Is there any way to test to see if the petcock is alowing the proper amount of flow?

 

There's really no specification on flow. It's either on or off. Hooking up a mighty vac to the line which goes to the nipple on the BACK of the petcock will give you on/off testing. Make sure the petcock is turned on. Use an 8mm wrench and I think fully CCW is on. If you pull the petcock off there is a strainer attached to it. That could be clogged.

 

I mixed up the attachment on the petcock when re-installing the tank.

After finding and fixing the problem, I tried pulling vacuum on the petcock to sync carbs and it will not hold vacuum. Is it vented or busted?

 

Try a different hose and make sure connections are tight. If everything seems to be holding and you're still losing vacuum then the diaphragm is busted and you need a new petcock.