01-2013 at 12:40 AM.