I pulled all of the hoses and fittings between the tank and the lock-off-didn't see any problems. I never saw any filters or screens, by the way- After I emptied the lines I tried running it and cracking the valve as suggested earlier. It didn't run at all with this method. I pulled the input side of the lockoff and observed the actuator arm while cranking the motor, and the arm moved and opened the port, so I guess that eliminates lockoff issues, and of course vacuum issues.

I think it is the regulator; tell me if this sounds right: I pulled the propane hose off the model J (the hose that goes to the mixer) and cranked the motor and there was no propane flow. I can get propane by pushing the button, but if I get it running that way the button pushes out so hard that I can't hold it in.