Hirth F23 twin has a redundant cylinder

After reading this, some of you will shoot me down.  It’s OK — I’m vowing to share what I know, no matter how embarrassing.

On this two cylinder engine, one cylinder is optional.

It seems like months ago — but in fact, it was only two weeks ago:  we had two days of glorious weather.

Our airport was alive with activity.  72 degree high temperatures (in January) brought out one helicopter, some gliders, along with their towplane, our ultralight, and a buzzing crowd of powered paragliders.

My crew was working hard on getting another plane ready for delivery; we’d just swapped a brand new F23 Hirth twin boxer into one of our Belites.  (This particular plane is headed down to Texas and Florida to be a dealer demonstrator on floats… stuff for another post…)  The F23 had already been running for a couple of hours, as we slowly cranked up the power and prepared the engine for first flight.

(Another person was performing the extended runup).

I did a careful preflight, ensuring that everything was OK.  The airframe had already flown, (briefly), but it would be a first flight for this particular brand new engine.

The electric start on the engine kicked it to life very easily.  A check on the 4 point safety harness, and I taxiied out to the active.

A quick runup, and I was accelerating down the runway — albeit slowly.  I shut it down and taxied back to the hangar.  Realizing that power was low, I quickly (and incorrectly) concluded that the propeller had the wrong pitch.  I had Gene fetch another with a better bite from our inventory of props, and a few minutes later it was torqued down and ready to go.

I taxied out again.  I applied full throttle, and the engine powered up smoothly.   Something was still wrong; RPM was quite low (at 800 RPM below full power); but the bird still wanted to fly, and I did to.  So I let it take off.

I made a couple of circuits of the field, enjoying the perfect weather, and also enjoying harassing the powered paragliders that were hanging around the east end of the pattern.  I sure wasn’t happy with the power, though – it just didn’t have nearly the snap I’d come to expect from our F33 installations in our Superlite models.

I landed the bird and taxied in.  As the bird rolled to a stop, I told Gene I still thought we had a real problem with the engine.

He pulled the top off each carburetor in turn, inspecting the linkage.

Well, it turns out one of the ball swages on the twin carburetor cables had failed (they’d passed a pull test in the vise!) and one of the two carbs wasn’t advancing past idle.

So I’d been flying with one cylinder.  With a F23 Hirth, the second cylinder is not necessary for a good flight.

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