Carburetor Operation

[lowNslow]

Nov 2 -"Leaning on the ground has no effect in our carbureted engines, actually. At least not at typical operating rpms. The idle jet is not affected by the mixture control at all, and the engine operates completely on the idle jet below 1000 rpms."

Nov 12
If the main hydrant valve (normally intended to provide a 20 gallon per minute supply capable of shooting 100 meters into a high-rise apartment fire) is carefully, slowly operated to a closed position... it will be possible to drop the pressure of the drinking fountain as well. Crude,...but possible. (Sorta like using the main fuel valve operated nearly to the shut-off position in order to lean the carb during cruise, such as Miles alluded to earlier.)
This is what we are talking about. No one ever intended this to be the method of operation, but it is possible.

Congratulations George. It takes a real man to admit when he is WRONG.

[GAHorn]

Never admit it if you can help it.
(The technique is not an intended one, as I stated. It is the manufacturer and designer of the carb who states that only the idle circuit is operational below 1000 rpm. My original intent was to provide the most "correct" answer, albeit a short-answer. I'd hate for folks to go around without knowing the whole story. If you're gonna try to regulate the drinking fountain with a fire hydrant, then I wouldn't want anyone thinking that is some refined, sophisticated technique. In reality it's pretty crude and contrary to the intended operation. It can, and most probably does, result in the operation of one or more cylinders operating in a too-lean condition during ground operations. (You can't turn this crude carburetor into a finely-tuned fuel injection unit (and you can't do anything about that horrid, leaky, uneven-length induction system and it's inability to distribute air/fuel mixtures evenly/equally to all cylinders) by twiddling the cockpit mixuture control.) While the technique suggested will not result in any measureable harm at idle, such operation inadvertently continued into flight can be.)

[cessna170bdriver]

Don't let George's over-exaggerated analogy scare you. The mixture control on Marvel Schebler carburetors isn't nearly as "crude" and George would have you believe. It is actually a precision CONTROL valve, and is quite capable of a much greater turndown ratio than a SHUTOFF valve (a very different design) like the fire hydrant, or even the fuel selector valve in your airplane. The ratio between idle fuel flow and full throttle fuel flow is hardly comparable to the ratio between 20 GPM and a drinking fountain. (A 1000 GPM hydrant is small, and it would take over 750 psi to shoot water up a hundred meters.)

You shouldn't have to worry about hurting the engine. If you lean at taxi power until you get a slight RPM rise, or lean to roughness and richen SLIGHTLY, you'll never "inadvertenly" maintain that setting into flight because as soon as you open the throttle for takeoff (or runup for that matter), you're going to be greeted with a deafening silence. (I know for exactly the reason you think I might. )

I don't propose that this is "some refined, sophisticated technique". It is simple thing and is doable by us mere mortals. As far as the "intention" of the design, these engines weren't designed to operate long term on 100 ocatane fuel either, but we play the cards we're dealt. All I know is that ever since leaning on the ground was suggested to me, I've not had a single fouled plug or a single stuck valve; and that's on 100LL and the occasional few gallons of 80. No MMO, no TCP, no snake oil.

Sorry to go on like this, but my mother raised at least one stubborn child.

Miles

[GAHorn]

... All I know is that ever since leaning on the ground was suggested to me, I've not had a single fouled plug or a single stuck valve...

Miles

Well that must be fairly recent and with scant operational data since only July this year you reported " My experience has been that while using straight 100LL I've never had a stuck valve, stuck rings, or a fouled plug that couldn't be cleared with a full power runup." And as regards your engine failure and emergency landing due to a swallowed valve you wrote, "The engine was still turning in the neighborhood of 2000 RPM, and was actually somewhat smoother than the couple of times when I’ve had stuck valves."

