There is a nut of some sort right behind the lever on the neck.

What's it for? Does it regulate the speed at which water enters the group with the lever group perhaps?
How do you adjust it?

Can it be adjusted on the fly?

Yes it controls the flow to the group, from a trickle to a shower. You just turn the slotted screw in to decrease and out to increase.

Primary use is to cut off the water flow to one group on a multigroup machine so it can be serviced while the other groups remain operational. I notice that this valve is removed on the Londinium levers.

It is only on dipper style groups. The thermosiphon style groups don't have it.

The range of effective adjustment is very small.

That makes sense. I played with it when I first got the Sorrento. Took a few measurements at different settings but ended up putting it back to the factory setting.

JohnB. wrote:Primary use is to cut off the water flow to one group on a multigroup machine so it can be serviced while the other groups remain operational. I notice that this valve is removed on the Londinium levers.

I believe Gaggia did this originally. Certainly made individual group servicing/troubleshooting a lot easier.

Thanks all, we played with it and got it to a good setting. The water was pouring out way too quickly; it is now a nice steady flow rate.

JohnB. wrote:Primary use is to cut off the water flow to one group on a multigroup machine so it can be serviced while the other groups remain operational. I notice that this valve is removed on the Londinium levers.

pizzaman383 wrote:It is only on dipper style groups. The thermosiphon style groups don't have it.

The range of effective adjustment is very small.

Right so I must be imagining it. Sure it cuts off flow but also regulates flow as desired. A quick and dirty video starting with the valve closed and then I open and close it a few times.

Paul_Pratt wrote:Right so I must be imagining it. Sure it cuts off flow but also regulates flow as desired. A quick and dirty video starting with the valve closed and then I open and close it a few times.

Yep: We used it to regulate the flow, the adjustment is quite finicky but we got it from puck thrashing fast to a very nice shower sprinkling.

Paul_Pratt wrote:Right so I must be imagining it. Sure it cuts off flow but also regulates flow as desired. A quick and dirty video starting with the valve closed and then I open and close it a few times.

Obviously you can use the adjustment to play with the flow rate but how many people do that? Most folks are buying complete lever machines not groups direct from the manufacturer. Hopefully the company building the machines at least knows enough to set the flow rate properly. When I asked Roberta Bosco about the adjuster she said the only time their customers mess with it is when doing maintenance. I'd still consider that is it's primary use once the group is set up & installed on a machine. I played with it anyways & found that Bosco's setting gave the best results so I haven't touched it since.