O-200 mixture overly sensitive to altitude

iamtheari

iamtheari

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My dad got a 2007 Champ last year. I ferried it home for him back in September. It's one of the most fun planes I've ever flown. Every time I get out of it, all I want to do is get back in and fly some more. But there is one quirk about it that I'm curious about.

The mixture seems to be overly sensitive to altitude. What I mean is that you have to lean it slightly about every 200 feet while climbing in order to maintain power for the climb. Climbing at 60 mph or so and full throttle, it makes about 2500 rpm. That will go down noticeably and a little pull on the mixture control always brings it back to 2500.

The first time I discovered this was on the first leg of the ferry flight home, when the engine suddenly started sputtering like it was going to die as I hit about 4,000 feet MSL. Since then, I've discovered that the best technique for this plane is to take off full rich and, as soon as you are stabilized with a positive rate of climb, start to lean the mixture for best power.

Is that normal in an O-200 or is there an adjustment we should be looking at when it goes in for annual this summer?
 
iamtheari said:
What I mean is that you have to lean it slightly about every 200 feet while climbing
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I've had O-200's and this is not right.
So if you are "leaning slightly every 200 feet" ie 1/8"?, the mixture would be out umm 5/8" every 1000' or 2 inches in 3000'?
 
I'd be bringing it in to get checked out now, that can't be normal. Sounds like a carb problem but I'm not an airplane mechanic
 
Let'sgoflying! said:
I've had O-200's and this is not right.
So if you are "leaning slightly every 200 feet" ie 1/8"?, the mixture would be out umm 5/8" every 1000' or 2 inches in 3000'?
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I haven't measured how much I'm moving the knob. Here are some additional observations in the hope they narrow down what's going on. During the climb, I occasionally just pull it back slightly to maintain 2500 rpm. It's nowhere near the idle cutoff point in flight. The engine does stop promptly if you pull the mixture back all the way while idling on the ground. The operation is very consistent during flights and between flights. Going full-rich for descent and landing has not resulted in any problems. The engine idles very smoothly on the ground at our home airport, which is about 2,100 MSL. It has never stumbled on me when adjusting throttle settings (from advancing for takeoff or go-around to reducing to idle when coming in to land).
 
Everything is guessing at this point. It could be a carb issue, maybe a really poor version of the pepperbox fuel nozzle that's even worse than most. That nozzle was a reaction to the rough running experienced with the one-piece venturis mandated by AD in the 1990s. The new venturi messed up the airflow and the old nozzle didn't like it, so they drilled a bunch of holes crossways thru it, which created problems of its own, leading to another amendment of the AD that allowed the installation of the old two-piece venturi and nozzle as long as the venturi was checked for looseness every 100 hours.

A poor nozzle won't atomize properly, making the cylinders get both mist and droplets. They don't like that. I could see some altitude sensitivity there.

Check the air cleaner. A dirty filter makes the engine suck more fuel. Make sure the carb heat is fully closing. Hot induction air makes the mix richer. Is the carb heat creeping on and you don't notice it? Make sure the primer is IN AND FULLY LOCKED. An unlocked primer lets the engine suck extra fuel thru it.

How long since the mags were opened and inspected? What sparkplugs are in it, and how old are they? How about those ignition leads? Been tested for leakage lately? Weak or mistimed spark is the single biggest cause of engine malfunctioning. A cylinder with weak spark gets fussy about mixture.

Electrical problems (ignition) cause about 90% of engine problems. Remember that, and save money. Lots of money is spent chasing nonexistent fuel issues. Many don't understand ignition, so they ignore it. They think that a spark proves all is OK. A normal mag drop at runup will fool them, too. It's not that simple.

Like I said, it's all guessing. Lots of fun for the mechanic.
 
Well, you are going to lose RPM as you climb without a constant speed prop as you're going to continue to lose power as you climb due to thinner air. You may be over leaning if you expect it to be able to stay at a fixed rpm.

Just how much lower than 2500 rpm every 200 feet are we talking here?
 