Now my friend, you're beginning to personalize this if by your statement "George would have you believe" you are insinuating that I'm deliberately misleading folks. I admit my analogy is an exaggeration, but only for the purpose of illustrating what the manufacturer states. (This is aside from my conditional agreement that idle can be crudely influenced by use of the mixture control.)
Anyone can look at the Precision website and see the multi-colored diagrams in which they show the fuel and air flows within this carburetor. As carburetors go, this one is pretty crude. (And it can also be seen by their diagrams that my analogy of using a large valve to control the pressure issued by a smaller valve...is directly applicable to the main mixture valve in this carburetor being used to influence the fuel delivered by the much smaller needle-jet of the idle mixture screw downstream.)
A miniscule movement of the large valve can have a disproportionate effect on the smaller. Coupled with the induction system shortcomings, it becomes another case of "measuring with a micrometer, marking with a grease pencil, cutting with a hatchet" as far as individual cylinder mixtures go.


http://precisionairmotive.com/Publicati ... 20Rev1.pdf

While the mixture control is adequate for the purposes intended while operating on the main venturi, ...this is only because the air flow is so much greater when it's operating on the main jet, and even then,...no one I know of who rebuilds carbs will claim this carburetor's mixture control is anywhere near the standard that modern carburetor technology is capable of providing. It's a crude mixture control. (Not as bad as the old rotating air-hole-disc ones on the A65's, etc., but crude nonetheless.)
Miles, your own experience with your own carburetor may not be consistently repeatable on the fleet to offer your technique as a standard operation to the public with the assurance that "you'll never "inadvertenly" maintain that setting into flight because as soon as you open the throttle for takeoff (or runup for that matter), you're going to be greeted with a deafening silence".

As the professional and perfectionist that I personally know you to be, you are well aware that generalizations without accurate, repeatable measuring/operating criteria cannot be offered carte blanche without risk. Advising everyone to get "a slight RPM rise, or lean to roughness and richen SLIGHTLY" hardly qualifies as accurate, scientifically acquired and repeatable data. I for one can do exactly that with my carburetor and then I can slowly advance the throttle and obtain takeoff power settings without my engine dying,contrary to your assurance to the public that situation absolutely will not happen. The different operating personalities of this model carb are notorious.
As a further point of discussion, even the rapid movement of the throttle may not kill an engine operated as you describe,...depending upon the performance of individual acceleration pumps. (As you are certainly well aware, there are three available settings for the travel/output of the acceleration pump generically, with variations amongst the thousands of carbs out there in world use.) I worry about it being prudent to categorically advise the world to utilize a technique which has been observed to be used successfully on your small sampling of the fleet, without also offering some safety caveats regarding a procedure which runs contrary to the mfr's recommendations.
It remains the fact that the idle adustment screw is the method by which the mfr intends the idle mixture to be controlled, and any burden of proof as to the safety of operation to the contrary is that of the commentator. IMHO
I've never bothered to lean this type carb/engine further at idle, and this includes over 3500 hours of operating them. The only stuck valve I've experienced that I recall was due to incorrect assembly.

[cessna170bdriver]

... All I know is that ever since leaning on the ground was suggested to me, I've not had a single fouled plug or a single stuck valve...

Miles

Well that must be fairly recent and with scant operational data since only July this year you reported " My experience has been that while using straight 100LL I've never had a stuck valve, stuck rings, or a fouled plug that couldn't be cleared with a full power runup." And as regards your engine failure and emergency landing due to a swallowed valve you wrote, "The engine was still turning in the neighborhood of 2000 RPM, and was actually somewhat smoother than the couple of times when I’ve had stuck valves."

Yes, I've had three stuck valves that I can recall; all before I started leaning on the ground and before I limited myself to avgas. The no stuck valve, no stuck rings, and no fouled plugs has been my experience for the last 700-800 hours over the course of 11-12 years.

To everyone: Your mileage may (and probably will) vary.

Miles

Miles

[johneeb]

I just reread this column from the AvWeb site and have copied a couple of paragraphs pertinent to the discussion here.

June 30, 1999

Pelican's Perch #19:
Putting It All Together

In recent columns, John Deakin has explained all you need to know — and more than some of you wanted to know — about the three engine controls: throttle (MP), prop (RPM), and mixture. Now, AVweb's resident pelican puts all that theory into practical perspective by taking you through each phase of a flight — start, taxi, runup, takeoff, climb, cruise, descent and landing — and offering specific tips for getting the most from your piston powerplant.

