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Pressure Carburetor?
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hawkins(at)cms.udel.edu
Guest

Posted: Wed Sep 26, 2007 8:43 am Post subject: Pressure Carburetor?

All-



Anybody have a good shop they'd recommend that's familiar with working on/overhauling the older pressure carburetors?  We sent the left carb on N2760B out to one shop - and it came back better in some ways - but mixture is either full rich or idle cut-off.  No adjustment in between.



We've gone through the cabling and linkage and all looks good.  We think it's internal to the carburetor at this point. Not sure, but the shop we used may have messed something up.



Engines are G0-480-D1A's



Thoughts?  Recommendations?



Matt



By the way, on our previous problem (engine skip/roughness when reducing power through 21" manifold pressure) was exactly as one of you suggested - improperly adjusted carb heat linkage.  Many thanks on that one!



********************

Matthew Hawkins

Director, Marine Ops

R/V HUGH R. SHARP

302-645-4341

FAX: 302-645-4006

hawkins(at)udel.edu

John Vormbaum

Joined: 21 Aug 2006
Posts: 273
Location: SF Bay Area, CA

Posted: Wed Sep 26, 2007 10:07 am Post subject: Pressure Carburetor?

Matt,



Isn't that exactly how a pressure carb is supposed to work? It's 

automatic mixture control with altitude. IIRC, you leave the levers in 

full rich for all segments of the flight and the carbs do the rest. 

Sounds like the shop did a fine job on your overhaul.



Cheers,



/John



Matthew J. Hawkins wrote:

 	  Quote: 	
		   



 All-



 Anybody have a good shop they'd recommend that's familiar with working on/overhauling the older pressure carburetors?  We sent the left carb on N2760B out to one shop - and it came back better in some ways - but mixture is either full rich or idle cut-off.  No adjustment in between.



 We've gone through the cabling and linkage and all looks good.  We think it's internal to the carburetor at this point. Not sure, but the shop we used may have messed something up.



 Engines are G0-480-D1A's



 Thoughts?  Recommendations?



 Matt



 By the way, on our previous problem (engine skip/roughness when reducing power through 21" manifold pressure) was exactly as one of you suggested - improperly adjusted carb heat linkage.  Many thanks on that one!



 ********************

 Matthew Hawkins

 Director, Marine Ops

 R/V HUGH R. SHARP

 302-645-4341

 FAX: 302-645-4006

 hawkins(at)udel.edu

 __________ NOD32 2521 (20070911) Information __________

hawkins(at)cms.udel.edu
Guest

Posted: Wed Sep 26, 2007 10:40 am Post subject: Pressure Carburetor?

Hmmm - now I'm going to have to show my (our) ignorance here!  Maybe the right one is what needs to be corrected!  The right WILL lean out as you adjust the mixture control.



Anybody else have thoughts?



Matt



--

dongirod

Joined: 11 Dec 2006
Posts: 140

Posted: Wed Sep 26, 2007 1:10 pm Post subject: Pressure Carburetor?

They don't call them Bendix auto pressure carbs. for nothing!



I have been told that when I level off, say 8K or above, to pull it back 

gently then push it back to full rich.  So that is what I do.



Don

---

CloudCraft(at)aol.com
Guest

Posted: Wed Sep 26, 2007 6:03 pm Post subject: Pressure Carburetor?

In a message dated 26-Sep-07 11:08:51 Pacific Daylight Time,  john(at)vormbaum.com writes:

  	  Quote: 	
		  Isn't    that exactly how a pressure carb is supposed to work? It's automatic mixture    control with altitude 	


 

 Kinda.

  

 The "Auto Lean" should lean with an increase in altitude and vice  versa.  These can also be manually leaned and sometimes need to be if the  auto lean isn't working.

  

 Evidence of auto lean not working is fogging of one (or both)  exhausts, excessive carbon or black soot in the stacks after a  flight.

  

 I once flew an AC-680E with one shot diaphragm and had to hold the  mixture of that engine in a lean position for as much as I could between  California and Florida.  The mixture control would not stay where I  wanted it in the manual leaned position and that got tedious.  Upon  delivery, the pressure carbs got sent to a rebuild shop with Mr.  Kernick's direction.

  

 The idea is to have properly operating pressure carbs that lean /  enrichen automatically.