Salty said:
Well, you are going to lose RPM as you climb without a constant speed prop as you're going to continue to lose power as you climb due to thinner air. You may be over leaning if you expect it to be able to stay at a fixed rpm.

Just how much lower than 2500 rpm every 200 feet are we talking here?
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Documenting that will be a good excuse to go flying. Given that the RPM comes back when I lean slightly, I don't think it's a matter of exceeding the altitude at which the engine can produce that RPM. The drop is not a lot and you can easily climb a couple thousand feet before the engine stumbles. I think I was at 4,000 flying off a field of about 1,000 feet when it really dropped on me that first time. But you definitely get a better climb if you start leaning promptly after takeoff and regularly through the climb.

Dan Thomas said:
How long since the mags were opened and inspected? What sparkplugs are in it, and how old are they? How about those ignition leads? Been tested for leakage lately? Weak or mistimed spark is the single biggest cause of engine malfunctioning. A cylinder with weak spark gets fussy about mixture.

Electrical problems (ignition) cause about 90% of engine problems. Remember that, and save money. Lots of money is spent chasing nonexistent fuel issues. Many don't understand ignition, so they ignore it. They think that a spark proves all is OK. A normal mag drop at runup will fool them, too. It's not that simple.
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Thank you very much for your post, which is definitely going to help us avoid monkeying with the carburetor needlessly. The plane and engine both have about 160 hours since new in 2007. Probably 20-30 hours in the past 8 months, with no change in behavior over that time. I do not believe the mags have been touched since new. I do not know which plugs it has but they are almost certainly original to the plane as are the leads.
 
iamtheari said:
Documenting that will be a good excuse to go flying. Given that the RPM comes back when I lean slightly, I don't think it's a matter of exceeding the altitude at which the engine can produce that RPM. The drop is not a lot and you can easily climb a couple thousand feet before the engine stumbles. I think I was at 4,000 flying off a field of about 1,000 feet when it really dropped on me that first time. But you definitely get a better climb if you start leaning promptly after takeoff.
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Now I think there is nothing wrong. What is your field elevation? An RPM increase is normal when leaning at full power at higher airports. If you can climb 4000 feet without stumbling there isn't likely much wrong with it.

Carbs are calibrated to run rich at full power. The extra fuel helps cool the engine and inhibits destructive detonation. Leaning to best power for an extended climb can damage that engine. See what your Aircraft Operating Manual has to say about it.
 
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iamtheari said:
My dad got a 2007 Champ last year. I ferried it home for him back in September. It's one of the most fun planes I've ever flown. Every time I get out of it, all I want to do is get back in and fly some more. But there is one quirk about it that I'm curious about.

The mixture seems to be overly sensitive to altitude. What I mean is that you have to lean it slightly about every 200 feet while climbing in order to maintain power for the climb. Climbing at 60 mph or so and full throttle, it makes about 2500 rpm. That will go down noticeably and a little pull on the mixture control always brings it back to 2500.

The first time I discovered this was on the first leg of the ferry flight home, when the engine suddenly started sputtering like it was going to die as I hit about 4,000 feet MSL. Since then, I've discovered that the best technique for this plane is to take off full rich and, as soon as you are stabilized with a positive rate of climb, start to lean the mixture for best power.

Is that normal in an O-200 or is there an adjustment we should be looking at when it goes in for annual this summer?
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Which Carb do you have MA3SPA or a Tillison?
 
Tom-D said:
0-200 is not the normal engine,, it could have several different carbs installed.
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I guess I don't know which engine it has for sure and got the O-200 information from an AOPA review of the new-production Champs. I'll take a look at the plane and report back on which carb it has.

Dan Thomas said:
Now I think there is nothing wrong. What is your field elevation? An RPM increase is normal when leaning at full power at higher airports. If you can climb 4000 feet without stumbling there isn't likely much wrong with it.
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Our field is at 2,100. The time it stumbled, I was leaving a field at about 1,000 feet and climbing up to 4,500. The stumble happened around 4,000. I wouldn't consider any of this to be high-altitude. Maybe I am just too out of touch with carbureted engines but it seems unusual to me for an engine to actually stumble due to being too rich at such a low altitude.
 
iamtheari said:
I guess I don't know which engine it has for sure and got the O-200 information from an AOPA review of the new-production Champs. I'll take a look at the plane and report back on which carb it has.


