I'm assuming you got gas?
In the prime position, gas should flow.
pull the lever straight back.
If it don't, somethings wrong

I'd remove and disassemble.
check the filter screen for clogging. It can varnish over and limit the amount of gas you're getting.
Next, 4 screws on the back of the petcock and peel the diaphragm off carefully.
Or get yourself a new one, raptor petcocks are a lot cheaper than stock. But are manual operation.