Flow rate differences that develop over time are usually attributable to buildup slowing things down. There's no mechanism that I'm aware of that would serve to make a group flow faster.
Buildup-related issues develop based on usage. What is the usage pattern of this machine? Oftentimes on a 3 group one group will simply never be used, or in some cases one group is used for most drinks. This will impact buildup rate and differences in flowrate over time.
My guess is that there was a serious flow-related issue with the leftmost jet in the past and only it was replaced. Your tech should really replace all of the jets whenever they do a repair that replaces one, to keep the flow rates matching. Though it may seem like only one group has an issue in reality all of them have slowed somewhat over time - you don't see it because they may just be reduced 25% (for example) compared to 75% on the "problem" one. If you replace the 50% reduced one, suddenly it's running like new and the other group or groups will appear slow. I learned this one the hard way, btw.
Looking into the service history of this machine will give you more clues.
had the same probleme look like the ruby gigleur need to be clean
good way to see is run time how long does it take to fill up on shot glass on ea group . us the same portafilter no coffee no basket
Some great info there thank you Brady and Olivier.... many thanks
Hey all... this is a really interesting thread... I have a similar issue with a 3grp 2007 GB5 AV model... however, the middle and rightmost groups run at the exact same flow rate e.g. reaches required volume in 26 secs whereas the leftmost group runs fast e.g. reaches required volume in 19 secs. This is consistently the case, was thinking if it was scale it would cause one group only to slow down rather than speed up... wondering if it is still scale buildup why the two slower groups run at the exact same rate.
Any ideas what would cause the one leftmost group to run fast?
If I were a betting man, I would bet on the gigleur (jig-Lee-air) being restricted. La Marzocco does not recommend cleaning these (.6 mm is hard to clean without accidentally damaging) and recommends replacing the gigleurs on all groups at once. They are only around $12 to $15 bucks, and an integral part flow rate control. IF you want ALL 3 groups to have the same flow rate, replace the gigleurs in each group at the same time. This would also be a great opportunity for a group PM (cleaning, gasket replacement) as the group needs to be opened to access the gigleurs.
How often do you change the baskets, screens and gaskets? Do you change them out all together as is recommended? And do you have more then one set? Could you have mixed and match baskets from different sets? Cause that is the most common cause of inconsistent shots between group heads - a worn basket and a newer basket being used together on a machine. Also the baskets can get a hairline crack at the edge that is almost invisible and will cause the shots to pour fast.
Also, this being a EE/ semi auto, the second most likely cause is a leaking brew/three way valve. Water leaking down the drain during a shot will drop the pressure and affect shot time. Most people don't notice it cause the barista is controlling the volume, not a computer. In an AV the shot volume would end up short - cause it's metered water you are loosing - and is easier to diagnose. Sounds more like your problem to me as you mention wet pucks.
And yes you can clean out the restricter. In emergencies, where we could NOT take the machine down, we would remove the screen and screw, and blow compressed air back through the group head to clear it out.