Air / Fuel ratios on Corvair carbs.

Builders,

Here are some short notes on the topic of carbs.  It is my hope that builders will read and think about them, consider the logic before jumping up to debate. The Comments are based on 25 years as a working aircraft mechanic and working with Corvairs since 1989. These comments are not based on a single planes experience, but take into account all types of testing, education, and practical experience.

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How Rich is right?  Recently, a builder has told people that correctly running aircraft carbs on Corvairs need to have black sooty tail pipes.  I can flatly state that this is way too rich, and there are a number of very good reasons why you should not fly a carb running that rich.

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As a logical base line for what exhausts should look like, perhaps we can all agree that an Exhaust of Certified plane, running 100LL fuel, with a correctly running engine, with by the book performance, a Certified aircraft carb running without adjustment for more than 20 years. is a standard we should use. This engine has never fouled a plug in 17 years, has never harmed the engine in any way. Notice that the inside of the exhaust pipe has a dusty light gray color, and that new paper towel was vigorously wiped on the inside of the pipe, and only produced that light stain between my thumb and the exhaust pipe. This is the correct color and soot content for any Corvair running an aircraft carb.  I know this from working with countless flying Corvair powered planes over the years.

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Why not black and sooty? A correctly running aircraft carb on an air cooled engine will have an air/fuel ratio of about 12:1 in normal cruise. This will automatically go richer, to some thing like 10.5:1 at wide open throttle, and in low power cruise at altitude, it can be leaned to 14:1 for maximum efficiency.  Any engine that is making black soot in the exhaust and can be seen to visibly smoke at 1,000 rpm is running an air/fuel ratio of 9:1 or so. I know this not just from books, and working on certified planes, but from directly reading a laboratory grade A/F meter while running an EFI Corvair on my dyno in 2007:

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Above, An exhaust evaluation as part of an Electronic Fuel injection test on a 2,700cc Corvair in 2007. It is shown running at power on my dyno. With this arrangement, a simple twist of a knob on the computer produced any A/F ratio you wanted to test. This is how I can say what A/F ratio produces visible smoke on a Corvair, and it is part of how I can speak about it’s relationship with power output.

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At any airport with a density altitude less than 3,000 feet, your Corvair should run perfectly smoothly and make good power with the mixture set full rich, just like any Cessna 150 with the same carb will do.  One of the reasons why I use MA3-SPA carbs is so they have the exact same ‘normal’ operation as any certified plane I have flown, and if the carb doesn’t work like it does on a Cessna or a piper, don’t fly it, period.

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A builder with an MA3-SPA carb reciently said his engine only ran correctly with the mixture pulled half way out. He was considering actually doing his first flight in that condition. His home airport elevation is only 516 feet. If I went to his airport, and got in a Cessna 150 and it took pulling the mixture out half way to run correctly, You could only make me fly that plane with a gun to my head. Something is wrong with it, and sane people do not fly planes with things wrong with them. It doesn’t suddenly become “O.K.” because the carb is now on an experimental. Wrong is wrong, time to correct the issue, not to find some condition where it kind of works for the first flight.

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Any guy who would consider flying a plane in that condition, has missed the point of this story: Risk Management, Judgement Error, money in the wrong place. Where Ken Lien was killed on the very first flight of his plane because he didn’t bother to correctly assemble the mixture control on his plane and it moved to idle cut off on its own. If you are in a plane, getting ready for the first flight, and the mixture has to be pulled half way out to run, please explain to me how you know that this isn’t the first sign that the mixture is assembled incorrectly.  You wouldn’t, and there is a significant chance the engine will quit.  People who want to die should step in front of busses, not fly planes that are not set correctly, as using a plane and poor judgment to end ones life only unfairly punishes those of us who practice intelligent flying.

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If the mixture was half way out on the first flight, and the new pilot had to do a go around on the first approach, most pilots would instinctively push the throttle, carb heat and mixture to the firewall.  This works, and it is the correct procedure. However if the pilot is tolerating a plane that must have the mixture half way out, when he does this, the engine will quit, he will overshoot the runway, and smash up the plane on the over run. All the local experts will then say “The Corvair quit, I told him not to use a car engine, he should have used an O-200” Neatly ignoring the fact that it is the same carb as the O-200, and it would have done the exact same thing.  If instead, the same pilot stepped in front of a bus, preferably while holding the hand of the ‘Expert’ who tells everyone not to use car engines, aviation would benefit, and the rest of us would come out ahead. Cold, but you know it is true.

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Engines running black soot are wasting fuel, prone to fowling plugs, can damage the cylinder walls, and will have excessive carbon build up. On the other hand……..wait, there is no upside.

