Hirth F-33, Not So Good Vibrations

I’ve run the engine about five hours and it has already managed to break parts from vibration. Above 3000 RPM she runs pretty smooth but below that the vibrations get progressively worse and quite bad. It vibrates so bad that it is uncomfortable, my compass will just spin in circles and trying to read the gauges is difficult. At 3000 RPM the vibration is acceptable, above 4000 RPM it is fairly smooth and only gets better as you go higher. But 3000 RPM is about 10 horsepower and is too high for idle, without brakes the airplane will roll along, even in grass. I suspect it may be too high for approach too. So I don’t think its possible to avoid the lower RPMs to keep vibration down.

The ignition coils were mounted on an aluminum bracket on the back side of the engine. That bracket has multiple cracks and in one spot completely broke.

Cracked coil bracket

Broken and cracked coil bracket

Broken coil bracket

When the bracket broke it allowed the ignition coils to move around. This resulted in one of the primary wires on a coil to chafe on it’s connector and break.

Broken primary coil wire

On the Hirth Engine Fanatics Facebook group I’ve read that some people have moved the coils and pulse pump to the fuselage to prevent failure of these components. I will also be making this modification.

I’m disappointed that the factory provided mounting method is so poor. Hirth should not have these components mounted on the engine or at the very least suggest alternate mounting methods in the manuals.