Carburetor Operation

[cessna170bdriver]

In a recent thread it was claimed that the mixture control had no effect on the idle circuit in our Marvel-Schebler carburetors. After studiying the manual, I can't come to that same conclusion.

I am looking at Section 14 of the "OVERHAUL MANUAL for C-125 C-145 AND O-300 SERIES AIRCRAFT ENGINES". Section 14 is titled INSTALLATION, OPERATION AND MAINTENANCE INSTRUCTIONS, CARBURETOR - MARVEL-SCHEBLER MA-3SPA.

The following is an excerpt from paragraph c. OPERATION (1) Idle System. Words in blue are direct quotes from the manual:

"Fuel from the fuel bowl passes through the metering sleeve fuel channel and power jet, and into the main nozzle bore where it passes through the idle supply opening in main nozzle through the idle fuel orifice in idle tube where it is mixed with air which is allowed to enter idle tube through the primary idle air vent."

Looking at the cutaway drawing the "metering sleeve" referred to above is the orifice whose size is adjusted by the "mixture metering valve" which is connected to the "mixture control lever".

An excerpt from paragraph (2) Metering:

"The idle system and the main nozzle are connected with each other by the idle supply opening. The amount of fuel delivered from either the idle system or main nozzle is dependent on whether the suction is greater on the idle system or main nozzle, the suction being governed by throttle valve position and engine load."

In other words, ALL fuel going to the engine, whether through the idle system or the main nozzle, gets metered by the mixture control.

Miles

[N170BP]

Thanks for posting this Miles. I was fairly certain (from practical
experience) that aggressive leaning during ground-ops helps
to minimize plug fouling, but didn't have the nuts & bolts data
to back that claim up (until now!).

[David Sbur]

Having just studied this topic for my MA3SPA here is a link you would be interested in for any questions on our carbs...

http://precisionairmotive.com/
click on Support
click on MSA Float Carburetors
click on MSA Float Carburetor Handbook
click on Read the Book
click on MA-3,4

Shows diagrams and info, plus a link to troubleshooting. My idle-cutoff is sometimes intermittent and I got ahold of this info plus a carb overhaul maunal which suggests a culprit or two. Concur with info above regarding the circuit being all together.

[N170BP]

Interesting reading indeed, thanks for posting the link.

I'm thinking the fact that ground-ops has us often somewhere
at or over 1000 RPM while running around on the ground, and
running the mixture full rich while doing so may (emphasis may)
lead to plug-fouling over time. I know if I don't lean on the ground,
I get to clean the plugs every 40-50 hours or so. If I do lean on
the ground, I can get well over 50 hours out of a set before I have
to pull and clean them (to be fair, this was back when I was running
100LL exclusively through the engine).

Just pulled the plugs during annual, and a combination of running
MOGAS and/or TCP added to 100LL resulted in very little
plug cleaning required (light tan deposits were found with no
lead fouling what-so-ever).

[David Sbur]

http://www.cessna120-140.org/forum/view ... 13a6a25071

This link will put you on the 120-140 site and see my discussion on it, plus a schematic that someone drew up for me.

I tend to run mogas/avgas blend and my annuals with about 80-100 hours on the plugs result in some pretty clean plugs, certainly little to no lead. I do tend to get sooty plugs on the ground while running up for something like a compression check. Funny, this last time the sooty plugs were the back ones on my 0-200, we debated whether it was a rich mixture or more likely they somehow were running cooler back there, who knows! Shutdowns I lean heavily for about 30 seconds with increased rpms, then full idle-cutoff, something I learned from local engine shop to reduce lead and general fouling, FWIW.

[Tom Downey]

""In other words, ALL fuel going to the engine, whether through the idle system or the main nozzle, gets metered by the mixture control. ""

If it didn't, how would you secure the engine with the mixture control?

[cessna170bdriver]

""In other words, ALL fuel going to the engine, whether through the idle system or the main nozzle, gets metered by the mixture control. ""

If it didn't, how would you secure the engine with the mixture control?

Thats the same question I asked George, who claims his manual says that the mixture control doesn't affect idle mixture on carbureted engines.

Miles

[kloz]

Wouldn't idle mixture be one thing, and idle cut off be another. The idle mixture controls the fuel-air mixture at idle. The idle cut-off controls the volume of the whole thing. No supply no run.

[GAHorn]

Miles and Tom, read either the section from the engine ovhl manual, OR from the official link provided earlier by David Sbur, and keep reading past the "operation" section...to the "metering" section, where it begins the paragraph with:
b. METERING, (Ref. appropriate figure for a particular model). – All fuel delivery on idle, and also as steady propeller speeds up to approx. 1,000 rpm, is from the idle system.

There follows, in that section, a detailed description of the idle system metering, and also the explanation of how vacuum smoothly transitions from idle metering to full throttle, as well as (in section 6, Mixture) how the mixture control rotates via a transverse slot, the amount of fuel metered during open throttle operation, and also provides a method of complete fuel shut-off for engine shut-down. The mixture control does not meter during idle operation at/below 1000 rpm. (It does however, have the ability to cut the fuel completely off, as described, ...but not intended as a method of "leaning" the idle circuit. Of course there are in aviation, many old pharrtts who have figured out ways to "trick" or "finesse" the systems by operating them in other ways than the mfr's intended...or at least think they do.)