  

 By the way, those of you with pressure carbs, when you shut down  the engine, do not leave the mixtures in idle cut off. This will stretch out and  dry out the diaphragms.

  

 So put them back in full rich when your Commander is  sleeping. 

  

 Wing Commander Gordon



Make AOL Your Homepage.

  [quote][b]

WINGFLYER1(at)aol.com
Guest

Posted: Wed Sep 26, 2007 6:13 pm Post subject: Pressure Carburetor?

I heard that, if you have neoprene gaskets,diaphrams etc. installed (at)  carburator over-haul, you can then leave the mixture in the off position. It  seems that I also heard that a bullitin was issued recommending that neoprene be  installed. Any one know for sure about this? Thanks Gil-  Wingflyer



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yourtcfg(at)aol.com
Guest

Posted: Wed Sep 26, 2007 6:39 pm Post subject: Pressure Carburetor?

HI MATT.  

   It sounds to me like the carb is working properly.  You should never manually lean your carbs except in case of emergency (like WCG described).  Many years ago I bought a 560A and as I flew it home I literally read the flight manual, on my lap, since I had no idea how to fly the thing.  The manual states that is is possible to manually lean the airplane above 10,000 feet.  So, as I crossed the Rocky Mts, at 12Kt, I decided to lean the engines.  They were running perfect, but I just could not leave well enough alone.  Well, I learned a couple of things about Commanders!!  First, you don't need to lean the stinking carbs!!  Second, whenever you do anything to a twin engine airplane, do it one engine at a time.  I reached for the mixture levers and moved them back about 1/2 way.  Nothing happened??  As I continued to pull them back a bit more, BOTH engines quit cold!!  The silence was defining.    It only took about one millisecond to get the mixtures back forward. 

   If you read the lettering on your power quadrant, you will notice that the mixtures are not labeled "rich" & "lean".  Instead, they say "normal" & "idle cut off" (Stop & go).

 I think you new carb is working great and I wonder why the old carb will allow you to lean it??  jb

 

 

 

 --

yourtcfg(at)aol.com
Guest

Posted: Wed Sep 26, 2007 6:45 pm Post subject: Pressure Carburetor?

The only gaskets available are neopream, (orenge)  You shoud still leave the mix forward (and the fuel "on") when parked for an extended time, overnight or longer.  The reason is the diaphrams are streched when pulled to idel cut off and will quiclky take a "set" if left there.  Leave the fuel on to "flood" the inside of the carb with fuel to keep all of the pars properly lubed.  Also, although not officialy sanctiond, you will likly find a can of Mavel mistery Oil in the hangar of an experianced, presure carb owner.  Used in the fuel system it will lube the inside of the carbs.  jb

 

 

 --

rsrandazzo(at)precisionma
Guest

Posted: Wed Sep 26, 2007 10:21 pm Post subject: Pressure Carburetor?

Jb-  

   

Good story… Made more so by the fact that you’ll admit to having done something that most of us would attribute to “a friend was flying a commander….”  

   

Rob  

     

From: owner-commander-list-server(at)matronics.com [mailto:owner-commander-list-server(at)matronics.com] On Behalf Of yourtcfg(at)aol.com

 Sent: Wednesday, September 26, 2007 7:39 PM

 To: commander-list(at)matronics.com

 Subject: Re: Pressure Carburetor?  

  

     

HI MATT.    

    

It sounds to me like the carb is working properly.  You should never manually lean your carbs except in case of emergency (like WCG described).  Many years ago I bought a 560A and as I flew it home I literally read the flight manual, on my lap, since I had no idea how to fly the thing.  The manual states that is is possible to manually lean the airplane above 10,000 feet.  So, as I crossed the Rocky Mts, at 12Kt, I decided to lean the engines.  They were running perfect, but I just could not leave well enough alone.  Well, I learned a couple of things about Commanders!!  First, you don't need to lean the stinking carbs!!  Second, whenever you do anything to a twin engine airplane, do it one engine at a time.  I reached for the mixture levers and moved them back about 1/2 way.  Nothing happened??  As I continued to pull them back a bit more, BOTH engines quit cold!!  The silence was defining.  It only took about one millisecond to get the mixtures back forward.   

    

If you read the lettering on your power quadrant, you will notice that the mixtures are not labeled "rich" & "lean".  Instead, they say "normal" & "idle cut off" (Stop & go).

