Don Maxwell
By Don Maxwell
Contributing Editor

Mooney Airplane Pilots Association
MAPA LOG, Published MAPA, Volume 24, Number 4
April 2001

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Ed. Note: We asked Don Maxwell to give his comments on the proper operation and maintenance of carburetor heat systems on Mooney models M20C and M20G. Don maintains lots of these airplanes in his shop and he sees all the problems with carburetor heat systems. We're having a high number of carb ice incidents this year. We were wondering if maybe the problems could be mechanically related - are the carburetor heat systems working properly in the majority of the fleet? We think the answer is no. As pilots, when we pull out that carb heat control, we assume the system is working properly to deliver the correct amount of heat into the carburetor. In many cases it's not and there is no way to melt ice if we get it. That's' bad. Here is what Don had to say on the subject of carburetor heat systems in Mooneys. The bottom line is this - get your carburetor heat system checked out soon by someone who knows how the system is supposed to work. Your safety could depend on it.

This article actually came about after a call from Bob Kromer. Bob mentioned that the Mooney community had experienced a high number of carburetor ice incidents this season and that the NTSB had called with some concerns. Bob asked me if it might be possible that the problems could be mechanically related. In other words, maybe the pilots were doing the right thing in the cockpit (pulling on carb heat ) but that the systems weren't working as they were supposed to. Bob mentioned that in the course of writing an article for a previous LOG on car ice, he had taken a look at the carb heat system on a randomly selected C model and found it missing some important pieces. Could this be a common problem with the Mooney fleet in general?

At the time he made the call, we had two “C” models and one “B” model in for annual inspections. I went out and took a look at the carburetor heat systems on these three planes. The Mooney’s we had in for annual had the three different carb heat systems that have evolved over the years.

The carb heat box on the “B” model is fairly typical of other aircraft types. One of the first things we learn as new pilots is that the purpose of a carburetor is to mix fuel and air into a combustible mixture. Unfortunately, while doing this carburetors also do a pretty good job of making ice under the right conditions and circumstances. To combat this problem, aircraft manufacturers have devised ways to introduce heated air into the carburetor to prevent and remove ice that forms inside. Over the years, several different variants of carburetor heat system have been installed on the carbureted Mooneys we fly.

The three Mooneys in my shop had three different variants of the carb heat system that had evolved over the years. None were the same. On the '61 model, the carb heat system is fairly typical of those found on other general aviation airplanes.

Intake air for the carb heat system is taken in at the back of the “doghouse” baffle around the engine. This air (not hot yet) is routed through a flexible hose to a shroud surrounding the muffler. The air is then heated as if flows past the engine's muffler inside the shroud. Once heated, the now hot air is ducted to both the carb heat valve located on top of the carb intake and cabin heat valve located on the firewall. Select carb heat and the hot air goes into the carburetor. Select cabin heat and the hot air goes into the cabin. Select both and the hot air goes both places.

On the “B” and early “C” models there was a problem with this system. If neither cabin heat or carburetor heat was in use. there was no place for the heated air to flow out. There was no "bypass" valve allowing for good circulation around the muffler with out carb heat or cabin heat selected. The extreme heat of the un circulated air around the muffler could cause muffler damage. Photo1 shows a distorted muffler and heat shroud caused by this condition and its something to watch for on your B or early C model.

The damage to this particular muffler wasn't related as much to the design as it was to an unapproved repair. The muffler was originally a Hanlon-Wilson studded type. Unfortunately, one side of the muffler was replaced in the field by the local Midas franchise. That's a no-no. The muffler was then distorted by uneven heating. This is really dangerous. You can't jury rig repairs like this. Cabin and carb heat comes from air circulated around the muffler. A crack or break in the muffler due to an unapproved repair can kill occupants due to carbon monoxide. A large exhaust leak under the shroud can cause the engine to run very rough when carb heat is applied. All exhaust systems are repairable, no matter how bad the damage. Companies such as Aircraft Exhaust Systems and Dawley can repair damaged mufflers correctly and properly. Go to them - not Midas.

To remedy the potential problem of muffler damage due to excessive heat buildup inside the shroud, later C and G models (from '62 to '68) incorporated a bypass valve located on top of the carburetor heat box. This valve allowed for the hot air circulated around the muffler to be vented overboard when neither cabin heat or carb heat were selected on. This arrangement consisted of two valves connected by a linage and operated by the carb heat control in the cabin. In normal operation, the carb heat valve is closed and the bypass valve is closed, keeping all the hot air directed into the carburetor.

