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carb heat question

M

martinair

Member
on my RV4 the carburetor heat never worked. The builder of that plane mailed: "it has always been a mystery to me, it never worked, but since I am living in California I didn't care too much".
Well I am living in Germany and yes, we do have winters.
We checked everything, the flap is working normally and correct. But the shroud around the exchaust- I believe it is too small and that might be the reason. I am asking for your opionions- what to do, probably I just order a bigger one from Van's?

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We need better photos to see exactly what's going on. It looks like the muff is just open which will let the hot air escape which will never work.
 
The muff is correct. The carb sucks in air through the opening in the muff, and that air is heated as it passes by the exhaust pipe.

How do you know your carb heat doesn't work? Did you have carb ice? Do you have a lever to actuate the door that chooses between fresh air and heated air?
 
Hi, thanks for quick answers.

I do know that it doesn't work because during run up checks, there is no drop at all in revs. Yes I do have a knob that operates the carb heat. And I had carb ice in descent prior landing once.
 
post pictures where we can see the whole air box and carb from the left and right sides. then post a pic looking into the front of the airbox with the carb heat off, and another picture with the carb heat on. That should be enough for a good internet diagnosis.
 
the last posting is the most interesting one:

"The Van's Aircraft construction and operating manual states that one method of building the
carburetor heat system is to 'run a 2-inch air hose from a heat muff and position it to feed into the alternative air inlet of the
carb[uretor] air box,' which is the method by which the pilot constructed the airplane. According to the inspector, the 2-inch hose is
not large enough to adequately supply enough heat to the carburetor to sufficiently melt the ice."
 
CATPart, thanks, will do- but next week. Aircraft is right now 1 hour drive away and I won't be able to make it earlier. Thanks.
 
The original RV series, RV-3, -4, -6, didn't even use a heat muff. It simply drew in air form the lower plenum. With this system, you will likely not see an RPM drop on run-up, but if you have a carb temp gauge, you will see a rise in temp. Many people didn't trust this so pretty much everyone went to the "muff" system. This is a very small muff so you still may or may not see an RPM drop, although it IS working.

As a reference, many L-19s also used lower plenum air for carb heat.

It's experimental, so you can use whatever you are comfortable with.
Point being, just because you don't see a drop doesn't necessarily mean it's not working.
 
Doesn't the usual Robins system take the heat off a crossover pipe in the front of the sump?

In the pic it looks like the muff is on a side pipe with a very contorted path to the valve.

Perhaps a more direct 90 degree elbow would work better to shorten the path and it's restrictions if a crossover pipe is not available.
 
I have a Vetterman 4 pipe on my -4, and fabricated my own muff installed on the L/H front pipe. My RPM drop is very little as others have said
 
The Heat is on...

martinair said:
I believe it is too small and that might be the reason. I am asking for your opionions- what to do, probably I just order a bigger one from Van's?
Click to expand...
Vie Gehts Martin,

I found in my RV4 while living in South Dakota that even with a carb heat muff on my Vetterman 4 pipe system, very little RPM drop was noted with heat applied. My RV4 was built in pre-FAB airbox times and utilized RAM-Air to the outside, filtered/warm air inside.
One tip I gathered in SD was to wrap a bed spring around the exhaust pipe inside the heat muff. It creates a better heat sink and greatly improved cabin heat as well. I rarely used carb heat except in visible moisture below 40F.

As Mel so eloquently mentioned, vertical sump Lycomings by definition and design slightly pre-heat the intake air as it's drawn through a hot oil filled sump with the Carb bolted onto the hot sump. This doesn't prevent Carb ice however, a Carb air temp indicator really helps identify icing conditions.
Unlike Carb equipped Continentals they are far less prone though not immune to carb ice.

Two options:
You could repair yours by closing the opening or: purchase one outright.

That said, here is a good one sold by Robbins Wings...
http://robbinswings.com/Carb_Heat_Muff.htm

Tcheus
Smokey
 
Last edited:
Hi Martin!

Lots of good info, here's one more data point:

I have the Robinswings muff rigged identically to yours, and I have a carb temp circuit on my EI engine monitor in my -4...it takes the guess work out of what's going on with the temperature inside the carb throat. The induction design, muff and alternate air intake do raise the throat temperature, but not always as much as I would like. I occasionally have to adjust power setting to keep things sufficiently warm. I have very little RPM drop with the application of carb heat during the ground run up prior to takeoff--you have to listen for it, it's so slight.

I've gotten in the habit of monitoring carb throat temperature during descent and landing...carb icing is definitely a concern in visible moisture when the temps are cool and the throttle at power settings below cruise.

Cheers,

Vac
 
I am pretty sure they have carb ice in California. They have it in Florida. The builder was lucky not to get bit.

My friend Julian has a 4 pipe exaust and he has 2 muffs on adjascent pipes connected by a scat tube. One pipe has studs welded to it for more heat transfer and on the 2nd one he put a dismantled stainless steel scrub pad in the muff for more heat transfer. He says it works well.
 
Carb Heat Muff

A very experienced RV builder and tech councilor in Oregon uses STAINLESS STEEL WOOL inside the heat muff wrapped around the exhaust pipe under the muff shroud. This makes a significant difference in the amount of heated air available to prevent carb ice.
 
Hi guys, I am impressed, thank you very much for all the helpful advice.

What we'll do: I am in contact with Rick Robbins and he'll make a 6" muff for me. Also, we are doing that anyway, we are installing a Dynon EMS D 10 digital engine monitor. This one has the carburetor temperature sensor.

I will report the outcome.
 
hi, quick update,

first one: I would never put stainless steel wool into anything which is leading into the engine. I just don't want to have these things sucked into my engine.

We installed the Robbins 6" heat muff and had a first test run & flight today.

The Dynon EMS D10 is cleary showing one thing:

when I pull the carb heat at 1700 RPM and the engine (exhaust) is definetly warm because the engine ran already 10 min: the RPM drop is zero. Absolutly no difference at all. I think I will give up on this.

We checked everything again and again.
 
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