I know a guy that always had issues with his petcock as well. He could try running it on prime but I don't think that's the issue.
I got rid of my stock carb as well. Put an old style FCR 39 on it.

 

Well here's the deal, when I first crank it up it'll start right up and run fine for about 1-2 mins, and then it just cuts off! and won't start back up unless you let it sit for while, and then again it'll do the samething....

 

that sounds like a cdi problem if it just shuts off then can start again after it sits for a while

 

sounds like a valve adjustment problem. it runs till the steel expands in the head then the valves are stuck open and it dies. then it cools down and does the process all over again. the ignition coil will do this too

 

When it shuts off is there fuel in the line?

 

I'm not sure I'll check it out tomorrow but I don't think there is any fuel in the line when it shuts off

 

so then its obvously a fuel related issue.make sure the vacuum line from the petcock to the carb isnt plugged, make sure carb is clean, and if all else fails clean the fuel filters and check the diaphram inside the petcock. hope you get it figured out!

 

Okay thanks I'll try those, man I hope I figure it out to because it just happened over night I got off the day before then when I got on it the next day this started... But Thanks

 

did you get it fixed yet? would be helpfull to know what it was...

 

anyone think about float level ? have had that happen those z carbs sit at an extreme angle if float level is off a little it will run till bowl empties