 I think you new carb is working great and I wonder why the old carb will allow you to lean it??  jb

 

 

 

 --

hawkins(at)cms.udel.edu
Guest

Posted: Thu Sep 27, 2007 3:34 am Post subject: Pressure Carburetor?

jb-

  

 What a GREAT  description of what we've seen!  Thanks so much for passing this on.   Somehow something so obvious and important has escaped us until now.  Guess  we've had so many things to deal with on this overhaul that this finally rose to  the "top of the pile".

  

 Both engines only  have about 12-15 hours on them and seem to be running rich - plugs fouling until  getting up to temp.  We could lean the right side out enough to get a  good mag check, but left wouldn't lean - though we had less problems with  the left mags than the right.  So this is what lead us to my  question.

  

 Thinking about going with the  wire-type plugs that don't foul.  Good  idea? 

  

 Sounds like the left  side is doing what it's supposed to do.  Right side may still not be  working properly with "auto lean" but sounds like it's not necessarily a danger  - just need to be aware of it for now.  We'll get it looked at - but at  least we understand (now) what the carbs are supposed to be  doing.

  

 Many thanks to all  of you!

  

 Matt

 [quote]   

   --

BobsV35B(at)aol.com
Guest

Posted: Thu Sep 27, 2007 5:40 am Post subject: Pressure Carburetor?

Good Morning Matt,   

 May I add a caveat or two?

  

 I have no knowledge at all concerning the specific pressure carburetors  with which your airplane is equipped, but there are many pressure carburetors  that DO need to be manually leaned.

  

 The ubiquitous Bendix PS5c is one such pressure carburetor. It has no AMC  (Automatic Mixture Control) unit involved in it's operation.

  

 The degree of automation for pressure carburetors varies from having  an electric switch to turn the whole shebang OFF or ON to having additional  settings for Takeoff, Climb, Auto Lean, Auto Rich, and Idle Cutoff.

  

 The number of variations is only limited by the desires of the engine user  and the fuel control designer.  The vast majority of the pressure  carburetors in use can benefit from manual leaning under many conditions.

  

 However, such manually leaning is generally only recommended when the  operator has adequate instrumentation  and time available to evaluate the  effects of that manual leaning process.

  

 Prior to the introduction of turbojet powered aircraft to the airline  industry, I spent fourteen years flying big round engines that were equipped  with "Pressure Carburetors". I also had a flight engineer whose primary job was  using the manual leaning capability of the pressure carburetors to gain the  greatest efficiency possible for our operations.

  

 I would suggest that anyone who is interested in manual leaning first equip  their engine with a modern engine monitor and a good electronic fuel flow  measurement device, then learn to properly interpret what that information is  telling us.

  

 Happy  Skies,



Old Bob

AKA

Bob Siegfried

Ancient Aviator

Stearman  N3977A

Brookeridge Air Park LL22

Downers Grove, IL 60516

630  985-8503



  

 In a message dated 9/27/2007 6:35:00 A.M. Central Daylight Time,  hawkins(at)cms.udel.edu writes:

  	  Quote: 	
		     What a GREAT    description of what we've seen!  Thanks so much for passing this    on.  Somehow something so obvious and important has escaped us until    now.  Guess we've had so many things to deal with on this overhaul that    this finally rose to the "top of the  pile".

	


 





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n395v

Joined: 10 Jan 2006
Posts: 450

Posted: Thu Sep 27, 2007 5:42 am Post subject: Re: Pressure Carburetor?

Let's slow down and think about this for a bit. Unfortunately most of the original TC drivers mechanics and designers are long gone and we have to rely on word of mouth OWTs and seat of the pants in flying these things but sometimes careful analysis of common sense and known physics can be helpful.



It is called a MIXTURE control, not an on/off control. And it is there for a reason.



This says that as you move the lever it alters the fuel air ratio. It does this by tightening a diaphragm which lessens the automatic input from the aneroid barometer and it also slides a precision needle valve in and out of it's seat adjusting the fuel air mixture/ratio.



The pressure carb also alters the fuel air ratio based on the changing altitude/ ambient pressure as you climb/descend.



While this takes some of the workload off the pilot we all know mixture should be adjusted relative to peak EGT. Pressure carbs adjusting for altitude only get a close approximation to getting the mixture to where the engine is happy and efficient. With AVGAS at $4+ a gallon efficient is becoming very important. Also the accuracy of the mixture is only as good as the tech that rebuilds or calibrates the carb. I do not believe there is anyone left alive who can do this properly on a commander. Last I knew there was only one working test bench left in existence.