Our field is at 2,100. The time it stumbled, I was leaving a field at about 1,000 feet and climbing up to 4,500. The stumble happened around 4,000. I wouldn't consider any of this to be high-altitude. Maybe I am just too out of touch with carbureted engines but it seems unusual to me for an engine to actually stumble due to being too rich at such a low altitude.
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It might not have been the carb. Might have a sticking valve, or an intermittent ignition issue.

Might have an O-200, or the Lycoming O-235. Both were being used around 2007, according to the TCDS. Is the oil filler cap a big thing, more than two inches across? Continental. Or a small cap a little more than an inch across? Lycoming.
 
Dan Thomas said:
Might have an O-200, or the Lycoming O-235. Both were being used around 2007, according to the TCDS. Is the oil filler cap a big thing, more than two inches across? Continental. Or a small cap a little more than an inch across? Lycoming.
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If the oil cap is a fair indicator, it's definitely a Continental. It's the same size as the C85 on the Cub and nowhere near what I see on the Lycoming O-360 family of engines I am more familiar with.
 
iamtheari said:
Our field is at 2,100. The time it stumbled, I was leaving a field at about 1,000 feet and climbing up to 4,500. The stumble happened around 4,000. I wouldn't consider any of this to be high-altitude. Maybe I am just too out of touch with carbureted engines but it seems unusual to me for an engine to actually stumble due to being too rich at such a low altitude.
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It sounds like it could possibly be carb ice, possibly in conjunction with an over rich mixture. Had it several times on my C150.
 
dmspilot said:
It sounds like it could possibly be carb ice, possibly in conjunction with an over rich mixture. Had it several times on my C150.
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Wow... I had not thought of carb ice. I have never knowingly experienced carb ice so it simply was not a thought that occurred to me. The conditions were very humid that day. I don't know the temperature aloft but I do know I stopped about 40 miles later and waited out two days of rain and low ceilings before I continued my trip. I was taught to use carb heat with low power settings in the Cub so I didn't think to try it with the wide open throttle, especially after leaning the mixture seemed to solve the problem.

My next flight will definitely involve a climb with full rich mixture here at home where the air is much drier to see how it goes.
 
dmspilot said:
It sounds like it could possibly be carb ice, possibly in conjunction with an over rich mixture. Had it several times on my C150.
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A good possibility, though leaning won't usually help that much. Those small Continentals are real ice-makers, even on hot days that are humid enough.
 
Wow! An O-200A in a Champ. I learned to fly in a Champ in 1992. He hand propped it and as I recall it was a 65.

Anyway, unless you have a carburetor savvy mechanic, I expect that a rebuilt carb is in your future. If it is a Marvel, it is about $600 exchange. I just put one on my Cessna, and sent the core back yesterday.
 
The plot thickens...

More experimentation shows that, with the mixture full rich, the RPM actually peaks with the throttle pulled back slightly from wide open. The plane just got its annual inspection and the mechanic says that the throttle is reaching the full throttle stop with a cushion gap at the lever. Any thoughts on why the engine would make more RPM with the throttle backed off slightly than wide open, all else being equal?
 
Dan Thomas said:
A good possibility, though leaning won't usually help that much. Those small Continentals are real ice-makers, even on hot days that are humid enough.
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Yes! I fly it’s cousin the c-85 with 0-200 jugs n crank, and I hit carb heat at cruise about ever 15 mins... I also turn it on all through run up... they like making ice... that is fact that’s not very arguable... lots of Loss of power after take off with no known causes (sounds like carb ice to me, or very possible minimally)as I read NTSB reports so I’m very vigilant about it.

