Hi,
I have a 1970's era 2 group Pavoni commercial machine with 110 volt and/or propane power. I am having trouble adjusting the temperature and maintaining stability with propane (with 110 volt power only, it works fine with adjusting the pressurestat). I plan to operate it exclusively with propane.

The gas travels first through a valve that has a large knob on the front that you push in to light (over-riding the safety mechanism) and then you rotate the same knob to increase or decrease the flame size. I am assuming that the purpose of this adjustment is to set the minimum heat level.

Then the gas passes through a second valve, which is the one I'm trying to understand. The gas arrives from the first valve on the right and leaves the valve to go to the actual burner on the left. This valve has a knurled adjustment knob on the front and a locked adjustment screw on the top. There is a copper tube on the back (larger in diameter than the safety thermocoupler on the first valve).

So I am guessing that the copper tube on the back runs to a fitting on the boiler so that this valve manages the gas flow to maintain a certain pressure in the boiler.

I am also guessing that the front knurled adjustment is the "fine" and that locked screw on the top is the "rough" adjustment.

I need to understand what the valve does, what the two adjustments do, and the proper procedure to adjust them, and test to see if they are working correctly. And if it is not working correctly, how to fix it or where to get a replacement.

I've flailed with the adjustment knobs, making changes each direction, but not seeing a pattern in the temperature I am observing at the group head. As I said, I was able to do this successfully with the electric power and pressurestat adjustment, but no luck figuring out how to adjust with propane.

Thanks!

Where does the large diameter pipe that extends back and to the right of the picture (behind the piece of frame) go?

Some systems (like the Astoria?) have a burner that is controlled by the water pressure off the bottom of the sightglass, similar to a pressurestat system.

If you're speaking of the copper pipe that runs straight back from the valve, I believe it goes directly to the boiler. Not sure yet if it goes to the top or bottom (it's buried under a lot of insulation I'd prefer not to disturb).

I wonder if that tube can get blocked and that might cause the valve not to function properly.

I'd bet it can, and it might be.

You could try unscrewing the pipe from the valve and see if water gushes out. If not, then it could be blocked.

Running descaling solution in the boiler might be the answer at that point; I can understand not wanting to mess with 1970's insulation. I'd either not touch the machine, or strip it down in a safe place with safety gear and remove all the insulation. Tough call.

Dunno if the valve itself can get scaled on the water side.

The Knurled nut contols the overall flame height and the screw controls the size of the flame on pilot status. The screw goes around and around without tightening...an eccentric. The pipe reads pressure from the boiler.

If there is a problem with the thermocouple the thing won't work...like any gas appliance.

The locked down screw, after loosening the nut, will tighten down to an end point. Can that be an eccentric? I thought an eccentric will spin and spin without end.

I opened up the tube connection that runs to the boiler from the problem valve and heated the boiler up. Water seems to run nice and clear through the tub, and spits steam without any trace of mineral or other deposits. So that tube does not seem to be the problem.

I heated up the boiler to operating temps (after refitting the tube back in it's proper place) and still do not seem to have any affect by rotating either the knurled knob or the screw that's locked down with the nut.

The first valve with the knob out in the front of the frame does adjust the flame size. Interestingly, when I rotate toward the far right, it lowers the flame size, and if I turn to the far left, it also lowers the flame size. The peak flame size is around the middle setting. I wonder if that is normal.

after you are done messing with it
I will just make sure that you have plenty of ventilation and you test the machine in a safe place
and by the way stay close to the main shut off valve and a fire extinguisher
been there done that

By your description the two screws are the opposite of the Astoria setup but the principle is the same.

Are replacements available if I'm unable to get this working properly? I'm wondering if the Astoria part would fit as a replacement.