If your carbs were not done on this bench and Jet A or deisel was not used for calibration then they are NOT working correctly.



Much has been said about "stretching" the diaphragm. This should convince you that the mixture lever does more than turn things on or off.



The reason to leave it full rich at shutdown is twofold.

1. The diaphragm will dry out and age quickly if you do this,Neoprene is better but does not eliminate this problem.

2. The mixture control also moves a needle valve that is fully seated against a brass boss in the ICO position. If left in  ICO the needle valve will leave an indent in the brass boss and leak.



The correct way to lean a normally aspirated pressure carb in a Twin Commander is......



When at altitude (over 5000') and you have full throttle and the props set, pull the throttle back till you get the 1st visible decrease in manifold pressure. then advance the throttle to max Manifold pressure obtainable.(this usually involve moving the throttle lever about a sixteenth of an inch forward from where the drop occurred)  The manual and many old timers would now say you are leaned. But you have that nice red lever that puts tension on a diaphragm and moves a needle valve both of which adjust fuel air ratio. Now why was it put there?



So in the best of all worlds you have an engine monitor that monitors all your EGTs and CHTs as well as a fuel flow indicator for each engine.



Then you can lean to peak if under 65% power (which you will be under at altitude) or if you are a wuss lean somwhere ROP or if a real man try to go LOP if the engine remains smooth.



If you do not have the fancy gizmos then lean till the engine gets rough then push it a skosh forward.



In my 560 this invariably saved me 2.5 GPH per engine at cruise or 5 GPH total. at 300 hrs a year this adds up to mucho deniro.

In the super charged versions your throttle will be set at wherever you set it for cruise so just lean till rough then enrichen. In the supercharged versions you need to be cautious and remain ROP as you most likely will be above 65% power and can cause harm at peak and LOP. To remain ROP do not dally when the engine gets rough bump that red rascal forward about an eighth of an inch quickly.



A caveat is the 560Fs for some reason the engine/carb combination on these setups will often do as JB described that is they go from cruise to silence. I suspect this has more to do with poor rebuild or calibration and maintenance than the setup it self.



I have found after several years of playing with LOP operations using engine analyzers that if your goal is efficiency and safe operation while running ROP then the lean to rough and a skosh forward on the mixture control will put your mixture lever in exactly the same place the fancy expensive engine monitor will have you put it by using their auto lean algorithms.



If you like to play with LOP and have FI and a super or turbocharger do not even attempt it without an engine analyzer.



If you lean correctly and control temps correctly not only does this save fuel but more importantly in these old birds it will greatly prolong engine life.



Well that was a long post and I hope helpful.



The short answer to the original post is your carbs (statistically) are most likely not calibrated correctly and while they may be safe I would not assume so.



I cannot remember the shop or tech that can do it but the name Czarnecki in Oklahoma rings a bell as does the word "precision" in the shop name. One of the old timers should be able to steer you there. It may also be listed somewhere in the old Commander list archives that I believe Nico is the keeper of.

_________________
Milt

Last edited by n395v on Thu Sep 27, 2007 5:48 am; edited 1 time in total

n395v

Joined: 10 Jan 2006
Posts: 450

Posted: Thu Sep 27, 2007 6:05 am Post subject: Re: Pressure Carburetor?

As an aside to this thread but same subject I am currently flying an M14PF (Russian Radial) that has a pressure carb and no mixture control.



These are great engines but the carbs are hard to come by and there are no parts available to rebuild them. There is nobody who can calibrate the and no equipment or instructions to calibrate them with. Not a good scenario.



So I put an Airflow Performance INc.                                        ( http://www.airflowperformance.com/html/site_html.html ) Throttle body on it that has a mixture control. The throttle body is a close 1st cousin to a pressure carb and a step up on the evolutionary ladder to fuel injection.



The thing is simplicity itself and required no engine mods or adjustments.



It jives me additional HP and full control of mixture at all altitudes.



Now I would never suggest violation of FARs but if I still owned a Twin Commander I would be sorely tempted to slap a couple of these on as opposed to rebuilding those old pressure carbs. They make several modes for Lycomings. (experimental of course)



In the pictures attached below the throttle body can be seen with a blue elbow attached to it.