If you aren’t in a dust bowl run it for a bit before take off... high power settings does not mean it won’t make it, in fact can increase its odds as the air is moving faster through through the venturi... you don’t obviously want to run it at take off for the sake of full power and for the engines health, but a good shot before take off... count me in...
 
I have ten years or so behind the O200A in my Cessna 140, Winter and Summer, dry and humid and have never had cause to think of it as an ice maker. I rarely use carburetor heat even on approach and landing.

That said I do have a carb temp gauge. I only use carb heat on approach when the gauge is in the yellow or colder. Obviously, cold carb temp is much more common in the Winter, but even sometimes in the Winter I don’t need carb heat. Our average Winter morning is thirties and forties, but sometimes down to the teens.

My previous Mooney, a carbureted C model had the same carb temp gauge. My habit of checking the carb temp gauge on approach served me well with it. It’s frequency for needing carb heat seemed to be the same as the O200.

I am all in favor of using carb heat when necessary, but not at all anxious to grind away the top rings with ingested dust when unnecessary. Flying around bypassing the air filter is not extending the life of an engine.

.....and, to the OP, I find myself leaning it a lot. Once off the ground it usually doesn’t see full rich again until approach and even then I sometimes shove in the mixture then pull it out a skosh.
 
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iamtheari said:
The plot thickens...

More experimentation shows that, with the mixture full rich, the RPM actually peaks with the throttle pulled back slightly from wide open. The plane just got its annual inspection and the mechanic says that the throttle is reaching the full throttle stop with a cushion gap at the lever. Any thoughts on why the engine would make more RPM with the throttle backed off slightly than wide open, all else being equal?
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Had a 172 that would do that. IIRC it was a faulty fuel nozzle that wasn't atomizing the fuel properly, and closing the throttle plate a little would result in the fuel striking the plate and getting atomized and spread out some, improving the burn. At full throttle the plate is lined up with the airflow and the fuel just shoots right past it.

These carbs are notoriously inconsistent. One can spend a lot of time troubleshooting them. The 70-year-old Stromberg on my A-65 ran far better than any of the MA3/MA4/HA6 carbs I've worked with.
 
Dan Thomas said:
Had a 172 that would do that. IIRC it was a faulty fuel nozzle that wasn't atomizing the fuel properly, and closing the throttle plate a little would result in the fuel striking the plate and getting atomized and spread out some, improving the burn. At full throttle the plate is lined up with the airflow and the fuel just shoots right past it.

These carbs are notoriously inconsistent. One can spend a lot of time troubleshooting them. The 70-year-old Stromberg on my A-65 ran far better than any of the MA3/MA4/HA6 carbs I've worked with.
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Yep. Dealt with that too on a 150 years ago. Spend lots of time putzing with it or send it to a reputable shop who can flow test it and tweak it before it gets installed. Also make sure the airbox is decent too.
 
Dan Thomas said:
Had a 172 that would do that. IIRC it was a faulty fuel nozzle that wasn't atomizing the fuel properly, and closing the throttle plate a little would result in the fuel striking the plate and getting atomized and spread out some, improving the burn. At full throttle the plate is lined up with the airflow and the fuel just shoots right past it.

These carbs are notoriously inconsistent. One can spend a lot of time troubleshooting them. The 70-year-old Stromberg on my A-65 ran far better than any of the MA3/MA4/HA6 carbs I've worked with.
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Interesting, I'm seeing that same thing on my 172. Especially above 4000'. Can pull the throttle back about 1/2" and no change on rpms at altitude. Noticed it on the way to OSH this year. The diagnosis makes sense. Is the solution a new fuel nozzle? I'm picturing that to be a like a needle jet in a Mikuni carburetor...
 
iamtheari said:
The plot thickens...

More experimentation shows that, with the mixture full rich, the RPM actually peaks with the throttle pulled back slightly from wide open. The plane just got its annual inspection and the mechanic says that the throttle is reaching the full throttle stop with a cushion gap at the lever. Any thoughts on why the engine would make more RPM with the throttle backed off slightly than wide open, all else being equal?
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That sounds like the flat cam on my O-300.
 
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