Carb Heat "Flapper" wear

[Bob Axsom]

After approximately 500 hours I discovered during the annual condition inspection that the the back face of the flapper valve and the underside of the metal top of the filter air box are worn about half way through. I do not know what the failure mode will be except reduction in power when that interface wears through but I intend to coat one surface with Dow Corning 736 (red RTV) at this point to reduce the wear. The hinge is also badly worn and I am replacing it with a stainless steel hinge. There is also a small crack in the filter air box metal top at the fwd L/H edge of the heated air inlet opening that I am working on my repair for (some form of doubler probably involving extruded angle stock). Strong temptation to do away with the whole carb heat system but conscience prevails - so far. Time to sit back and think elegant fix thoughts.

Bob Axsom

[kentb]

I wonder where all that metal went.
I assume that it is the black (smoker) powder and would stop in the air filter. What is the effect to powdered AL in the engine?
Maybe when it is that fine it just ignites and gives you more power.

Kent

[plehrke]

Originally Posted by Bob Axsom

The hinge is also badly worn and I am replacing it with a stainless steel hinge.

I had the same thing after 200 hours. Did what you are with replaced the hinge with SS. No wear since in the next 50 hours.

[Rick6a]

Originally Posted by Bob Axsom

..... .There is also a small crack in the filter air box metal top at the fwd L/H edge of the heated air inlet opening that I am working on my repair for (some form of doubler probably involving extruded angle stock)...

I just completed the first significant repair since "Darla!" first flew in 2005. At 250 hours, what can be loosely considered the "carb heat" function failed. A 2" long crack was noted at the bend on the FWD R/H side of the VA-130 airbox top plate. Especially noteworthy was the hole in the VA-122 arm was enlargened and the shaft on the Bowden cable bug nut was worn down from its original diameter too! I repaired the local area on the FAB using a combination of .063 angle stock and an .063 strap doubler sandwiching the fiberglass FAB in between the two pieces of repair stock.

I was not too keen to replace the steel VA-122 lever arm because to do so I would have had to virtually disassemble the entire FAB to get at it.

I decided to ream the badly enlarged hole to .280 which was close to what it was anyway, then press fit a .282 O.D. bushing into the hole. I installed a new Bowden bug nut using washers on either side of the "bug nut" and operation is once again normal.



While I was at it: I have always strongly disliked the stock carb heat muff. It is next to useless. Upon application of carb heat, I never could observe any noticable RPM drop so I removed and replaced it and I am glad I did. Upon removal, I noted that the "muff" left obvious wear marks on the exhaust cross over pipe and conceivably could have eventually worn through the cross over pipe necessating a truly expensive repair, not to mention potential carbon monoxide issues. Piece of junk! I replaced it with an EC-100 SS muff from Wicks. With shipping, it cost $68.43. The subsequent test flight revealed a definite 30-40 RPM drop now but I am still not quite happy. I reviewed the archives on VAF then called and ordered the well regarded 6" heat muff made by Robbins Wings. It's cost with shipping is $45. When it gets here, I'll quickly replace the Wicks unit and note any meaningful improvement in performance.

[Alan Carroll]

Originally Posted by Rick6a

...the hole in the VA-122 arm was enlargened and the shaft on the Bowden cable bug nut was worn down from its original diameter too! ...I decided to ream the badly enlarged hole to .280 which was close to what it was anyway, then press fit a .282 O.D. bushing into the hole. I installed a new Bowden bug nut using washers on either side of the "bug nut" and operation is once again normal.

Rick,

I had the same thing, and my fix was to drill another hole farther down on the arm. Now that hole is enlarging, so some more permanent solution is clearly needed.

Any other solutions to this problem?

[vlittle]

I just inspected this area yesterday, at about 250 hrs tt and there is no problem on my airplane.

One thing I did was fabricate a stiffener for the FAB that attaches it to the cranckase and limits vibration. It runs from the most forward attach bolt up to the crankcase.

Sorry, no photos. I installed it just before my first flight because I had heard that this was a problem area.

Vern

[MrNomad]

Rick:

My build partner insisted on the ROBBINS WINGS carb heat. He was right, it works great.

Barry
Tucson RV9A

[Vlad]

for good info.
Vern can you explain in general what kind of stiffener you installed. Material, appr lenght, what point on the case to attach? I am about to start working on my FAB, already have Vans carb heat kit.

[pierre smith]

....if you'll pull the cable housing forward in the Adel clamp to slightly bend the cable. It seems to stop any vibrating motion.

Regards,

[AlexPeterson]

I too had the hole wearing larger, so I put a piece of reinforced silicone tubing over the lever. The bolt now rides only on the silicone, so it can't wear the lever any more.

At around 1050 hours, the hinge is getting fairly worn though. I'll probably replace it in the next year or so. The flap itself is also wearing a little as Bob described, not sure what I'll do there. It might make sense to make the flapper from some non-metallic material, but I need to think about that a bit. Perhaps reinforced rubber sheeting backed by aluminum? Use the rubber as a hinge? Hmmm.

And yes, I will fall directly out of the sky because the stop nut is not engaging enough threads...