None at all, Barry.

Transition is good.

....I have some bad news for majortom and DarZeelon....your ideas, theories, concepts,
understanding....or whatever you want to call it, about the operation of these simple
aero-engine carburators is incorrect. I don't know any other way to say it.

Both of you are in disagreement with the explanation offered by Webra Engines, are you not ?
Both of you agree that the low speed, or idle adjustment is the primary fuel adjustment, that
controls the carb operation. You say the high speed adjustment is just the secondary adjustment....yes ?

The explanation offered by Webra Engines differs with your explanation. Would either of you
like to change your story ?

FBD.

Dave,


I would definitely not!

The explanation offered by the Webra manual is, due to a poor choice of words, ambiguous and inconclusive.

Reading down, the text does refer to the 'influence' of the so called "idle needle's tapered tip", but does very little (diddly squat, actually) to explain how it actually controls the size of the fuel orifice, at various throttle settings.

This lack of a clear explanation for the operation of the low-speed needle's tapered tip, together with the use of the incorrect terms 'idle range' and 'idle needle', make this manual less than useful... despite the fact that the original TN carburettor's design and operation, was 'copied' by nearly everybody in the model engine industry.


And Dave, it serves little purpose if you go into a civil discussion (with or without your 'shadows'...), and 'transform' it into an argument...

You should simply agree with what I and Tom wrote here.

It will not 'demote' you in any way, shape, or form.

Hi Dave--I do not wish to change my story. Right now it's Saturday morning, I have a contest starting in a few hours, so will have to come back when I have the time to expand on my remarks. The only argument you offer in opposition is the Webra manual. The diagram is fine, but limited (only 2D, whereas a 3D cutaway animation would be a lot more helpful). The Webra product is a fine piece of engineering, at least as far as my own satisfaction with it goes. Our argument is really over the little statements accompanying the diagram in the Webra manual. Dar says they are misleading, and I agree. More later.

It's most likely a bad choice of words in the translation. Happens all the time. This isn't something to argue about.

You can probably prove the manual wrong by setting the low speed very rich, and watching for the smoke to taper off when the high speed needle valve begins to take effect, and low speed metering ends. I've done that before to show a buddy where he was wrong about something.

Let's keep it friendly!


edited for grammar

Thanks blw, your demonstration illuminates the matter very neatly, and I will let it go at that. I'm not mad, my nose is not bent, and I don't wish to quarrel over semantics.

The information given by Webra in this case is spot on, 100% accurate. It is not easy to grasp
new factual information about things if you have your own theory already set in stone in your mind.

Looking at these types of cross section pictures are not very helpful. It simply doesn't explain things
clearly, in fact from my own experience, it has really mottled the waters instead of clearing thing up.
It is hard to see what's what looking at a cross section drawing.

If you Guys want to lean something about these carbs, and how they work, I will devote some time to it.
Many others can learn as well. I will also dispel some carb myths that have already infected you Guys.

I will not argue about it with you. The point is, if you guys already have all the carb knowledge you
want, I won't waste my time trying to teach you something you don't want to hear. The question is,
is there anything you could possibly learn about these carbs....or do you already know everything
about them ?


The instruction from Webra about the "idle needle (2) only effects the idle range" is where you guys
are missing the boat.

This is your stance:

FBD.

Dave,


The point is: Do you think the throttle at 1/2 - 3/4 open is a part of the 'idle range'?

...I can just imagine that model of yours, with the Tower .75 and a Master Airscrew 11x8, adjusted to "idle" at 3/4 throttle, coming in for a landing...


I believe even you agree that the taper of the low-speed needle, in conjunction with the spray-bar/fuel nozzle of the Webra TNII carburettor, does control the mixture strength up to that point.

Yet the manual calls this 'the idle range'... Maybe the translation to English was wrong, but the fact is it can be misinterpreted and the method, by which the tapered tip of the so-called 'idle needle', in conjunction with the 'fuel supply opening in the sparybar', influences the 'idle range' is not explained to the least... even though it is supposed to make you 'grasp the principle'...


You see, Dave? I don't care if half the world calls this second needle in two-needle carburettors the 'idle needle'.

It is much more than that and deserves to be recognized as such.


'Idle range'? there is no such thing.

There are three points to consider, as stated by the Webra factory instructions. There
are plenty of "Internet myths" associated with these points. Let's have a look at them,
shall we ?