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Anyone who says that an MA3-SPA needs to be jetted differently for different displacement Corvair engines is wrong. Think of how many different engines have run on my test stand, all with the same, untouched in 15 years, MA3-SPA. Note that I have the mixture set full rich on the stand, and it runs cleanly on all engines. And yes, my stand has both EGT and O2 sensors. Beyond this, Dan Weseman and I recently took his 3,000 cc and 3,300 cc Corvairs to one of Florida’s most respected dyno shops and ran them both is a day long session.  What carb did we use? Why the same one off my run stand. It ran perfectly on both motors and the shops very elaborate instrumentation showed that the air/fuel ratio stayed correct through out the power range on both engines, without any kind of adjustment. Aircraft carbs work like that.

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Would you like to know how aircraft carbs are supposed to be operated? Read this story: Cylinder Head Temperature measurement and learn what a Lycoming Operations Manual is.  Down load it, print it, read it and know it. This is what successful people will do.

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Conversely, You could get advice from a guy who is neither an pilot nor an A&P, who has never owned nor flown a plane with a mixture control, teamed up with a guy who has never seen a Corvair turn a prop in person, and another guy who damaged his engine by using a carb no one ever head of so he could save some bucks. Take your pick, but if someone doesn’t like the concept of listening to the professionals and people with experience, again, I am going to suggest that bus thing again, I know it sounds mean spirited, but people willfully doing dumb things shouldn’t even be called ‘accidents’ because they are not really. an accident is someone trying to do the right thing. Willfully choosing not to do the right thing is not an accident.

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This ends the technical part of this story.  No valid technical information follows.

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I am not listening to William Wynne because:

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One: He sounds arrogant, and although I have never met him, and he wrote stories about people he loved: Risk Management reference page in hopes that others could avoid being hurt, I still say he is a jerk because I found two sentences in the 855 stories that are on this site that offended me, and I refuse to learn anything from him since.

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Two: I own a Prius, and he is always mocking people who own Priuses, and I can tell he isn’t kidding, and he feels superior about this, which is stupid because as a Prius owner I alone have a right to feel superior to all other car owners because I know the best way to protest the use of fossil fuels is to buy a car that you can feel superior about.

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Three: When I was in his tent at Oshkosh pontificating about how America has been ruined because no one follows the Ten Commandments anymore, he asked me to name them, and I couldn’t. The year before I said the problem with America was no one followed the Constitution, and he asked me how many articles it had and I said 10, and he said “guess again, you are off by three” , and I guessed 13.  I don’t get the connection that I should read more before being sure I am right.  I never listen to people with long hair, even though William has essentially the haircut as Jesus and everyone at the Constitutional Convention of 1787.

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Four: I don’t listen to people who sell things, because they are trying to make money off me. I only listen to people on the net who’s opinion about how to do things can’t get them a job doing it, nor is it apparently worth money to anyone. Those are the people I trust.  Yes, I know that I should trust William because he has a vested interest in my success even if he actually likes me or not, But I would rather trust people I have never met, who write in nicer tones, who I have a simplistic childish belief are motivated to tell me the truth, unstained by their limited experience, personal bias, and ego.

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If anyone read the above for points and didn’t find them funny, you probably have good taste, and I remind you I am a mechanic, not a comedian. I have a small but consistent group of people, most who have never met me, who remain quite sure that I have a “Condescending tone” and a “Giant ego”.  Before anyone is temped to say those things, I ask that they read the two paragraphs below, which appear both on my website and in every manual we print, and please share with me how this isn’t adequately honest and frank:

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“If you have never met me, but read this and think that I am charmed with myself, you got it all wrong. I know countless humans who are better people than I. They are kinder, smarter, and harder working. I can’t sing nor dance, I learn slowly, and I can’t stand to hear my recorded voice nor see my image on film. If I was once handsome, all trace of it is gone along with my uncorrected eyesight. I can be a conversational bore, and I deeply wish I had given my parents more moments to be proud of me. At 50 I look back on my life with a very critical eye and stand on the far side of a very wide gulf from the heroes of my youth. Even our dog, impeccably honest and loyal as canines are, Loves Grace and only tolerates me.

Honest evaluation leads to harsh thoughts like this. I spend a lot of time alone and have long bouts of insomnia, which can lead to thinking about things excessively. But the secret I would like to share with anyone who at times feels the same way, is that I have a sanctuary where I am insulated from much of my self-criticism, and a have a front, where at 50, I am much better on than I thought possible in my youth. When I am building things with my hands in my shop, I rarely feel poor. Although I now need glasses to do any close work, and my hands have lost a lot of dexterity, I am a far better craftsman than I ever was in my youth. I am not a great craftsman, but over a very long time I have worked to develop these elements in my life, and I compete with no one except who I was last year. While all else fades, these things flourish. It is a gift I am most thankful for.”

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ww.

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