Massive engine problem

"I fitted a new Pierburg pump today." I hope you don't mean the old gold fuel pumps. Those have been grounded years ago after numerous failures. The newest version Rotax ones will have a Rotax part number embossed on them (not Pierberg!). Also, they have a provision to attach a hose to the drain hole for leading fuel safely away in the event of a diaphragm rupture.

Have you done a differential compression check, both hot and cold?

Have you done a borescope inspection of the cylinders?

Some of your issues remind me of a stuck valve on Continentals & Lycomings, usually from lead buildup. A valve problem here could be from air in the lifters from an incomplete initial purging of the oil system. Since the engine and oil system started dry, I'd do another, very patient and thorough purge of the oil system. Before purging, I'd also pull off the valve covers and check for mechanical damage, like for example broken springs, missing spacers and such.

I've had an inlet valve fail on a 912UL and at anything above part load the vibration was terrible to the point where the plane beacem scary to fly and I had to land. At low throttle settings it was hardly noticeable. The fault was not detectable with a leakdown test, it was only after stripping the engine that the faulty valve was discovered. It was like the sealing area of the valve had been hammered flat, in fact it was so flat that the edge of the valve was sharp. I guess it stopped sealing at temperature with high cylinder pressure.

A borescope (if available) would be a quick way of eliminating cylinder/valve problems.

I’ve been following along with this and some very good suggestions have been made. Looking at the video the manifold pressure never exceeds 26 even at full throttle, so it appears the engine is down on power.  I was on board with the possibility of an exhaust blockage from a loose baffle, or a stuck valve, or a bad lifter. But those don’t explain the one instance when the engine stopped completely and forced an off field landing. That one instance points me toward a fuel or spark problem. 

Kevin Stewart wrote:

Assuming that you can create the symptoms on the ground, could you just stop the engine and inspect the fuel level in the float bowls? That might show if you have insufficient fuel flow.

Are the fuel tank vents operational, i.e. not creating a vacuum under full load?

Here's a question for some of the other forumites: what is the effect on the EGTs if the mixture becomes lean?

How I could do this? If there is a low fuel level during the vibrations and I stop the engine, the fuel will fill the chambers immediately. Or what you mean?

Chris

Sean Griffin wrote:

Chris

The way you describe this "vibration" could be resonance - occurring within same rpm range.

I still feel that this may be something to do with the propeller;

How long since the prop was dynamically balanced?

Has the prop suffered a strike - no matter how minour?

Does the prop track straight/all blade tips the same?😈

Because I can initiate the problem by using the HACman, I am quite sure there is a problem about the mixture. Too much air or not enough fuel. 

Yes the prop wasn't dynamically balanced ever, but it's the same prop like before the engine was overhauled and everything was fine. 

Chris