By John Deakin

Carbureted Engines

Engines with the usual Marvel-Schebler carburetor (as found on Cessna 182s) have an accelerator pump just like your car engine does. Well, let me qualify that, because I haven't the faintest idea if modern car engines have such devices, and virtually no interest, either! I lost track of what's what in car engines in about 1964, I think. Dreadful, dirty things, I never did really like working on them. Now airplane engines, that's another story! Works of art, mostly. But, I digress. Back to pumping the throttle.

Any fuel produced by pumping the throttle is squirted into the carburetor throat, at a point too far from the cylinders to do much good at cranking RPM. Instead of helping the start, the squirt of raw fuel just dribbles down the low point in the intake system and puddles there, creating a major fire hazard in the event of a backfire.

It may also dribble out a drain tube onto the ground, making a puddle of fuel. This becomes an extreme fire risk if any torching occurs. I once very nearly lost a Lambert Monocoupe (an old wood and fabric airplane) from this very thing. Luckily, I saw the smoke, leaned out the door, saw the puddle burning merrily away with flames licking at the belly, and was able to leap out (nearly took the seatbelt with me!) and push the airplane back away from the fire.

Leaning on the Ground

Finally, you may find it beneficial to lean your engine after start, and for all ground operations. In theory, a properly set up engine will run at "taxi power" without fouling plugs, but the reality is that most general aviation piston engines are set up so rich (for easy cold-starting) they do often foul plugs. You can test this on your own engine by setting minimum idle RPM, then leaning until the engine quits. Watch the tachometer very closely for a small rise just before the engine quits. The more rise you see, the richer your idle mixture setting. On the big radials, this rise should be almost imperceptible, or "barely detectable," but some of the flat engines call for as much as 100 RPM rise. Again, this is mostly for optimum starting, not running. It's perfectly safe and often desirable to correct this with manual leaning, once the engine is running.

The downside of leaning on the ground is the very distinct possibility of attempting a takeoff that way, so if you lean on the ground, lean it brutally! You can't hurt the engine by leaning at "taxi power," but you sure can cause some heavy damage if you take off with the mixture partially leaned! If you attempt a takeoff while "brutally leaned," the engine will simply wheeze and die when you try to apply throttle. If you enrich at any time, for any reason, either go right to full rich and leave it there for takeoff, or re-lean it "brutally" once again.

[GAHorn]

If you get very much rise...then you should adjust your idle mixture screw towards lean. (Precison says 25-50 rpm max.)

[c170b53]

Hey this thread is great, just like the Nov 11 coverage of WWII, there's conflict. action and incoming fire. Can't wait to meet you gentlemen at the next convention. Is not the standard practice to avoid lead fouling on 80/87 engines running 100LL to idle at 1000 RPM and lean? In other words at 1000 RPM is not some of the fuel coming from the main jet.

[cessna170bdriver]

Is not the standard practice to avoid lead fouling on 80/87 engines running 100LL to idle at 1000 RPM and lean?

I used to think so....

[Tom Downey]

I just reread this column from the AvWeb site and have copied a couple of paragraphs pertinent to the discussion here.

June 30, 1999

Pelican's Perch #19:
Putting It All Together

In recent columns, John Deakin has explained all you need to know — and more than some of you wanted to know — about the three engine controls: throttle (MP), prop (RPM), and mixture. Now, AVweb's resident pelican puts all that theory into practical perspective by taking you through each phase of a flight — start, taxi, runup, takeoff, climb, cruise, descent and landing — and offering specific tips for getting the most from your piston powerplant.

By John Deakin

Carbureted Engines

Engines with the usual Marvel-Schebler carburetor (as found on Cessna 182s) have an accelerator pump just like your car engine does.

Remember when John Deakins talks about the 182 vhe is talking about a 0-470-/// MA 5

NOT the MA3 used on 0-300/0-320 and some 0-360s

In the MA3, the accelerator pump nozzle is pointed right at the opening side of the throttle plate, as the throttle is pushed in, the plate opens and stream of fuel is directed at the opening and the suction above the throttle plate will draw the fuel into the vertical chamber above the carb.

AS LONG AS the engine is cranking the fuel above the throttle plate will stay there or be drawn into a cylinder.

IF the engine is NOT cranking gravity takes effect and ANY fuel in the manifold, "Y" pipes etc is dropped into the airbox which drains onto the ground.

