I just bought a 99 savage 650 from a guy who had it sitting for over a year in his garage. when i picked it up he said everything was fine it just may need an oil change, he said he had put carb cleaner in the tank. it ran well until i filled it up with gasoline. i used 89 octane unleaded (which shouldn't matter). now when i start it up it runs for a little while then starts to backfire and die when i give it gas. if i run it on reserve it doesn't do this, i don't know if its because it is still running off the old gas or if its a problem with the valve or the carb. any suggestions? thanks.

*update, keeping the bike on reserve actually does NOT remedy the problem, the only way to keep the bike running is to leave the fuel valve on PRI

Both RES and ON require suction from the engine to open the diaphragm.  If it only runs on PRI than your diaphragm is not functioning properly.  When you filled the tank do you remember moving the petcock from PRI to ON?  I wonder if the original owner just kept it in PRI for this very reason and then when you filled it up you changed it to ON (as you should).

Upside down?

I sat here staring at the old petcock I have in a million pieces on my desk.  "Upside down?  hmmm, maybe, no.  Upside down?  How the he!!"  Thanks for clearing that up.  Now I'll be able to sleep tonight.