Ideas?

Just a simple test to see if the petcock works, get a piece of vacuum line (make sure it's vacuum line because regular fuel line etc may collapse under vacuum) put it on the back of the petcock on the "VACUUM" nipple, turn the fuel to "ON" or "RESERVE" and SUCK!!!!!
Put a piece of fuel line from the fuel nipple into any suitable container. If fuel flows then you know the petcock works.
A simple test to see if your petcock is buggered!

 

Oh yeah...^^ what he said!

 

Yeah, ... you gotta test that Petcock.
Applying vacuum to the Membrane and see is the Petcock is functioning right.

If you suck on the vacuum hose and taste a little bit of Gas, ... the Membrane is punctured and not opening the valve.

But, ... you should also look for:

The Petcock "Tower" clogged or even missing.
Non-Vacuum Hose in place of Vacuum Hose
A Fuel Filter of sub-standard manufacture; malfunctioning.
Low Level of Petrol in the Petrol Tank.
A kinked Fuel Line

 

Oh yeah...^^ what he said!

 

Ok, Sorry for the lapse. Will check vacuum on petcock.