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M20C carburetor adjustment


DXB

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So my plane is in annual and I wanted to address the slightly low peak fuel flow (17ish gph rather than 18ish gph) at takeoff under standard conditions, figuring this may help my engine temp issues in climb.  I had assumed there were separate controls to set idle mixture (which I understand is relevant only at low rpm) versus main mixture settings, but per my shop this does not appear to be the case- only an idle mixture setting.  I think I have the Marvel MA-4-5 carburetor.  Carusoam posted a handy guide to these carbs on another thread:

http://www.insightavionics.com/pdf%20files/MA-4%20Carb%20Manual.pdf

I am reading but am still confused on how mixture could be enriched at WOT on takeoff for these carbs, and how it differs between carb models.  Can anyone shed some light on this, and if there is an easy way to address my peak fuel flow issue?

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First, you should ensure your mixture control is actually making full travel to the full rich position.  The knob in the cockpit should not contact the instrument panel, it should instead hit the stop at the carburetor.

To change the WOT fuel flow you would have to change the main metering jet, or have your overhaul shop ream the jet to the greatest allowable size per the carburetor overhaul manual for the specific carburetor part number that corresponds to the O-360 A1D engine.

How high are your CHTs in climb?  More than likely your carburetor is correct, or that your fuel flow gauge is slightly off at the high fuel flow rates of WOT.  If that is not the case, you're looking at removal of the carburetor and sending it to an overhaul facility for flow checking.

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10 minutes ago, N1395W said:

First, you should ensure your mixture control is actually making full travel to the full rich position.  The knob in the cockpit should not contact the instrument panel, it should instead hit the stop at the carburetor.

To change the WOT fuel flow you would have to change the main metering jet, or have your overhaul shop ream the jet to the greatest allowable size per the carburetor overhaul manual for the specific carburetor part number that corresponds to the O-360 A1D engine.

How high are your CHTs in climb?  More than likely your carburetor is correct, or that your fuel flow gauge is slightly off at the high fuel flow rates of WOT.  If that is not the case, you're looking at removal of the carburetor and sending it to an overhaul facility for flow checking.

Great info, and excellent suggestion to make sure the mixture cable pushes the lever on the carb all the way forward.  My fuel flow seems spot on based on how much I use to top off the tanks- of course a small error at highest flows at low altitude may not throw it off much.  My hottest CHT is 430ish in climb if I watch carefully and climb a bit faster than Vy- if I merrily climb at Vy and ignore, it can get up to 450. Oddly it alternates between #3 and #4- a flip of a coin - with a high associated EGT in whichever one is running hot, and then other one runs in 390s, with a lower EGT also.  #1 and #2 run in the 390s-410s range usually.  No issues is cruise.  Plugs are good, mags are getting IRAN'd, and baffling is getting tightened up at this annual.  Not sure how much the fuel flow is worth chasing if issues after that.  Pulling the carb looks like a PITA- It was last overhauled in 2000.

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Accurate information is required first.  You can check the fuel flow indicator by pumping 5 gallons of fuel into a jerry can and comparing it to the fuel flow reading.

What type of CHT thermocouples do you have? Bayonnet or rings, rings generally read incorrectly, if rings on top plug or lower plug?  Bayonnets in the factory provided location are the only correct method.

The condition of your engine baffles and sealing tapes will make a bigger difference to your temps than anything else.  They should be tighter than a bulls butt in fly season.  New sealing tapes and silicone caulking are your best help.

Full throttle operation results in additional fuel flow at max power, reducing throttle (MAP) reduces fuel flow and raises temps.

Clarence

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3 hours ago, M20Doc said:

Accurate information is required first.  You can check the fuel flow indicator by pumping 5 gallons of fuel into a jerry can and comparing it to the fuel flow reading.

What type of CHT thermocouples do you have? Bayonnet or rings, rings generally read incorrectly, if rings on top plug or lower plug?  Bayonnets in the factory provided location are the only correct method.

The condition of your engine baffles and sealing tapes will make a bigger difference to your temps than anything else.  They should be tighter than a bulls butt in fly season.  New sealing tapes and silicone caulking are your best help.

Full throttle operation results in additional fuel flow at max power, reducing throttle (MAP) reduces fuel flow and raises temps.

Clarence

Thank you for your expert input Clarence.  I just had a fresh install of a JPI 900 as primary a couple months ago, with all bayonnet probes for CHT in the factory slots, so I believe the data - ignorance seemed like bliss before that.  I guess I could also formally fine tune the K factor for fuel flow per the JPI manual - I hadn't bothered because the amount I need to fill up vs amount used per the JPI seemed spot on in my early experience with it. I was previously also reducing power because of the temps until else someone pointed out to me why this is stupid, as you note above- it's certainly not any worse when I climb at WOT.   

