Since you changed the oil, then prime being left on won't hurt anything. The reason one should'nt leave the petcock on prime is the chance of the float needle not seating and leaking fuel into the engine, which would leak past the rings and fill up the crankcase with fuel. If the engine is run like this the oil is dilluted and won't lubricate properlly. Although there is an overflow in the float bowl that should allow the excess fuel to go out the carb and not into the engine, its possible. As for the not starting, did you store the bike like the manual says? Extended storage can reak havoc on a bike. Check battery,fresh fuel,spark plug, air filter(vermin), muffler outlet for obstruction, corrosion on eletrical conections. Did you leave fuel in the carb for extendid time? If so take the fuel bowl off to see if varnish has formed, if so remove carb and clean it. Get a can of knome-away and spray into every orifice to chase em out.

1) change the oil
2) Drain the fuel bowl (screw on the bottom ot the carb)
3) Take out spark plug, turn engine over with plug out to dry the cylinder.
4) buy new plug and install.  They're cheap ($2 to $3)
5) Battery?? If original battery, check fluid level.   If you did not have a battery tender on it over the winter, charge it.    Please note, if you have not used any type of winter storage charge methods to top up the battery, then a 3 year old battery the has gone thru 3 long storages like that may be on it's last legs......

If you do these simple things first, chances are it will run.   If it doesn't run, then start tearing things apart to clean.

KIS  - Keep it simple

I charged the battery, drained the carb, and it fired up right away!

Thanks a lot for the great help. You guys are the best.