So since you bought it was running fine for a while, than started acting up.
For start, I would first drain the tank 100% and remove it, fill some gas in it, splash and flush, then take the petcock out and check for dirt on the strainer. If you can, switch to PRI and blow some air through it direction opposite the fuel flow. While you're at it, replace the vacuum hose as a good measure (look posts above). Make sure it sits tight.
Also, if petcock can be taken apart, do it. Check for cracks in the membrane, to cover everything that can be done on the gas tank/petcock.
Assemble, fill up, switch to PRI, wait 20secs, switch to ON, test drive, scream for help if it doesn't work

Again, that's just me.