Concentrate on the orfice plug in the regulator valve assembly. It is the screw driver headed plug right in center with cover off. If it has never been replaced you will see a roll pin with a steel ball under the pin. That little ball bangs back and forth and closes off the orfice hole. Also, if it is a later ball and pin style, that seat will be hardened. They still peen them closed but makes it real hard for a tiny drill to clean them out.
New orfice screen assembly did not have the ball and pin. Hole originally is .031 and , like said, usually drill to about .040. Too big, won't go on pressure, too small will surge or stay on high pressure.
Also, you will no doubt find the screen on orfice assembly collapsed. You can clean that screen by cutting off the end, cleaning it and then just roll the end up a little to close it off. Good share of the time though, screen is damaged. New ones are MADE OF GOLD according to the price.
That cover has a very heavy spring on relief valve. Remove corner bolts of cover and install longer bolts to prevent problems taking cover off and do same to reinstall. Regulator valve piston has a 0-ring and also a orfice that you should blow out. It is also spring loaded and should not be rattling around inside.