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Engine cuts with carb heat


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#1 srosenzw

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Posted 22 October 2013 - 04:12 AM

Any experts on carb heat out there?

I recently got my '65 C model back from annual.  During a recent run up check, I applied full carb heat and it resulted in the engine cutting off.  I immediately turned carb heat off and the engine recovered normally. 

 

I opened up the cowling and inspected all the ducting to the heat exchanger and to the carb air inlet.  All clear. 

I checked the position of the carb heat flapper door in the air inlet (pic attached) and it closes all the way with carb heat applied.  Feels like the mechanic may have greased that cable and now I seem to get a little more throw.  As such, the door closes pretty tight.

 

My question is, is the flapper door suppose to remain open a little even with carb heat fully on?  The engine will only cut out as I apply the last quarter inch or so of carb heat.

 

thanks

 

Steve

Attached File  IMG00933-20131020-1039.jpg   155.92KB   15 downloads

 

Ok, thanks for all the good ideas.  But here's a couple more pics and observations.  But previously my carb heat check during engine run up was always a 50-75 RPM drop, I have no carb temp gauge.

the second butterfly valve mentioned several times is connected to the small screen at the nose below the prop.  Isn't this an alternate air intake incase the air filter becomes blocked??

 

The other pic shows the carb heat cable fully retracted (carb heat on and flapper door in the full down position) and the second butterfly valve is closed too.  Looks like when the flapper door is closed, it allows heated air from the heat exchanger to enter the air intake from behind the closed door. To me, it only looks like the second butterfly valve is for alternate air (scat tube shown disconnected and you can see the valve closed with carb heat on).

Steve

 

 

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#2 BigTex

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Posted 22 October 2013 - 04:23 AM

Steve, that's the way it's supposed to work. It sounds like the duct coming from the cuff around the muffler has a some sort of blockage.
Gary Franks
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#3 srosenzw

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Posted 22 October 2013 - 04:32 AM

I did remover the scat tube from the muffler/heat exchanger and verified it was free of obstruction.  But sure seems like the engine is getting choked off somehow.



#4 BigTex

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Posted 22 October 2013 - 04:48 AM

There's a secondary butterfly valve that vents the hot air. Do you know if that's working correctly?
Gary Franks
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#5 daver328

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Posted 22 October 2013 - 02:29 PM

Yeah, it is a little unsettling.

What did it do before the annual? Do you have the carb-temp gauge? (Ours has one up on the headliner above the windshield) If you do, have you watched it?

The Operator's Manual says to apply carb heat on every descent and landing? I do that. Some say carb ice is never a factor on Mooneys and they don't use it ... while others say Mooneys are way more suseptable ... and they always use it?

About a month ago, (doing patterns) I pulled my Carb Heat (full) out turning base and someone who lives in one of the houses under the pattern called their friend at the airport to see if I was ok! I guess it sounded so rough to them on the ground ... they thought I was in distress and was making an emergency landing?

I have been watching the carb temp guage and considering using partial rather than full carb heat?

Interested to hear others thoughts?
David

#6 daver328

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Posted 22 October 2013 - 02:39 PM

There's a secondary butterfly valve that vents the hot air. Do you know if that's working correctly?


That's a good question!
David

#7 Shadrach

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Posted 22 October 2013 - 02:40 PM

David, If the carb temp gauge shows a temp above freezing, then you don't have carb ice...unless the gauge is wrong! :o

 

My dad had a C for a while and would use partial carb heat to keep carb temp above whatever number he felt comfortable with. 37df seems to ring a bell.

 

Srozenzw,

 

You are looking at your system with the engine off. I suggest you check to see if there's enough play in any of the components to choke off the engine under vacuum.  Is it possible a hose is collapsing/kinking or a door/butterfly is being pulled closed by airflow while under vacuum?


Cheers!

Ross


#8 daver328

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Posted 22 October 2013 - 03:27 PM

David, If the carb temp gauge shows a temp above freezing, then you don't have carb ice...unless the gauge is wrong! :o
 
My dad had a C for a while and would use partial carb heat to keep carb temp above whatever number he felt comfortable with. 37df seems to ring a bell.
 
Srozenzw,
 
You are looking at your system with the engine off. I suggest you check to see if there's enough play in any of the components to choke off the engine under vacuum.  Is it possible a hose is collapsing/kinking or a door/butterfly is being pulled closed by airflow while under vacuum?


Yeah ... I agree.

There are some REALLY SMART pilots who have these really scientific discussions about Venturi effect and low-pressures and lower temperatures occurring in places like intakes where air is channeled and accelerated. And they have stories about someone who knew someone who knew someone that had Carb ice when it was 65F out?

