If you have the original type vacuum petcock they don't like sitting for more than a few days.
As you probably know, the vacuum petcock has 2 valves, a manually controlled valve that works the same as a non-vacuum petcock, allowing you to select between ON, OFF and REServe and a vacuum valve that prevents fuel from entering the manual valve when the engine is not running.
The carbs on these bikes have a tendency for the fuel in the float bowls to evaporate and fast enough that if they are not used for a week or so the level can get low enough that there isn't enough for the engine to start. The problem is that the vacuum petcock doesn't let any fuel into the carbs unless the engine is turning fast enough to produce vacuum so you have to crank the starter long enough for the engine to supply enough vacuum to the petcock to open the valve for a long enough time for the float bowls to re-fill and it is pretty easy to run the battery down doing that.
You have a few options when that happens:
- If it has been sitting for a few days and you expect that it will be hard to start, before you touch the Start Button, vigorously whack the throttle open several times to operate the accelerator pump and squirt raw fuel into the carb throats.
- If it has been sitting for a week or more and whacking the throttle doesn't help there are 3 ways to fill the bowls:
1) Disconnect the fuel line from the petcock, connect a funnel and pour 90cc of fuel in directly.
2) Disconnect the vacuum line from the petcock (this is the small barb farthest from the petcock's main body), connect a short piece of clean tubing in its place and apply vacuum (with a vacuum pump or just suck with your mouth) for about 10-15 seconds.
3) Disconnect the drain/vent line from the petcock (this is the small barb between the vacuum line and the main body), connect a short piece of clean tubing in its place and blow gently into it for 15-20 seconds.
