914 full power loss in flight

Sure sounds like a fuel delivery issue.

That was a thought I explored, but after a lot of time spent with the fuel pressure regulator circuit on this engine, I feel like this isn't a fuel delivery problem. The fuel pressure sensor is at the regulator, and the only thing downstream of the regulator are the two carbs. Since the engine didn't run rough, whatever happened affected both carbs equally. My conclusion-- perhaps wrong-- is that if it were a fuel delivery problem before the regulator than the pressure gauge would have shown this, and if it had been after then the engine would have run unevenly.

Also, wouldn't the manifold pressure still increase as I opened up the throttle, even if I had a fuel delivery issue? From the brief seconds I looked at it in the air, it seemed to me that the MP dropped to at-or-below idle MP every time I asked for more than a bit of power.

The motor requires air and fuel. If it stumbles when opening the throttle, giving it more air, it’s not getting enough fuel. If it smooths out reducing the throttle it’s because the air/fuel mixture is right for combustion, less fuel wants less air.

@Sam that's a cogent point. I was showing positive fuel pressure, and I know from experience with this plane that the engine will fail due to upstream fuel starvation only when the gauge shows 0 fuel pressure. Is there a way that both carbs could simultaneously stop delivering fuel? Maybe if the choke flopped open or a compensation tube cracked?

Or maybe the new gas was contaminated? The plane has a ~6L header tank, so if there was bad gas 15 minutes of 100% power could be just about the time that the header tank would have been completely refilled from the main tank.

Problem found. A pressure hose had blown off the carb. This instantly caused both carbs to lose their float bowl pressure, which in turn meant the venturi air was higher pressure and thus forcing its way into the carbs, instead of carb fuel going into the venturi. That explains the immediate and total loss of power.

I think what had happened is that when the mechanic replaced the pressure hose, the new hose was very, very tight, and so (I'm guessing) the thinking was that no hose clamp was needed. However, as the engine compartment heated up, the hose got softer and softer, reducing its grip. At the same time, with increasing altitude the difference between the hose pressure and ambient increased. The ultimate outcome was the hose couldn't hold on as hard at a time when it needed to hold on even harder than before.

Hose reseated, this time with a hose clamp. Engine performed flawlessly to 13,500'.