Me? I just do it because the book says so ... but yeah, just a little common sense would tell you if it's 90F and sunny clear w/ no moisture ... Then yeah, you don't need it!

Now it's getting cooler and misty moist air ... Yeah ... Maybe use it ...
David

#9 Shadrach

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Posted 22 October 2013 - 05:50 PM

Dave, I'm referring to a "carb temp" gauge which gives you a direct reading of the temp of the carb venturi. It has nothing to do with the OAT or OAT gauge (thermometer)

Cheers!

Ross


#10 piperpainter

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Posted 23 October 2013 - 01:15 AM

There's a secondary butterfly valve that vents the hot air. Do you know if that's working correctly?

 

I agree you need to look at the scat. This valve will not stop the engine. I had my butterfly valve for the dump fail and it didn't stop the engine, it just didn't give me as much of a drop. That's how I knew something was wrong. I would look at all the SCAT lines and check for damage or possibly a bird nest or something inside. This is the most likely answer. Hopefully it's just a kink! Goodluck.



#11 carusoam

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Posted 23 October 2013 - 01:51 AM

My '65 C had a challenge with that second butterfly. The small wire that operated it bent/broke...

Just another vote for getting it looked at to make sure it is working properly.

Carb temp is definitely a good idea. I only had ice once, in the summer. The solution of full carb heat was painfully slow....

Having the carb heat gauge gives better options....


-a-

#12 Hank

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Posted 23 October 2013 - 01:51 AM

I rarely use carb heat in the pattern. In clouds, I use enough to keep the carb temp needle above the orange stripe.

My OM says to apply full carb heat unless equipped with the optional carb temp gauge. What does yours say?
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#13 daver328

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Posted 23 October 2013 - 03:39 AM

Dave, I'm referring to a "carb temp" gauge which gives you a direct reading of the temp of the carb venturi. It has nothing to do with the OAT or OAT gauge (thermometer)

Yes, I probably didn't word my response clearly. I know what it is, what it measures and where it measures. I have finally realized it's up there and include a "look-see" up there in my scan from time to time and now watch it as I apply carb heat.

I was referring to guys who do not have that gauge and go from one extreme to another debating as to whether you need to use carb heat? That carb temp guage removes all the guessing ... I'm just realizing how good it is that it's there!

I guess it is probably overkill - turning on carb heat to land each and every time? In fact, the verbage in the owners manual (1965 M20C Mark 21) let down procedures actually specifies you use full carb heat rather than partial. I come from a military pilot, airline pilot background where you follow the checklist ... period. I wish on the landing procedures in the manual instead of saying only "carb heat" it added "as required" or such!

As a guy who has been flying nothing but turbine jets and helicopters for the past fifteen years, I am having to learn or re-learn proper thought and techniques for carbureted, piston air cooled engines. I quickly discovered to lean aggressively in taxi. Proper technique for: cooling in the climb, cowl flap management, leaning in cruise, cooling (I.e. Not shock cooling) in descent (or "let down" as Mooney calls it) and use of carb heat ... are all things I am working to understand and master right now.

Until, now I just turned on carb heat because the book said so ... No matter what that gauge said. I am realizing that I probably didn't (really) need it ... but now as winter approaches I probably will need it.

And I think your point is ... Just look at the gauge ... It will tell you if you need it.

Now that I am getting more and more familiar and comfortable in the Mooney (about 25 hours now) I actually have some time and brain cells free to think about and look at the carb temp gauge. About two months ago we almost moved it down to the panel from its present location - when considering a 201 windshield conversion. Ended up going with a one-piece (non 201) windshield instead so we left it where it is. It's not located in a place where one sees it and thinks to include in the scan ... Way up there on the ceiling / headliner panel ...
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David

#14 triple8s

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Posted 23 October 2013 - 05:47 AM

N78880 ( 65 "C" Model ) had a carb temp probe on the UGB-16 engine monitor and when I first had the monitor installed I was trully amazed at what the carb did, the temp swings it made during different phases of flight. One thing i was surprised at is how little OAT had to do with carb temp, so remember this, summertime or wintertime has little bearing on whether or not you can get carburetor ice. Its all about if the carb is cold and how much moisture is in the air and that doesnt take much either. I dont own 880 anymore but if I were to own a C model again I definately would invest in a way to monitor carb temperature.



#15 Shadrach

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Posted 23 October 2013 - 11:16 AM

P
And I think your point is ... Just look at the gauge ... It will tell you if you need it...


Pretty much. Or....parish the thought...you could amend your check list to say "verify carb temp" instead of "carb heat on". If you have an instrument that tells you your carb temp is 52 degrees, it just seems silly to pull the carb heat. Rote behavior has its place in GA, but it really shouldn't take the place of analyzing the situation and acting accordingly!


