Won't start, coughing through carb

I finally was able to try to get my 1982 GPz750 to start today. It's been sitting for about 4 months, not running. It ran like a champ before, but I had a bunch of electrical issues.

All that has changed (as far as fuel goes) is a new petcock. Today when I started to crank it, fuel was dumping from the petcock and backfiring through the carbs. It wanted to start, but just wasn't getting anything.

What would cause the carb cough? And how do I make the petcock fully close?

what kind of petcock are you using? the old one is vacuum fed. backfiring thru the carb sounds like maybe timing issue