912 ULS no more than ~4800 RPM

Well Sean, I think that the things fail when they fail and if they don't fail, they would be fine... if they were fine.. I'm just following the same line of your analysis. 

It's evident that something changed but at least not because, I have changed something intentionally like the pitch on the prop.

True Carlos - I knew what you meant - just being pedantic?.

An engine that fails to acheive its usual rpm, is possibly experiencing one or more of the following:

Air restriction

Fuel supply deficit

Fuel quality /contamination

Ignition problem

Throttle mechanism has changed/slipped (not opening fully)

Choke mechanism is failing to open fully (over rich air:fuel mixture)

There are other possibilities, eg loss of compression but the above, in no particular order, likely covers the most common causes.

My advice, as always, is start with the simplest/easiest/ usually cheapest possibilities.

Thank you all for your comments and ideas.

As I mentioned at some point, I hope I did, I'm not a mechanic.

I have received the plane today from maintenance, I haven't test ted it yet, but I wanted to share what they did just to see if, on your expert eyes, this makes sense.

Work done: Carb-sync. 

Again, I'm not a mechanic but, I understand the basics of a carburate combustion engine and I don't see how this can fix the problem. I never noticed and engine roughness and I think that that is one of the indications to identify if a carb-sync is required even if the compensating tube can help hiding that. The only thing I noticed is that the engine, constantly was no giving more that ~4800 RPMs (of course, small fluctuations) and, seems to me that the last 1/3 or 1/4 of the throttle is not doing anything (no RPM changes). By the way, that day the engine almost quit when I was vacating the rwy. 

I was expecting a carb refresh, a hole in the carb-diaphragms or something more based in observation. I can't see how, using a basic logic, suddenly the engine changed their regular ~5100 RPMs on take off to a constant and roughness free ~4800 RPMs and the reason is a problem in the carb-sync. I'll go today afternoon to test the plane again...

Do I'm suffering some kind of paranoia?

Carlos,

Sean has given you a good list of items to look for.

A carb sync can reveal more than just the balanced condition.
What was the MAP at idle? <10" ?
What was the Idle RPM?  >1800 ?
What was the MAP at WOT?  >27" ?

It is nice to know that both sides are the same, but the Actual values matter!
The devil is in the details, and the details can tell the story.

Thanks Bill, 

I’ll check those numbers today and I’ll double check the Sam post. I’ll be updating the thread later just in case you guys have some time to check ☺️