At what pressure does it prenfuse ??
if its dipper = what boiler pressure
if its a hx feed from line , how much pressure on the machine

if its to low , you want be able to press much water into the cylinder , but still it would be plenty to flush big amounts of water , due to no resistance

It is not a dipper. I think the pressures is about 1.1. But it seems that even if the pressure was a bit higher It wouldn't gulp more in as it is pre-infusing. I did test the water coming into the portafilter and it was just below 200. I saw a video on orphan espresso about the CMA groups and there is an adjustment to regulate the speed of the water going to the portafilter. I might try that but I'm not worried about the speed as much as the volume. How many ounce shot is usual with one pull of a commercial lever group with a 58 mil basket?

I would try the adjustment. Speed may be the wrong term, in just that it allows for higher pressure, which would equate to a higher volume with more air displaced or compressed.

I'm not sure what's typical, but looking at the size of the group, you should be able to get close to a 2oz shot I would guess. There should be plenty of water to push through before the shot blonds with typical coffee dosing at your 14-18g. Have you checked your water line pressure? It could be that this machine uses line pressure to pre-infuse and that your pressure is low.

I may be fighting a similar issue on mine. I just rebuild the lever assembly but I am not getting the shot volume that I was getting previously even when pulling the lever multiple times. There was a small imperfection in the new seal which may be causing an incomplete seal.

The good news is I have spares and should know soon if it is indeed the seals.

Replacing the seal seems to have fixed the issue. Since 1 ounce is typical, I am seeing 1.5 ounces after two pulls.

Since the group was compared to a Zodiac group, a Zodiac group can do a roughly 2 oz shot with just one pull. If you are only getting ~1oz, then there is still something strange going on...

I'm still at the same place with my machine. I can't see how outside water pressure could be an issue since it only happens when the boiler is being filled. The "gulp" opens the group up to the pressure of the boiler his has nothing to do with line pressure as far as I can see. But maybe I can't see far enough.
Any other ideas are welcome.
thanks again.

On my President (with Zodiac groups) right behind the group head there is a little nut that controls the flow of water into the group. If you tighten it all the way you effectively choke off water from the group. I couldn't see from any of the angles of your groups, but its possible they have that nut or a similar mechanism that may be restricting the flow too much?

Doug from Orphan Espresso briefly mentions it in one of his videos explaining how the group on a Faema Lambro worked: https://www.youtube.com/watch?v=c6CWzFDHKXM