Starting GS500F 2007 after winter

[_eggdealer]

Hello, I'ts my first time posting here so apologies if i'm doing it in the wrong place. However, I wanted to ask on any help/info anyone might have about my gs500, I bought it recently and it's been sitting in my garage since November and I just attempted to start it and it starts on Prime and reserve perfectly well but when i turn it to On it dies after a minute and it wont start if its on the On position before starting it. The tank is like ... Under halfway full, could it be that? I just changed the battery and it works well so nothing of that sort.

[Armandorf]

On has a diaphragm that uses vaccum from the left carb to automatically open or close, check hose.
In new style gs petcock is rebuild-able with a kit.
Diaphragm is usually torn or dried rotten after sitting too long or 20years+