To grasp the principle of the TN II carburettor (the most common carb on our r/c engines)
it is important to understand that the:

a. the high speed needle (1) only affects the high speed range

b. the idle needle (2) only affects the idle range

c. the tapered tip of the needle (B) influences the idle range in conjunction with the fuel
supply opening in the spraybar (A).

Let's consider the low speed needle first, and this statement:

Webra Engines makes two references to an "idle range" in their factory instructions. What is
this "idle range" ? The idle "range" is the portion of the lower speed of the engine that is
"controlled" by the low speed needle...the taper of the needle being the controlling
part. When the throttle (barrel) of the carb is cut back to an idle, the low speed needle goes
inside the spraybar nozzle. When the LS needle is outside the spraybar, it serves no function.

Only when the LS needle taper comes into the spraybar does it have any effect on the fuel flow.

Let's backtrack for a second, so we can understand what is taking place. When the carb barrel closes,
even if it is closed all the way, there is always a small amount of fuel that flows into the spraybar.
This is to allow fuel through for the idling of the engine. This is by design. There will always be this
trickle of fuel....if there were not, the engine would not have any fuel necessary to have an idle.

To be continued....

....relax, DarZeelon....

The idle "range" is the portion of the lower speed of the engine that is "controlled" by the
low speed needle...the taper of the needle being the controlling part.

The "idle range", which you continually argue does not exist:

....is the "range" controlled by the taper of the LS needle. This "range" is quite small. It exists
from zero throttle opening (no air entering the engine) to about 1/8 throttle opening, certainly
no more than 1/4 throttle....probably not even that much. Keep in mind, as the throttle barrel
opens, it is moving away from the LS needle.

The taper on the LS needle works the same way as the taper on the high speed needle. As the
LS needle runs out of it's "range"....the taper of the HS needle is already allowing more fuel into
the spraybar. The difference between these two functions is known as the "transition". This
function is not accomplished by accident. When all things are well, the transition from the
LS, to the HS is not noticeable. This is what carb tuning is all about. The carb myth about the LS
needle comes in right here.

To be continued.

Dave,


The low-speed needle affect fuel flow, even when it is completely out of the nozzle.

Since fuel is a liquid, its flow will be reduced even if the needle is 0.5-1 mm out from the nozzle, because its presence there will force the outgoing fuel to alter its route.
It is true, however, that its major influence is when its tapered tip interacts with the nozzle, effectively changing the size of the fuel orifice.
I.e.; increasing it, as the throttle opens and it moves progressively out of the nozzle; and decreasing it, as the throttle closes and it moves progressively into the nozzle.


As to your 'back-tracking'... i.e.; ' the 'idle range' is any carburettor setting, in which the low-speed needle's tapered tip affects the fuel flow...
It is akin to statements like; 'wherever I lay my hat, that's my home'...


This is the part-throttle range and using the term 'idle range' to describe it is wrong and misleading.

This is where the misinterpretation begins. Most guys think that because there are
two adjustments on the carb....the low speed, and the high speed....that there are
two avenues of fuel supply. There is only one. The myth is that "if the LS is set too
rich, it will effect the HS throughout the function of the full range of the carb". This
does not happen.

Once the carb goes past the "idle range" there is only one place where the fuel can
come into the carb. In fact, this is the only place where fuel enters the carb, ever.
The fuel comes in through the HS needle....no where else.

Another Internet myth is that there is an "idle circuit" associated with the
LS needle. There is no such thing. In order to have an "idle circuit" there has to
be a separate channel for fuel to enter the venturi of the carb. This of course
does not exist in these type carburetors. This is where most guys go wrong,
thinking that there are two "fuel adjustments", and therefore two places
where fuel comes into the carb.

This is why the Webra manual correctly states:

b. the idle needle (2) only affects the idle range

The fact is....even though it may be hard to accept....the LS needle does
not, and cannot effect anything but the "idle range". Because once the LS
needle is "clear" of the nozzle (spraybar) it can no longer restrict the flow
of fuel....and the important thing to remember is....the only function of the
LS needle is....to restrict the small amount of fuel that is allowed to pass
past the HS needle....when the barrel is near closed, or in the "idle range".

Think about it.

FBD.

Mostly true, Dave.

In a two-needle carburettor:

1. There is only one fuel circuit and it is used to supply fuel to the engine, over its entire operation range.

2. ...But, this circuit has two controls, placed in series to one another. They are controlled obstructions to the flow of fuel.

3. The first is 'the high-speed needle', which for the great majority of R/C carburettors, is fixed, once set by the user (the effect of higher fuel pressure at high throttle is minuscule).