I gave my shop a blank check to get the baffle into "bull's butt" condition...hopefully that'll do it!

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Work in progress- will post the outcome.

 Interestingly, the mechanic /shop owner pointed out to me that are 3 flavors of the Marvel Schleber MA-4-5 carb that are approved for the O-360-A1D in the C model:

P/Ns are:  10-4164-1, 10-3878, 10-3878-M

I have the first, and apparently the three differ in how rich they run at WOT.  But, neither he nor I can find any info on the precise differences among the three.  Does anyone here know? 

 

Dev

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2 hours ago, Shadrach said:

Do they make different sized economizer jets? It seems that if the problem is just in climb a higher flowing economizer jet would be a logical 1st step.

I looked at overhaul manual:http://www.expaircraft.com/PDF/marvel-schebler-OH.pdf

It looks like they have a standard air metering pin setting for the economizer on each of the MA-4-5 variants, but seemingly only one "correct" setting for each model.  Looks like you may have to pull the carb to access it too so I'm not sure.

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Carb supposed to be OH every 10 years. Look up the reason the NTSB reported the issue to be of Harrison Fords last plane crash. I just put an OH carb on mine, have not flown yet since other mods are still taking place, but anxious to see the difference. When I had the old & OH side by side, moving linkages, examine throttle plate, etc…wow big difference! Mine had not been OH in 18yrs…I have owned my Mooney less than 1 yr.

Sherman

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1 hour ago, ShermAv8tor said:

Carb supposed to be OH every 10 years. Look up the reason the NTSB reported the issue to be of Harrison Fords last plane crash. I just put an OH carb on mine, have not flown yet since other mods are still taking place, but anxious to see the difference. When I had the old & OH side by side, moving linkages, examine throttle plate, etc…wow big difference! Mine had not been OH in 18yrs…I have owned my Mooney less than 1 yr.

Sherman

Yes thanks - this seems like a reasonable point- i just looked up the OH interval from Marvel Schleber - "10 years or engine TBO."   Overhaul might be worth considering- how many hours were on yours?   Mine is at 15-16 years but 700 hrs.  Also do you happen to know what the typical problems and failure modes are for our carbs, and how often they occur?  Not sure how to balance these risks with the maintenance induced failure risk, but would like to learn more.  

I just read the Harrison Ford NTSB report- seems like part of the main metering nozzle came off, making the mixture way too rich?  It's unclear to me if it was a wear-related failure or it was assembled incorrectly in the first place at last overhaul- the language in the report is noncommittal, but it had <200 hrs on it since overhaul in 1998.  Also this is 419 Holley carb- a pre WWII design, not sure how it stacks up against what we have. 

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On December 24, 2015 at 0:19 AM, DXB said:

I looked at overhaul manual:http://www.expaircraft.com/PDF/marvel-schebler-OH.pdf

It looks like they have a standard air metering pin setting for the economizer on each of the MA-4-5 variants, but seemingly only one "correct" setting for each model.  Looks like you may have to pull the carb to access it too so I'm not sure.

Thanks for the link.  Man is that a simple carb compared to the Rochester Quadrajet I had on my 82 Z28.

I know a carburetor guy with excellent references.  Ron Stryker in Clearwater Florida.

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On ‎12‎/‎26‎/‎2015 at 6:05 PM, DXB said:

Yes thanks - this seems like a reasonable point- i just looked up the OH interval from Marvel Schleber - "10 years or engine TBO."   Overhaul might be worth considering- how many hours were on yours?   Mine is at 15-16 years but 700 hrs.  Also do you happen to know what the typical problems and failure modes are for our carbs, and how often they occur?  Not sure how to balance these risks with the maintenance induced failure risk, but would like to learn more.  

I just read the Harrison Ford NTSB report- seems like part of the main metering nozzle came off, making the mixture way too rich?  It's unclear to me if it was a wear-related failure or it was assembled incorrectly in the first place at last overhaul- the language in the report is noncommittal, but it had <200 hrs on it since overhaul in 1998.  Also this is 419 Holley carb- a pre WWII design, not sure how it stacks up against what we have. 

