Not sure... but I'd start by removing that (what looks to be) 1/8" pipe that goes from the valve to the bottom of the burner, then carefully open burner valve to see if you can determine if gas is coming out of that. (Please don't use a match to see in there, ha-ha....) Those are probably flare connections, but maybe compression fittings.
Next, check that tube you removed for any restriction: can you blow through it? As far as the orifice, I'm guessing that's either in that tube you removed, in the brass fitting that goes into the base of the burner, or inside the base of the burner assembly itself.
Needless to say, when you go to put everything back together, please use a soap solution on all connections to ensure they're gas-tight.
Sorry I can't be of more help! Please post pics as you go --