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Cheers!

Ross


#16 BigTex

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Posted 23 October 2013 - 01:33 PM

For those that currently have a way of monitoring carb temps... Do you tie it into your engine monitor or have a discrete gauge?

Also, when I enquired about adding it when I had my EDM 700 installed, they said my carb wasn't ported to accept a probe. Sounds like I need to get a second opinion.
Gary Franks
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1965, Mooney Mark 21 Model: M20C

#17 daver328

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Posted 23 October 2013 - 01:55 PM

Pretty much. Or....parish the thought you could amend your check list to say "verify carb temp" instead of "carb heat on". If you have an instrument that tells you your carb temp is 52 degrees, it just seems silly to pull the carb heat. Rote behavior has its place in GA, but it really shouldn't take the place of analyzing the situation and acting accordingly!

I fly just fine and all ... But for the first few hours I was just trying to SMOOTHLY put the gear up and down with the bar, make sure I didn't pull the prop back before the power ... and land on the mains ... if you know what I mean ...? Not just new airplane saturation, but new airplane class and category saturation and returning to the world of GA saturation.

Heck ... I was "just a little concerned" the first time when that one red light remained on after I retracted the gear. On all the other planes I've flown that means it's not locked up properly. Took me reading on here afterward that it stays on ANY time the gear is not down and locked (missed that one when reading the manual).

Regarding amending a checklist ... we do amend it and procedures in the airlines (and did it in the military) more than you might think ... but it would be a fleet-wide amendment, issued by a training department and endorsed by the FAA. For a base-line, I just abbreviated, condensed and copied verbatim the procedures section of the Owners Manual to produce a small usable checklist -which I then laminated. I have modified that a few times now. For example: I realized the manual does not include a specific numbered item prompting you to extend flaps prior to landing (Mark21 M20C Owners Manual page 22) in the landing procedures ... Only mentioned in the paragraph verbiage ... Referencing the maximum extension speed. So I added an extra bullet with "Flaps .... Extend" to the landing procedures section ...

I think more changes (read -improvements) and better practices will come as I get more time and experience in the Mooney. Honestly I am pretty happy with where I am at now, for having not more than 25 hours it in so far ... That is ... and already taken the wife to the Bahamas in it ... Can't tell you enough how much we LOVE it!
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#18 Shadrach

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Posted 23 October 2013 - 02:06 PM

It's funny, a new airplane is a new airplane...bigger..smaller...heavier...faster...slower...whatever, you're busy when you have to think about everything you have to do...no muscle memory of how things should feel or where they are. It's challenging.  I am currently embarassing myself in my father-inlaw's Decathlon. I've got 2 hours of dual and 20 landings, 15 of which were unassisted all of which were ugly.  I have no swagger around his hangar at all. I listen, keep my mouth shut and maintain a low profile. He wants me to do a little X wind work before turning me loose.  I am nervous as hell about flying his airplane (been married 2 months) whether he's in it with me or not. 

 

So look on the bright side, you just have to get used to a new plane. Trying to get used to a new plane with a 15000hr+ AA Captain/Check airman in the back that happens to be your wife's father is far more stressful...I promise!


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Cheers!

Ross


#19 BigTex

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Posted 23 October 2013 - 02:57 PM

It's funny, a new airplane is a new airplane...bigger..smaller...heavier...faster...slower...whatever, you're busy when you have to think about everything you have to do...no muscle memory of how things should feel or where they are. It's challenging. I am currently embarassing myself in my father-inlaws Decathlon. I've got 2 hours of dual and 20 landings, 15 of which were unassisted all of which were ugly. I have no swagger around his hangar at all. I listen, keep my mouth shut and maintain a low profile. He wants me to do a little X wind work before turning me loose. I am nervous as hell about flying his airplane (been married 2 months) whether he's in it with me or not.

So look on the bright side, you just have to get used to a new plane. Trying to get used to a new plane with a 15000hr+ AA Captain/Check airman in the back that happens to be your wife's father is far more stressful...I promise!


This is absolutely true. The bulk of my hours are in Decathlons and I feel they're absolutely pussycats to land. Everything comes easier with time and experience. For me, I'm still embarrassing myself in my Mooney. Maybe someday I'll know I've landed when I feel the tires rolling like the good old days in the Decathlon.
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Gary Franks
Based at: KDTO, Denton Texas
1965, Mooney Mark 21 Model: M20C

#20 Shadrach

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Posted 23 October 2013 - 03:28 PM

They're like pussycats in that the spring steel gear make the airplane "pounce" onto a spot of runway about 75 feet down from my initial touch down point... :P  


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Cheers!

Ross





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