That's the way the system is supposed to work. Unfortunately, over years and hours of vibration and heat, the system tends to develop problems. On the B model in our shop, the engine mounts were sagging, allowing the carburetor intake scoop to rub on the bottom cowling. The scoop was worn enough from this interference that the carb heat valve was not sealing properly when car heat was selected (see photo #2) Even though the valve was in good condition, without a seal surface the car heat on this Mooney M20B would not be effective if needed. Avery bad situation. The early C model in our shop incorporated the Mooney design of the time that had the bypass valve on top of the air scoop. This system design require the most maintenance of all carbureted Mooneys for the carb heat to work properly. Photo #3 shows the carb heat system on this early C model before being removed for inspection. Photo #4 shows the wear we found to the carb heat valve shaft. The shaft was worn out. Another bad situation, allowing for partial carb heat at best.

Photo #5 shows in detail the bypass valve and bearing sleeve we talked about earlier. Remember, this valve allows for hot air to be ducted when neither cabin or car heat is selected, It is not uncommon to find this valve missing or even plugged off with a cap. On several occasions, I have found where some mechanic decided to attach hose to this valve and route the hot air to such places as the magnetos, the generator or the fuel pump. Can you imagine that - to the fuel pump! The correct thing to do with the hot air coming out of the bypass valve is to attach a short piece of tubing to it and to vent it overboard as shown in photo #6.

Photo #7 shows the car heat components removed from our early C Model disassembled on the workbench. It is obvious that many of these components have been repaired in the past by welding and brazing. Owners are under the impression that these pieces are not available any more, so they undergo these home repairs .These are normal stocking parts for any Mooney service center. Check with them for availability and price before resorting to home repairs. It's your neck if you need carburetor heat and the system doesn't work properly.

In 1968, Mooney removed the bypass valve from the top of the intake scoop and moved it to the firewall at the heater control valve (photo #8). This was the configuration we found on our third airplane. This configuration the troublesome linkage arrangement seen on the earlier M20C models. With this arrangement, the bypass valve was now controlled by the cabin heat valve. During normal operation the cabin heat valve is closed and the heated air from the muffler shroud is vented overboard through a hose from the cabin heat box. When cabin heat is applied, the valve opens and directs all the heat from the bypass valve into the cabin. on our third airplane to inspect, everything seemed to be in good working order. But two out of three randomly selected carbureted Mooneys in our shop had problems with the carburetor heat systems. Problems bad enough that partial to no heat was being routed to the carburetor when the pilot selected carb heat full on. That's not good and is reason to suspect that the Mooney carbureted fleet in general could be suffering from the same problems.

Mooney service instruction M20-14 dated 6/6/66 refers to maintenance and inspection of the carburetor heat system. Make sure your mechanic has a copy during you next 100 or annual inspection. And I know this sound like self promotion, but there is nothing like getting your airplane into a shop with knowledge and experience working on Mooneys. There are areas on our airplanes that need someone with type-specific information and experience. The Mooney carburetor heat system is one of these area's. Take your carbureted Mooney to an experienced Mooney shop the next time you need a good inspection and repair don on the carburetor heat system. There would be nothing worse than getting carb ice pulling out the carb heat control and having the system not work. Without carb heat, you're going down with ice in the carburetor. Call me if you need help or support in the maintenance of your Mooney.   I can be reached in Gladewater, Texas at (903-643-9902) or e-mail me at

Damage to muffler and heat shroud due to improper repair to the muffler from
an auto repair shop.
Photo 1   

Due to interference with the lower cowling, there was a lot of leakage past the valve when carb heat was selected on the B model. Cold, unheated air was allowed into the carburetor as a result.
Photo 2

Detail view of the side of the carburetor air box on an early c model. Note the two valves activated by the carb heat control. The forward one is the carb heat valve and the aft one is the bypass valve.
Photo 3

Damage to carburetor heat valve
Photo 4

Detail of the bypass valve, shown here in the open position with carburetor heat selected off. Proper operation of this valve is critical for adequate heat if carburetor heat is selected on.
Photo 5

Correct way to duct hot air out of the bypass valve. 
Get this hot air out of the cowling.
Photo 6

Carb heat valve components disassembled on the work bench. The old valve is shown on the bottom. A new replacement valve is shown at top.
Photo 7
Details of the bypass valve mounted on the firewall beginning in 1968
photo 8