Need help diagnosing a idle issue.

Bike: 1980 xj 650 maxim (in my signature)

Took it out to ride today for the first time since purchasing and noticed that when i start it, it idles fine but soon starts to die down and eventually shut off. After turning up the idle screw and riding for a bit the idle gets high when i stop and then again eventually dies down again. After it dies its really hard to get it to start.

-Bike will start on first try after sitting for a long time.
-valves are all in spec.

The only issue that i can think of is the petcock. On my bike im pretty sure its a different one from stock because the outlet on it is angle down right at above my carbs causing the fuel line to be pinched. The reason i think this might be a issue is maybe because when the bike sits for a bit it has enough fuel but after idling the kink in the hose makes it so its not getting enough fuel. ???

Thanks for any help

I can post a pic of the petcock issue tomorrow.

 

Clasic signs of vacuum leaks.
There are several ways to find them. My favorits is spraying a little starting fluid on the suspect area, when you find it the idle will increase. Spray one spot at a time and wait a little before continuing. Throttle shaft seals or intake boots will be the most likley areas, fuel vacuum shutoff valve is also possible.