Terrific information from Sir Ken W. for sure. And, I'm guessing something that most do not take the time/effort to do.
I migtht also add however, that some early models had a brass "limiter nut" found on the other end of the Brass Regulator Knob. This was employed to help limit the amount of pressure an Operator wanted their employee to use.
Carefully, as Ken mentioned, unscrew the brass knob off and see if you can identify the limit hex nut. If there is one, take off.
Anyways sounds like an easy fix.
The JAGUAR 6.6 --"A TRUE INDUSTRIAL SYSTEM"; not a generic squirt & suck!