Priming the 0-300/C145, or the 0-320 series while not cranking does little good. because fuel in the manifold requires air flow to carry it to the cylinders. even the Lycoming engines that prime at the intake valve.

Yep,,,,,,,,I know a bunch of you guys pump the hell out of the primer before you start, and get a start every time...... but think about this, You wet the pipe enough, you get some gas where you want it.

But what happens to the extra fuel you pumped into the manifold? most of it is on the ground/airbox/ cowl. the rest? when the engine starts, here it come up the maniflod to foul plugs, wash the cylinder walls, deluting the oil there. right when we need the lubrication the most ( on start up).

[GAHorn]

I'd also remind folks that when Capt. Deakin (who is an experienced and knowlegeable pilot whom I admire and agree with quite often) Mike Busch, and others write for periodicals which promote the products of their advertisers. Anyone who has read AvWeb, or any of the other periodicals which publish his work, also carry in the margins the very products which are applicable to his writings. Example: Articles about engine operations (such as the one mentioned) virtually always have right next to them in the margins...ads for Digital Electronic Engine analyzers, digital engine intruments, etc. etc.
The philosophies of operations and the products promoting those philosophies are usually hand-in-hand on the same page.
While I'd love for TIC170A to have the income from those mfr's, it's not the promotion of those products which influence me to recommend the techniques I've believe in or have found useful or in the best interests of maintaining our machines.
In the case of this particular thread, ...I've found it very useful to lean during ground ops most turbocharged/fuel injected Continentals I've operated, because those systems lend themselves to that due to their more accurately controllable metering systems...which by nature of their type operations and turbocharging...also notoriously are adjusted grossly over-rich at idle. Those engines truly are contemptuous of excessive fuel at idle, particularly after landing when the engines are still quite hot and rich mixtures are definitely not warranted.
In these C-145/O-300 engines we operate in our 170's,...I've already stated my opinion that the mixture control is simply too crude a method to accurately meter the idle circuit. I can't help but feel this should be put in better perspective.
I've never measured it, but I'd be very surprised if more than a cup of fuel is consumed during the typical start-up, taxi-out, and run-up at a non-controlled field. I'd be very surprised if more than a glass of the stuff is used even at fields with control towers and busy traffic.
When comparision of that amount of fuel is made to the amount of fuel consumed over the next hour's flight.... the amount of fuel and lead that we're discussing is negligible. Yet, most of us would agree that (excepting the high-altitude takeoffs that occur several thousand feet ASL) we should take-off and perform at least all initial climbs at Full Rich. That take-off and initial climb likely consumes a gallon of fuel within the space of the first .1 hour on the meter.
With that in mind, I doubt that all the aggressive leaning in the world during idle/taxi-out is even noticed by our engines.
I'd much rather advise a person whose engine idle is so rich at typical operating elevations, that it behaves like Capt. Deakin's example "...as much as 100 RPM rise..." that their carburetor is simply very poorly adjusted and in need of maintenance. It needs it's idle mixture properly adjusted. The correct fix is with the knob on the side of the carburetor, not the knob in the cockpit. Just my opinion.

[kloz]

I am no expert, but we sometimes forget that these carburetors must be set up to allow the engine to run correctly at sea level. Anything above that would need "some" mixture control. Just wanted to add this to the discussion.

[cessna170bdriver]

I am no expert, but we sometimes forget that these carburetors must be set up to allow the engine to run correctly at sea level. Anything above that would need "some" mixture control. Just wanted to add this to the discussion.

I think George alluded to that a few posts back, saying that the idle mixture needs to be set for the lowest density altitude likely to be encountered. For most folks that would be on the ground on a not-too-hot day. For me, based at 4000 ft, my mechanic set it a bit on the rich side of the 50 RPM drop so it wouldn't be too lean near sea level (Bakersfield muni is under 400 MSL, less than 30 miles away.)

Miles

[GAHorn]

I think George alluded to that a few posts back, saying that the idle mixture needs to be set for the lowest density altitude likely to be encountered. For most folks that would be on the ground on a not-too-hot day. For me, based at 4000 ft, my mechanic set it a bit on the rich side of the 50 RPM drop so it wouldn't be too lean near sea level (Bakersfield muni is under 400 MSL, less than 30 miles away.)