[cessna170bdriver]

George,

The "smooth transition" between the idle system and the main nozzle is due to the different locations of the OUTLETS of the two systems. The idle system's outlets are above (downstream) of the throttle plate where the suction is greatest with the throttle at or near the closed position. The main nozzle outlet is in the throat of the venturi where the suction is greatest at wide open throttle (maximum air flow through the venturi). As the throttle is opened, the suction above the throttle plate decreases, and the suction in the venturi throat increases (due to greater air flow), thus causing a transition from the idle system to the main nozzle.

However the idle system and main nozzle have a COMMON INLET, and that is through the mixture control valve and power jet. The idle supply opening is actually located in the bottom of the main nozzle bore.

Miles

[GAHorn]

Correct. One could go a step further (for the sake of illustration) to say the entire carburetor shares a common fuel source...the aircraft fuel tanks. But I believe their comment that due to the airflow (which you describe slightly differently, but nonetheless the same as the mfr) that only the idle mixture controls fuel metering below 1000 rpm. It's the way they designed it to work. You could block off the entire exit of the main jet, and it would still idle completely on the idle jet, exactly as before. The bottom of the main jet source, while allowing the idle jet to acquire fuel during idle, does not supply fuel to the main jet at idle below 1000 because insufficient vacuum is coming from the main jet. Therefore, only the idle mixture meters fuel below 1000 rpm.
(Did you notice that the idle mixture control is downstream of that fuel source you mention? Therefore it is the idle mixture control which is soley controlling during idle.)

[cessna170bdriver]

Correct. One could go a step further (for the sake of illustration) to say the entire carburetor shares a common fuel source...the aircraft fuel tanks.

True, but it would be extremely difficult to adjust the fuel selector valve finely enough to control the mixture.

(Did you notice that the idle mixture control is downstream of that fuel source you mention? Therefore it is the idle mixture control which is soley controlling during idle.)

Yes, George, I did notice that; I even quoted the part of the manual that says so. But the cockpit mixture control is in SERIES with the idle jets (there are two, one fixed, and one adjustable by the idle mixture screw). When the cockpit mixture control restricts the orifice upstream of the idle jets to the point it is smaller than those jets, it becomes the controlling orifice. That is why, even at idle, with a properly adjusted idle mixture screw, you can slowly move the cockpit mixure control toward the lean position and get a rise in idle RPM. The engine will maintain that RPM until you either lean further, or add throttle, either of which will cause the mixture to become TOO lean and the engine will either stumble or quit.

Miles

[Tom Downey]

There are 2 ways for fuel to get out of the float bowl of a MA-3/4 carb. that is thru the mixture valve, and the accellerator pump.

Both Idle Cut off, and all mixture is accomplished by the mixture control valve. Idle cut off is the same as saying the valve is fully closed.

[GAHorn]

We have this fire hydrant on the corner. It has a huge valve on it that turns the water on or off for the firemen's use.
It also has a small drinking fountain plumbed into it.
When the main hydrant valve is open, then the drinking fountain has water pressure sufficient to allow a small trickle of water for us to drink from.
If the drinking fountain lever is operated, a 1/4" column of water about 3 or 4 inches long will "arc" up-and-over for us to drink from. This is analogous to the idle mixture circuit of the carb.
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.
It's a technique that anyone can observe if they idle their engines and very, very slowly pull the mixture to cut-off. A very slight rise in rpm will occur before the engine dies. This is because the idle mixture is deliberately adjusted slightly rich to accomodate low density altitude days, so the engine will not idle too lean and so it will not stumble during acceleration. (What Miles is describing.) This can and is used to test for a correctly adjusted idle mixture. (In fact, if a small rise does NOT occur, then the idle mixture is determined to be adjusted too lean.) The technique is not recommended as a regular method of operation, but only as a test of the idle mixture.
It is a technique...rather than a documented or recommended procedure, because if the throttle is advanced slowly the engine may still accelerate and allow a takeoff...yet because of the overlooked/forgotten leaned-position of the mixture control, the engine is running too lean during takeoff for it's best health. (The economizer circuit "override" is defeated and the engine will actually run excessively lean during full throttle climb. This is not good because the engine uses the extra fuel to keep cool during initial, open throttle climb.) Conversely, if the accelerator pump is activated sufficiently during a rapid acceleration,...again the engine may achieve takeoff power, yet actually be running too lean for it's own good, once it acheives a steady-state climb. Just FYI.
(Of course, all this talk about micro-leaning the engine is seriously flawed when one considers how crudely designed this engine and carb actually are. We talk as if we are dealing with some finely-tuned, pressure regulated, fuel injected engine,...rather than a loosely designed, hose-coupled, multiple-length induction system being fed by an updraft carburetor similar in complexity to the Marvel Shebler on my 39 cast-iron Ford tractor.)

[Tom Downey]

GA, I lean the 0-300/C-145 at all altitudes and power settings.It isn't that difficult to pull the mixture out untill it stumbles and then push it in a wee bit.

And OBTW, the smooth transitition from idle to cruise jet is accomplished by the accellerator pump. With out it, the best adjusted carb will allow the engine to quit.