Massive engine problem

Hello community :)

I have a CTSW stationed in Switzerland with a 912 ULS (5651087), which was completely overhauled (incl. gearbox and carburettor) at 2,200 hours about 2 years ago. It was a private engine builder, not a Rotax-certified company. Until then, the engine ran perfectly without any problems.

After a few test flights, the following occurred: At almost full throttle, the engine suddenly started to vibrate (clearly noticeable, 30-40 Hz). I took the throttle out completely 2-3 times and gave it back again and the problem disappeared for several months or about 25 flying hours.

It reappeared nearly 1 year ago with a total of 30 hours of flight time and did not disappear so easily, I recorded the following video of it in flight:

https://www.youtube.com/shorts/wEDteyF-ths

It occurs here from just above half throttle and starts suddenly, the threshold at which it starts was always the same here. When the throttle was reduced below this threshold, the vibrations remained. Only when the throttle was idled they disappear, then I was able to accelerate again up to the same threshold without vibrations. You can't hear the vibrations in the video very well, but you can clearly see it in the vibration of the throttle lever. The whole plane also vibrates noticeably.

Then a new test flight and it occurred immediately after take-off at full throttle and even repeatedly easing off the throttle did not make it disappear. Then the engine stopped completely, started up again after a while, stopped again... This was followed by an emergency landing in a field without damage. 

Checks in the hangar:
Float chambers were both full, all floats perfect in terms of weight, fuel filter free, fuel comes out of the line in front of the pump after loosening it, so it doesn't seem to be a fuel problem.

Both ignition boxes replaced, no improvement.

Both carburettors run evenly from idle to full throttle without hitching, the bowden cable connection turns freely in the lever.

Synchronisation checked pneumatically, OK.

Gearbox spring tension checked in both directions, all OK.

Ground connections measured using the 4-wire method (1 A measuring current), total motor-battery resistance 0.15 Ohm, all connections checked.

I have installed an HACman, I have removed it, no improvement.

I have replaced both carburettors with a pair that work perfectly, no improvement.

Carburettor flange checked, they are in perfect condition.

After I did all this the problem disappeared at about May last year.

I thought that I had accidentally eliminated the fault during all the tinkering, but now it's back. In last November, a completely normal take-off and suddenly the same thing happened again during the initial climb. I was able to get back to the aerodrome with a shortened circuit and packed the plane away in the hangar, completely frustrated. I then went on holiday for 2 months so that I was only now able to deal with the problem and, to be honest, only now found the motivation to do so.

The error is now practically permanent, which is a quite positive thing to finally eliminate it.

The following is very interesting and should be enough to perhaps ring the bell for someone here:

- The error only occurs at low altitudes

- The error only occurs at low temperatures

- The error can be provoked at will by leaning the mixture

On the first point: I have never been able to observe the error above 3-4000 ft.

On the second point: The error first occurred in the cold season. Then during I was trying to figure it out it got warmer again in the meantime and the error disappeared completely. Now it's cold again and tadaaa, there it is again. Is that just a coincidence? I don't think so.

To the third and imho most exciting point: I have installed an HACman. Before anyone points the finger at it: No, it's not the cause, I've already completely removed it twice and the error occurs in exactly the same way, as it did again today. BUT: I can provoke the error pattern 100% exactly when it is installed! To do this, slowly reduce the mixture during a test run at the ground at WOT and it ALWAYS occurs, it is exactly the same vibrations. It can also be provoked in summer in this way. In flight you should only activate the HACman above 5,000 ft, which is what I do, if I activate it below the vibrations turns on. If I leave HACman active and descend slowly from over 5,000 ft, the vibrationes starts again at around 4,000 ft. However, this was not the case before the engine overhaul, when the HACman had no such influence.

So I think it must have something to do with the mixture.

What was done in this according to the mixture:

- Checked the restrictor (0.35 mm), which is installed

- 2 other carburettors were fitted, without modification

- I have installed a fuel pressure sensor. Here are the numbers:

Idle: 0.37 bar

4.000: 0.36 bar

4.500: 0.36 bar

5.000: 0.33 bar

5.200: 0.29 bar

5.400: 0.28 bar (here the lever is approx. 20 mm before the end stop)

5.500 (WOT): 0.24 bar

Today at WOT the vibrations starts, sometimes immediately, sometimes delayed by several minutes and almost impossible to provoke. When it does start, you have to go to idle to get it to go away, then you can accelerate again until the next time.

- I fitted a new Pierburg pump today. This pump brings significantly less pressure (0.22 bar) but it is still in the green range. The error pattern hasn't changed, although I have the feeling that it was worse with the Pierburg pump and started more quickly. But not sure about it...

- Today I also removed the two big intake hoses from the carburettors so that they can breathe 100% freely, no improvement. There was fuel in the hoses from the shaking before...

That's the current status. Well, who can guess??? ;)

Honestly, I'm close of buying a new engine.... :((

Greetings from Switzerland
Chris