Miles

Well, in light of this discussion regarding idle mixture as it relates to engine health, it's probably worth while to consider the reason this subject offers the very symptoms being discussed. Because someone noticed that little phenomenon that allows the engine speed to rise during shutdown, when the mixture is slowly pulled, it eventually occurred to many that it might be a useful technique to operate the thing at idle "brutally leaned".
If that technique has merit,...then why all the concern about the idle circuit being adjusted so "brutally rich" to begin with? re:

...these carburetors must be set up to allow the engine to run correctly at sea level. Anything above that would need "some" mixture control...

Actually, ...no. They don't. They don't need to be so danged rich. Why are they typically adjusted richer than absolutely necessary?

It must not be in order to avoid burned valves,...for we all know that valves are not injured below about 65% power due to the low combustion temps involved at low power settings. Idle is certainly low enough not to be concerned with that problem. (Therefore it's not harmful if the idle mixture knob on the carb is set fairly lean. It does not have to be set for a standard sea-level day for fear it'll hurt the idling engine.)

It must be in order to provide ready and reliable acceleration at engine speeds too slow to allow the throttle linkage to activate the accelerator pump which is the real "kick in the rump" when it comes to acceleration. (It'd not be a good thing for the accel/pump to operate too actively at very low throttle settings, otherwise the fuel sprayed at low mass-air-flows would tend to be way excessive, of high concentration and interfere with lubrication as Tom pointed out.) In fact, the accel/pump ideally operates after two other idle-circuit functions have excersized...the secondary and third (tertiary) idle circuits.
As the mass air flow increases due to an opening throttle, additonal fuel is brought into play from the secondary idle circuit, and air flow being still further increased by initial throttle opening brings still more fuel into the throat via the third/tertiary idle circuit. It is, in fact, these secondary/tertiary idle circuits which are primarily denied fuel by "brutally leaning" the carb with the cockpit control. That is why such technique stalls the engine ( as a desireable action according to proponents of the technique.) And, in my opinion, it is another reason why it is not necessarily a good idea to use that technique.
While it is best to always plan ahead, operate slowly and cautiously, with regard to taxying and manuevering in close proximity to other aircraft,...it is also a good thing to have available rapid-acceleration capability should one need it. I can think of a couple instances in my flying experience that it was handy to have rapid acceleration capability while taxying. I'll bet most of you can too.
I'm not convinced the amount of lead in that cup of fuel used for taxying to the runway is significant when compared to that which is used in the first few minutes of flight. Not to mention the amount that is suddenly sprayed into the throat the moment the acceleration pump is activated by the pilot on takeoff. No. If the carb idle is adjusted correctly, (and that means not excessively rich), then no further leaning action should be necessary at idle. Obviously, I even think it's inadviseable in most circumstances.

[David Sbur]

Any fuel produced by pumping the throttle is squirted into the carburetor throat, at a point too far from the cylinders to do much good at cranking RPM. Instead of helping the start, the squirt of raw fuel just dribbles down the low point in the intake system and puddles there, creating a major fire hazard in the event of a backfire.

It may also dribble out a drain tube onto the ground, making a puddle of fuel. This becomes an extreme fire risk if any torching occurs.

Try this, with a full carb bowl, and protect your eyes! With carb in hand (MA3SPA) try actuating the accelerator pump with short strokes or long strokes. With short strokes you will find that the fuel shoots up through the center and strikes the throttle plate and consequently dribbles down the center of the carb and back out the bottom to the airbox. I.E. you get a bunch of fuel on the ground and in the cowl. Yesserreee, a big fire hazard.

Try the same thing with long strokes and notice that the throttle plate 'gets out of the way' and the shot of fuel goes straight up and hits the intake spider, kinda splatters around, and lo and behold some of the fuel kinda gets sucked up into the cylinders albeit not terribly atomized.

Bottom line for me is that primer is better than accelerator pump, and if you use the pump make sure you are cranking the engine and that you use quick full strokes. Less strokes better, let's face it this method of priming doesn't seem to be the greatest. Better yet in my case, if I must use supplemental starting fuel I use the primer while cranking.

0-200A with MA3SPA on a Cessna 140 FWIW