The petcock is the fuel valve on the bottom of the tank. Trace the hose on that down between the carbs. Where that connects to a "T" fitting. Inside that "T" is a very small filter that is easily clogged. Remove that and replace the line. Farther up the hose closer to the tank find a good spot and cut that and install an inline filter. If you read posts concerning the hidden filter you will find pictures and suggestions for an inline replacement.

Given what I gather about your lack of experience I suggest a shop look at your carbs for you. It isn't reall to hard to do and if you have a manual or do a really good read on Dinqua's (Pat) website and maybe even print out the pages you need you can do it yourself and you always have us for guidance. Maybe you have a mechanically inclined friend who could help too or as stated above if you live close to one of us we could pop over for a few hrs and help.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!