Yes different carb, but the point is, the OH periods are neglected by many owners, maybe because they are not educated on the parts that help make up the whole. Manufacturers put out these intervals & Service bulletins for a reason, and so many owners think if its not an "AD" it does not apply to them....a discussion I'm not going to get into here. My carb had 1200 hrs on it, but to your question....the float is a potential issue as well as accelerator pump. I will attach the SB information & letters I have on the carb as well as a bulletin for the mechanical fuel pump, good info to have. You may already be aware or have all this info.

lyc 366B Carburetor Throttle Body Screw Inspection.pdf

Lyc SB582 Reprint of Precision Airmotive Mandatory Service Bulletin No. MSA-13.pdf

Reprint of Precision Airmotive Mandatory Service Bulletin...Supplement 1.pdf

SL_15-17.pdf

SL_031_Volare_SB9s.pdf

Lyc 548A Diaphragm-type Fuel Pump Inspection-Replacement.pdf

Lycoming SB621 Fuel Pumps.pdf

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33 minutes ago, ShermAv8tor said:

Yes different carb, but the point is, the OH periods are neglected by many owners, maybe because they are not educated on the parts that help make up the whole. Manufacturers put out these intervals & Service bulletins for a reason, and so many owners think if its not an "AD" it does not apply to them....a discussion I'm not going to get into here. My carb had 1200 hrs on it, but to your question....the float is a potential issue as well as accelerator pump. I will attach the SB information & letters I have on the carb as well as a bulletin for the mechanical fuel pump, good info to have. You may already be aware or have all this info.

lyc 366B Carburetor Throttle Body Screw Inspection.pdf

Lyc SB582 Reprint of Precision Airmotive Mandatory Service Bulletin No. MSA-13.pdf

Reprint of Precision Airmotive Mandatory Service Bulletin...Supplement 1.pdf

SL_15-17.pdf

SL_031_Volare_SB9s.pdf

Lyc 548A Diaphragm-type Fuel Pump Inspection-Replacement.pdf

Lycoming SB621 Fuel Pumps.pdf

Thank you for this GREAT info!  Will go through carefully and ask my shop about it - I honestly haven't dug into the ADs and SBs on my plane too much as the MSC that did my prebuy / annual last year seemed to do a thorough job.  

It's interesting that carbs never seem to come up much in overhaul discussions - on this board or elsewhere.  There's lots written by folks like Mike Busch for the average owner to help think about IRAN or overhaul intervals of other components (e.g. doing a 500hr mag IRAN seems like a no brainer, but engines are complicated and may be may be reasonable to go beyond TBO in many cases).  Don't see much about carbs.

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  • 2 years later...
On 12/21/2015 at 7:50 PM, DXB said:

So my plane is in annual and I wanted to address the slightly low peak fuel flow (17ish gph rather than 18ish gph) at takeoff under standard conditions, figuring this may help my engine temp issues in climb.  I had assumed there were separate controls to set idle mixture (which I understand is relevant only at low rpm) versus main mixture settings, but per my shop this does not appear to be the case- only an idle mixture setting.  I think I have the Marvel MA-4-5 carburetor.  Carusoam posted a handy guide to these carbs on another thread:

http://www.insightavionics.com/pdf%20files/MA-4%20Carb%20Manual.pdf

I am reading but am still confused on how mixture could be enriched at WOT on takeoff for these carbs, and how it differs between carb models.  Can anyone shed some light on this, and if there is an easy way to address my peak fuel flow issue?

Were you able to find the solution for your problem? I am exactly at the same point you are at now... 

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9 hours ago, Oscar Avalle said:

Were you able to find the solution for your problem? I am exactly at the same point you are at now... 

Sorry short answer is no.  I am routinely in the 17-18gph FF range, which is pretty standard from talking to others.  I have the richer carb.  I really don't think it's the carb, I think it's intrinsic to the cowl design. I do notice there is adjustment in the economizer setting per the manual above. Let me inquire in the other thread.  

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  • 11 months later...
On 3/19/2018 at 8:14 PM, DXB said:

Sorry short answer is no.  I am routinely in the 17-18gph FF range, which is pretty standard from talking to others.  I have the richer carb.  I really don't think it's the carb, I think it's intrinsic to the cowl design. I do notice there is adjustment in the economizer setting per the manual above. Let me inquire in the other thread.  

I am having this problem in my C172. It has the MA-4-5 10 I adjusted the economizer all in and it helped but not enough. I also have a Mooney variant of the carb but it is the 10-4164 model. I wish I knew what nozzles were in each. I would install the Mooney carb on the Cessna except that looks like it wouldn't be legal.

You can adjust the economizer with the carb on the plane, well I can on my Cessna - probably not on a Mooney. I just couldn't  get enough adjustment.

