I've read as many relevant threads as I can find, but few mentions of changing the Gicleur-Jet on vibratory pump equipped machines.

They did seem to establish that vibe pumps MAY not flow enough for it to matter, but I'm not sure.
So I'd like to approach it directly.

My understanding is that rotary pumps flow many times more than the needed range,
and need to be "tamed down". (to oversimplify in the intrest of brevity)
Vibe pumps (my Ulka at least) can flow more than needed, presumably even with
the outflow from the OPV.

I also gathered that the effect of a somewhat smaller jet, is to "slow down and calm".
Among other things, this is said to be "more forgiving" of bad technique.

The reason for my interest, is that the puck is always pretty beat up on top, no matter
what other changes.... and there's a certain... "violence and a chaotic nature"
(best I can describe it) compared to many other machines I've seen during a pour.
Particularly the first several seconds.

The machine came equipped with a .80mm jet.
The maker (grimac/royal falcon) was pushing pods at the time it was made (1993/4)..
and I believe it was calibrated for them.
Also it came with pod "adapter" showerhead only, and a non adjustable OPV set at ~13bar.
(since remedied)

My pulls are inconsistant... fair amount of channeling to much of the time, and "spritzers",
in spite of a LOT of effort perfecting technique.
Lowering brew pressure to 8.5bar DID help.... but only to a point.

I'm convinced a "calmer/slower" process would be very helpful.

Would experimenting with smaller jets be inherently a bad or moot idea, or worth trying?

Thanks in advance.

Certainly worth trying. An unrestricted Ulka E5 pump, for example, will give about 110ml/10s, which is on the high side. With a 0.6mm gicleur the flow is about 60ml/10s (and I'd imagine it would be around 90ml/10s with a 0.8mm). I've switched to a gear pump now, but I successfully used the combination of a 0.6mm gicleur and E5 pump for some time.

Glenn...
Thanks much.... very helpful.
Gonna give it a try.

The pump is an Ulka EAX5.
I only found .5 and .8 jets in the type I need....
I have on order 2 units of .5, and 2 jet drills, in .711 and .635 (out of stock on .609)
(If anyone needs the source, feel free to contact me)

Think I'll try the .635 first... based on your comments.

I'll post on the results when I have some..

Maybe the pod adapter shower head on your machine might also be giving you inconsistent results when using ground coffee. You could swap this for a regular shower head?
Cheers.

A 7 mm nut driver (or socket) loosens the existing jet. 1/4" ID flexible plastic tubing fitted with a 3" length of a 1/4" wooden dowel will "slip fit"over the hex head of the jet to make removal and replacement much easier.

Jet sizes are not that exact - the "standard" jet for an E-61 group is 0.70 mm and the actual size can vary by +/-.05 mm from measurements a friend once took. Why not just start with the purchased 0.50 mm jet?

BUT, I'm heavily leaning towards what Gor said re the pod adaptor dispersion block/screen as being the culprit of your brew problems.

Gents...
I apologize for not being clear.
The pod adapter was removed long ago.
Kind of a funny story too:
Back in 1993 when I bought the machine, neither I nor the seller thought to look up inside the group.
As I'd mentioned, Grimac was pushing pods at that time, and the machine came
only with pod adapter.
First time I tried pulling a shot, it was a mess. More like internal combustion than extraction.
Seller immediately gave me a standard E61 distribution screw and showerhead.

I mentioned it only due to my speculation that the entire system was calibrated for pods.
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Eric..
The 1/4" tubing and dowel tip is very helpful! Thanks!

The reason I didn't choose to start with a 0.50mm jet is that in reading other threads and talking to a friend with a rotary pump machine who'd tried a 0.50 jet
had said it was to much.
Hence, I selected a 0.70 and 0.60(ish).
I'll try the smaller first, to maximize the effect and see.
*IF* I like the result, I'll get a hold of a 0.50 jet too, and try all 3 eventually to experiment.

Thanks for the replies