Carb heat hardware dance

scard · #1 07-02-2009, 03:51 PM

As many know, us carbureted RV drivers have a little rite of passage that we must go through. Usually the mantra of "build it like the plans say" works, but not in this case. I've been on this learning curve now for a little over 400 hours. There are a few basic, per plans, problems with the carb heat hardware setup. Not enough heat for my comfort, B-nut with cotter pin in steel bracket wears the hole severely for increased slop and eventual breakage, extruded door hinge wears the hinge loops for more slop and eventual breakage. All this conspires to make for one big sloppy, about to break, mess. To date, my efforts have focused on the carb heat control cable to door lever attachment. I removed the standard B-nut very early on at around 150 hours. That obviously wasn't going to work long term. I then installed a proper rod end bearing with a little attachment "pin vice" type thing to put it on the end of the solid wire cable. This has the effect of breaking the cable right where it goes into the adapter like clockwork at around each 80-100 hours of service. Duh.

I finally decided I was ready to try something else. This time I would address each of my misgivings with the setup. First, was to install a Robbins Wings carb heat muff to hopefully produce more heat. Along with that, I installed his airbox flange that has the sides closed off to only draw air from the heat muff. There have already been a couple of times when we were in the clouds in cooler weather that I really wished we had more effective carb heat. Next was to deal with the door hinge that was continuing to wear. While talking with Gary Zilik over a beer at his kitchen table (at 8000' and reduced O2 levels), I presented my little set of issues. Note that he has a lot of hours on his carbureted RV compared to me. He used a piece of baffle seal material riveted in place of the hinge. He also simply wrapped the control cable around the shank of a bolt for actuation. The ultimate in simplicity and many fewer parts to wear. So those are the solutions I'm adopting for this next step in my learning curve. I really like the looks of this installation and have high hopes.

click the pictures for bigger.

L.Adamson · #2 07-02-2009, 05:55 PM

The one item I get concerned about, is the flange with the sides closed. And I don't know if the concern is worth worrying about at all.

Years back, an RV owner that I know, had to make an off field landing onto a country road with "fence posts" in his 6A. Carb icing was suspected. The NTSB concluded that the Van's scat tube wasn't large enough to effectively produce enough heat. As I remember, this is the reason that Van's went to the flange with the sides open..........for more airflow. This particular RV had no flange openings at the time.

So what I'm curious about; does the extra heat the Robbins muff produces, offset the volumn of air, that the Van's setup was producing? With the same airflow, but with improved temps from the muff, it should. I suppose I'm just wondering how it's been tested.

L.Adamson --- RV6A

scard · #3 07-02-2009, 08:03 PM

Yep, eyes wide open here. I was aware of the change. Kinda' a problem each way. If you're sucking air through the sides, it isn't heated... There has to be a whole lot more heat with this muff. I haven't flown it yet so we'll see.

zilik · #4 07-02-2009, 08:40 PM

Wow, it looks just like mine and you never saw it. Just an explanation over a beer or two. Seems like the low O2 didn't slow the mind down a bit.

petehowell · #5 07-02-2009, 09:22 PM

Scott,

500 hours on my stock setup with no cable issues. The heat is good enough for LOP ops, but I don't fly in the clouds. My carb temp sensor says that I get do get a heat rise when I use it and I do get an RPM drop.

I wonder why my cable is not breaking. I may pull the cowl and take a good look at it tomorrow.

Great post - looks like a dandy solution!

Alan Carroll · #6 07-03-2009, 06:51 AM

Scott,

I'll be interested to see how your mods work out.

I've had to redo the carb heat control arm twice in 500 hours, due to enlargement of the "b-nut" hole. This may have happened because I set it up slightly tight when in the normal (non-heat) position (ie the cable is pushing the door up against the top plate when the knob is pushed full in). In the current iteration I've set it up looser, with the idea that incoming air will keep the door pushed up out of the way.

I think that certified planes require something like 60-70° temp rise (to compensate for similar temperature drop through carb). I mounted a temp sensor in the back of my airbox (though a hole in the fiberglass), and could see only about 10° rise with the stock carb heat setup.

N941WR · #7 07-03-2009, 09:30 AM

Scott,

Try using a Bowden cable wire end. They are $ 8.00 from Van's (part #: HW WIRE NUT KIT). They are much lighter and the vibration won't cause the cable to break like that.

Here's a picture of mine:

scard · #8 07-03-2009, 09:45 AM

Quote:

Originally Posted by N941WR

Scott,

Try using a Bowden cable wire end. They are $ 8.00 from Van's (part #: HW WIRE NUT KIT). They are much lighter and the vibration won't cause the cable to break like that.

Here's a picture of mine:

Yep, that is where I started. I called it a "B-nut". I didn't have a problem with the cable breaking, but the B-nut was wearing the hole in the arm at a very rapid rate.

N941WR · #9 07-03-2009, 10:06 AM

Quote:

Originally Posted by scard

Yep, that is where I started. I called it a "B-nut". I didn't have a problem with the cable breaking, but the B-nut was wearing the hole in the arm at a very rapid rate.

Can put a small iBolt on the lever and connect to that with the B-nut?

I did something similar on my rudder horns to keep the tailwheel springs from wearing.

PS. I think mine has lasted so long w/ the b-nut is because I simply don't use carb heat. Watching the carb temps go over 100 when I slow down to land w/o carb heat was good enough for me.

L.Adamson · #10 07-03-2009, 05:45 PM

Quote:

Originally Posted by N941WR

PS. I think mine has lasted so long w/ the b-nut is because I simply don't use carb heat. Watching the carb temps go over 100 when I slow down to land w/o carb heat was good enough for me.

Me neither. I'm not even sure that I've pulled the knob twice!

I learned to fly in Pipers, which had Lycomings. Unlike Cessnas which were using more Continentals at the time; Piper didn't recommend carb heat for every landing. It was only if you really think you need it. The Lycoming manual reads the same.

Secondly; I live in higher altitude mountain country. The potential of leaving carb heat on for a go-around is worse than not using it, because of a power reduction.

And third, since I know the potential of the cable connection breaking; as is being discussed; I don't pull carb heat everytime during the pre-takeoff runup.

As I'm doing my condition inspection at the moment; the carb heat cable still looks like original. Can't say the same about that auxiliary air intake on the bottom of the filter box though. Some rivets have loosened and the pro-seal has broke.

L.Adamson --- RV6A (flying)