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Welcome aboard, Stephanie.

Which engine do you have in your Cessna, O320?

The FF is most likely different than the O360 of the M20C... necessitating different nozzles..?

Then again, you might have the 180hp C172XP... :)

PP thoughts only, not a mechanic...

Best regards,

-a-

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3 hours ago, Shiny moose said:

100LL has BFSC of .5 gallon per hour per HP, weight 6.02 lbs at SL @59 degrees, to get your 180 hp 15GPH  200HP 16.6  just my .02

So why does the Owners Manual for my 180-hp C specify at SL, WOT/2700 that fuel flow should be 18.2 gph?

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5 hours ago, Shiny moose said:

100LL has BFSC of .5 gallon per hour per HP, weight 6.02 lbs at SL @59 degrees, to get your 180 hp 15GPH  200HP 16.6  just my .02

 

 

 

 

 

Not really relevant for the reason stated below. Also, how does 100LL have a BSFC?  BSFC will vary from engine to engine depending on thermal efficiency. An IO360 with an 8.7:1 compression ratio will get a little more horsepower per unit of fuel than an O360 with an 8.5:1 compression ratio (leaving the lack of precision inherent to carbs aside). It will get significantly more horspower per unit of fuel than than a TSIO360 with a 7.5:1 compression ratio. Fuel energy is measured in BTUs. 

2 hours ago, Hank said:

So why does the Owners Manual for my 180-hp C specify at SL, WOT/2700 that fuel flow should be 18.2 gph?

Because the mixture setting that is specified for take off fuel flow is not a stoichiometric.  In fact it's quite a bit rich of stoichiometric.  Simply speaking, some of the fuel is not being used to make power. The additional fuel is being used to slow the combustion event, thus producing lower peak cylinder pressure, thus producing lower CHTs.

Edited by Shadrach
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I just wonder how  much of this is folks scaring themselves silly over new instrumentation.  These engines have been banging around in these airplanes for over half a a century, honestly how bad can it be?  That and how accurate is your information?  What is the error in your measurements?  17ish vs. 18ish is only a 6% difference.  Are these instruments really that accurate?

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15 minutes ago, steingar said:

I just wonder how  much of this is folks scaring themselves silly over new instrumentation.  These engines have been banging around in these airplanes for over half a a century, honestly how bad can it be?  That and how accurate is your information?  What is the error in your measurements?  17ish vs. 18ish is only a 6% difference.  Are these instruments really that accurate?

Reading from the beginning, Dev simultaneously had high CHT indications and low FF indications on take off and in climb.  He could sell his engine monitor on eBay and take the ignorance is bliss route but I don't think that's how he's wired.

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3 hours ago, Shadrach said:

Not really relevant for the reason stated below. Also, how does 100LL have a BSFC?  BSFC will vary from engine to engine depending on thermal efficiency. An IO360 with an 8.7:1 compression ratio will get a little more horsepower per unit of fuel than an O360 with an 8.5:1 compression ratio (leaving the lack of precision inherent to carbs aside). It will get significantly more horspower per unit of fuel than than a TSIO360 with a 7.5:1 compression ratio. Fuel energy is measured in BTUs. 

Because the mixture setting that is specified for take off fuel flow is not a stoichiometric.  In fact it's quite a bit rich of stoichiometric.  Simply speaking, some of the fuel is not being used to make power. The additional fuel is being used to slow the combustion event, thus producing lower peak cylinder pressure, thus producing lower CHTs.

I can understand your point and trying to help this guy figure out a maybe problem, or maybe not a problem, but my point was that he needs a min FF to get the full power On an O360 180 HP  200HP or any other  piston powered engine, more than that will be for cooling, to much and you’ve caused another issue, to less fuel flow than needed to produce the rated power and you will not get to that HP no matter what you do to the engine. The manufacturer gives you what they need for FF for the additional cooling.  In our world  I dont think you can pull more HP out of a .5 pound of fuel than it has in it. If someone said they have an IO360 200HP that they have modified to burn only 10 gph on take off and 5gph per hour at 70% power I would call it a scam. 

An IO360 200HP pushing 60% or 120 HP will burn 10gph or a O360 180HP pushing 67 % or 120HP will burn the same 10gph, or how about a 540 400HP, if it is ran at 30% or 120HP ( dont know why we would do this ) you would burn 10gph. It’s about the HP not the engine size.

I agree BTU is what is what energy it takes to raise one pound of water by 1 degree, and fuel has BTUs , but BFSC is directly realated to this question as it is fuel flow per hour (pounds per hour) divided by HP . 

 

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