I start to pull shots out of my Gaggia Orione two group and I noticed a big difference in the shower output of both groups. The left one (from the barista view) seems to have a more powerful output meaning by the sound and the amount of water but the volumes and time are pretty close for the both of them.
Should I play with the valve at the back of the group in order to get the same from the both of them or is something wrong that I should correct
TIA

You could try adjusting the valve however consider also,

-that one of your groups pistons may have been replaced at some point (the original and newer Ascaso manufactured ones have slight differences)

-your groups and/or pistons may have lime scale buildup inside

-your groups may not be original to the machine and thus subtle changes in manufacturing over the years may account for the slight difference in sound

-or it could just be one of those things..

Thanks dmccallum for your input but I would like to know if there is a procedure I must perform to balance the both groups in order to have more control on the machine and gain more consistence
Thanks

There's not a procedure as such beyond ensuring your machine is clean and configured uniformly. Your boiler will be delivering a uniform pressure to both groups, so it's the config & condition of these that will determine the rate of flow.
If one of your pistons has limescale build up in side it for example then you must either address or live with it.

Another cause might be a difference in strength of the pistons inlet valve spring (you can find it on the pistons underside underneath the shower screen).
So quite a few variables and you have to investigate each if you want to account for why one group delivers at a different flow rate.

Have you taken the groups apart yet yourself?

After I bought the machine a former Gaggia technician toke care of it disassemble clean it replace some spare including piston gaskets, he told me that the pistons and coils was in good working order but since the machine is circa 1963 I guess pieces have been changed along the way.
I did not take apart anything else yet since I am a newbie here but thanks all the info in this forum I am getting more confidence and planning some work in the near future.
I will check the the pistons inlet valve spring to see is there is a remarkable difference and starting point.
Thanks

it's either the shut-off valve on the neck on the backside of the group (partially turned down or clogged), missing shims/washers between screen and piston (preventing the valve to open completely) or the level of the piston in the pre-infusion position is not correct.
if the piston is not raised high enough in the position where the catch snaps down on the lever shaft cam (holding it in its place), the intake holes in the piston skirt don't go completely past the seal lip. you have to raise the piston by turning the top group head assembly counterclockwise until you get the full flow rate. one turn equals 1 mm.
you can then synchronize the groups with the shut-off valves. i usually have them completely open. the actual purpose of the valves are for shutting down the group without necessitating turning off the entire machine (for repair and maintenance on the fly). that's what i do in my espresso bar.
the only parameter which changed from 1954 to 1975 is the intake hole diameter (besides thread size of the rack bushing). newer pistons have an one-way valve with an o-ring, the older ones don't have an o-ring.

Thank you very much cafebmw I will try your suggestions to see if I achieve some improvement in the operation.
I just want to minimize the amount of wrong things in order to have not a perfect shot just a better one.
Some update of the new setup