angel

[HansLab]

flew some 30 minutes. Fairly icy conditions, so pull the CVV (carb heat). Suddenly the engine shook as h..%$%$..ll and I lost RPM. Quickly looked where the wind was coming from, and were to land if necessary, while closing the CVV. Sound and shake stopped. Without further delay flew some 10 minutes to the next airport. In landing pulled the CVV again, and there it was again: sudden shake (very, very much), and loss of power. Could land safely, though. Had an angel on my shoulder..
afterwards checked the engine and it sounds like heavy disbalance: as if one cylinder is totally flat.
Today I dismanteled the engine together with some friends, and I'd like to share my experiences:
After taking the cylinder head off, it became clear the outlet valve wasn't working, and was stuck in 1 mm open position. Loss of compression cleared. But why - and why in such a sudden. And what was the relationship with the CVV?
after carefull search we found the following:
A ½" piece of 1 cm tube from the carb-heat system was the problem. These tubes are used to center the exhaust pipe in the carb-heat tube. There should be 6 of those little b...s in the pipe.
And I only got 4 left. One got loose during flight, and was sucked up in the carb-heat box. After that it was sucked into the airhose to the carburetor, through the carb, into the cylinder (yes, through the inlet valve), took some time there (the 10 minutes the engine looked OK again), and tried to leave through the outlet-valve. There it got stuck, and I lost power and experienced a heavy disbalanced engine.

We removed the piece, and looked at each others shoulder for the angel that should've been sitting there. Not only could we land safely, but the 1,5x1,5 cm piece of aluminum visually did not damage piston, valves, rubber carbframes or anything else. Not even my confidence, for I really like this engine and hope it will stay with me and Foxy a long time.

This week will bring thorrowly looking at the cylinderhead and valves, then build her up again, testrun and fly back to her own hangar.

In my annual a throw check for all rivets has been written down. And some comfortable chairs to put onto my shoulder. For my angel.

[TahoeTim]

I'm glad to see it worked out well but i would toss that carb heat system. That is a horrible design on so many levels. Eventually you will have holes in your exhaust pipe from rubbing.

[Esser]

Glad to hear you are ok. What engine was that on?

[Dave S]

Hanslab,

That was exciting!

At least, in retrospect, it probably would not have quit on you....just the wild vibration & approximately 25% power loss.

I was considering installing a carb heat of this design...so far all I have done is stare at the parts and not put them in the plane....I am now considering the carb heat apparatus of this design might be a much bigger problem than what it is intended to solve that doesn't seem to be much of a problem on a Rotax anyway. Probably better to stare at the parts on the bench than use them in the plane

6 years of flying without carb heat......the old 912 never misses a beat. Currently running outside cold air for intake into the Rotax Airbox - I use the carb heat intake on the air box as an alternate air intake from under the cowl behind the engine (it has a screen over the hole).

Thinking that the electric carb base heaters might be a better call if a person is required to have carb heat due to regulations or personal preference.

Sincerely,

Dave S
KF7 Trigear
912ULS Warp Drive

[Mcslamma]

I'm a fairly new Kitfox owner, and I've wondered about those little tubes and the rivets that secure them, I have locking wire to keep the muff from vibrating, but I know some rivets are slightly loose, and have been keeping my eye on them, but I think now I shall put a screen, not a filter in the Carb heat box between scat tube and box for carb heat, just in case of such an event. I'm glad you got her down safely and shared this with everyone.

[SkyPirate]

I'd drill a little hole in that piece of debri and make a necklace out of it..the bullet that didn't get you

[bbs428]

Close call. Glad it all worked out.


all the best to your angel.

[HansLab]

Thank you for all your thoughts.
Wanted to share the solution with you:
We've had the cylinder head fixed (cleaned, valves treated), ready to build in next sunday.
The carbheat pipe around the exhaust is such a Kitfox-like item: I sure wouldn't miss it. So, we took out the little centering-tubes and put in tubes that are so long that it'll be impossible for them to take the curve that is in the carbheat pipe.
next to that I'm thinking about some kind of metal net in front of the carbheat-box. Haven't figured it out, though, 'cause you want to stop pieces of rivet. And still have some air going through...
However when I think of it: these carbs sure suck the h..ll out of it, so a fine-mazed net might do the job, as well.
Will do some experiments when back on my home field. Keeping you posted.

[Spook712]

Hello Folks,

I'm feeleing really bad, because I had the exact same thing, only that the part justmade it into the carburator, not the engine, luckily.

I had a resistance in the throttle, so I started investigating and fount the tube already on the engine side of the carb !!

I use a fine mesh at the carb heat box that prevents any debris from entering the box from the carb heat site.

Only thing I'm concerned of is the rubbing on the exhaust.

Greets from Germany,
Tuna

[Paul Z]

Your poor Angel is going to have to go in for wing therapy, that poor Angel will never be the same.