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_________________
Milt

WINGFLYER1(at)aol.com
Guest

Posted: Thu Sep 27, 2007 6:07 am Post subject: Pressure Carburetor?

Milt, When I leave the mixture forward The right carb on my 680 leaks about  a gallon or two over night. Since I am a new 680 owner I am trying to learn as  much as I can. Any info would be helpful. Thanks Gil  Wingflyer.



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WINGFLYER1(at)aol.com
Guest

Posted: Thu Sep 27, 2007 6:19 am Post subject: Pressure Carburetor?

Had a problem with the left engine not starting easy, spitting ,  spurttering and running rough. After the first so called mechanic adjusted  and rebuilt the carb on my 680 another mechanic found the problem. The intake  manifold gaskets needed to be replaced. Cured the problem. Sometimes all the  adjusting and re-building may not be the true answer. You have to check it ALL.  Gil



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WINGFLYER1(at)aol.com
Guest

Posted: Thu Sep 27, 2007 7:57 pm Post subject: Pressure Carburetor?

Thanks for the info. When I leave the mixture rich on my 680, fuel leaks  out of the right carb. Know of any fix for the leaking. Thanks  Gil



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bowing74(at)earthlink.net
Guest

Posted: Thu Sep 27, 2007 8:44 pm Post subject: Pressure Carburetor?

Caulk.  

bb  

         

  

From: owner-commander-list-server(at)matronics.com [mailto:owner-commander-list-server(at)matronics.com] On Behalf Of WINGFLYER1(at)aol.com

 Sent: Wednesday, September 26, 2007 10:56 PM

 To: commander-list(at)matronics.com

 Subject: Re: Commander-List: Pressure Carburetor?  

  

     

Thanks for the info. When I leave the mixture rich on my 680, fuel leaks out of the right carb. Know of any fix for the leaking. Thanks Gil  

  



 

         

  

See what's new at AOL.com and Features Subscriptions page, Photoshare, and href="http://www.matronics.com/Navigator?Commander-List">http://www.matronics.com/Navigator?Commander-List the Web href="http://forums.matronics.com">http://forums.matronics.com ===========   

  

       [quote][b]

yourtcfg(at)aol.com
Guest

Posted: Sat Sep 29, 2007 7:47 am Post subject: Pressure Carburetor?

HI MATT

   Yes, fine wire plugs are worth the money, I have them in both my airplanes.  Also, All Commanders left the factory with fine wire plugs, even in 1956.  You have two adjustments on your carbs.  If they are rich on the ground, they are probably rich in flight.  There is an idle mix adj at the throttle arm (see you maint manual) and an "off idle" adj.  Both will need to be "tweeked" if the carbs have been off the airplane.  Call me at 360-903-6901, I talk faster than i type.  jb

 

   	  Quote: 	
		  Thinking about going with the wire-type plugs that don't foul.  Good idea? 	


 

 

 --

yourtcfg(at)aol.com
Guest

Posted: Sat Sep 29, 2007 8:03 am Post subject: Pressure Carburetor?

Sorry Milt, but you are wrong.  Your straight 560 may have had manually leaning carbs, the AUTO lean carbs need to be left in the "normal" position for all flight and ground operations.  They are NOT to be routinely adjusted in flight.  That is why the factory lab;ed the quadrant "normal & Idle cut off" rather than "rich & lean".  jb   

 

 

 --

yourtcfg(at)aol.com
Guest

Posted: Sat Sep 29, 2007 8:35 am Post subject: Pressure Carburetor?

HI MILT.  I am not picking on you, but need to make one other correction.  This may be why you were saving gas by manully leaaning you carbs.  On normally asperated engins, equiped with auto lean carbs, it is important to retard the throttls in cruse.  These carbs are equiped with a enrichment valve that is opened at throttle settings above 80%.  This feature uses fuel to cool the engine durring climb and high power settinsg.  Once you reach you cruse altitude, you should retard the throttls as far as possible and still maintane the desired MP.  This closes the enrichment valve and alows the carb to adjust the mix properly.  If the throttle is left fully forward, the ericment valve will reamin open, defeating a portion of the auto lean circut.  jb

   	  Quote: 	
		   	  Quote: 	
		  

When at altitude (over 5000') and you have full throttle and the props set, 	
	


 

 

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