4. The second is 'the low-speed needle', or 'fuel metering valve', which changes its degree of flow obstruction, as the throttle position is changed, making it easier for the fuel to flow as the throttle is opened further.


The second control thus affects the amount of fuel drawn by the engine, as long as it is a greater obstacle to its flow.
The first control takes over, only at very high throttle positions, as the second control ceases to impose a limitation to flow; and thus controls ONLY the maximum flow of fuel the engine can draw.

Providing the high-speed needle is set for a peaked top-end, opening it further, when the engine is at a part throttle setting (i.e. fuel supply is limited by 'the low-speed needle-nozzle interaction'), will hardly have any effect on the mixture strength. Closing it will not have much effect either, until it; instead of the low-speed needle-nozzle interaction, becomes the greater obstruction to fuel flow...
Closing the throttle further will again 'cancel' the effect of the high-speed needle.


In engines with two-needle carburettors that have full-range fuel metering, those that employ a longer nozzle with the fuel orifice at the side of the nozzle, in the middle of the venturi; such as the ST, or the MVVS carburettors; it is possible for the low-speed needle's position, if it is closed too far, to make it impossible to adjust the high-speed needle for full throttle operation.
Opening the high-speed needle more will have no effect in such an instance, like at a part-throttle in other carburettors.

My posts from this point on will be a tutorial for those who want to learn. It is important for
those who wish to learn about this carburetor operation not to get sidetracked worrying
about other types of carbs, lets stick to this one basic style first. Once we learn the basics,
we can branch out and look at variations and exceptions.

Lets look closer at the parts of the carb, shall we ? When we examine the parts carefully,
we can see how they work....and how they interact with one another. The interaction of the
parts is simple, and at the same time very clever indeed.

The parts we need to concern ourselves with are the:

1. carb barrel, the part that rotates inside the carb
2. nozzle, the round brass tube that is pressed into the barrel (1). I will refer to this tube
as the spraybar when we talk about it's function in the carb.
3. main needle.
4. low speed needle.

You have seen how the barrel inside the carb slides back and forth as you
move the throttle arm ? There is a screw on top of the carb body that threads
into the body. On the end of that screw is a portion that is not threaded. That
non-threaded portion fits into an angled slot in the barrel. When you open and
close the barrel, the carb barrel moves sideways too, because of the angled slot.

Look at your main needle. It is referred to the main needle because it's adjustment
controls about 90% of the carb function, hence the name "main needle". See the thin
taper on the main needle ? This taper, as it rides inside the nozzle, controls all the
function of fuel metering, except for the very small amount in the "idle range" that is
controlled by the low speed, or idle needle. When the main needle is in position, it fits
inside the nozzle/spraybar. As you open the barrel to allow air flow, the nozzle pulls
away from the taper on the needle.

As we increase the amount of air going into the carb, the taper on the main needle lets
more fuel in as well. The taper has been carefully designed so ratio of fuel/air is good
throughout the full range of operation. The amount of air going in at any given throttle
setting remains the same, but we can adjust the amount of fuel by turning the main
needle in, or out. We can adjust the over-all "mixture" of fuel/air.

When we have the barrel all the way open, or in full throttle, the amount of fuel we let
in is still controlled by the taper on the main needle. Once we get the fuel flow adjusted
at full throttle, the taper on the needle takes care of all the "partial throttle settings", except
for the very bottom, or "idle range". The adjustable main needle serves as an adjustable
"main jet" would in other applications.

Lets think about the other end of the nozzle/spraybar now. The low speed
needle is screwed into the carb barrel as well, on the opposite side from the main needle.
It moves out of the spraybar as the barrel is opened, so the amount of fuel changes anytime
the barrel is rotated. Think of the low speed needle as a "plug"....an adjustable plug. The LS
needle will act as a plug, in fact, if you screw the LS needle all the way in....it will completely
plug the nozzle, and no fuel will flow.

The small taper on the low speed needle works just like the taper on the main needle, except
the distance of travel is very small. This distance is called the "idle range". The statement in
the Webra manual is correct when it said; "the tapered tip of the idle needle influences the
idle range in conjunction with the fuel supply opening on the spraybar". The "fuel supply" is
the small amount of fuel, or trickle of fuel that I mentioned earlier. This small amount of fuel
is allowed to flow past the main needle, even if the barrel is closed. There will always be this
small amount of fuel, otherwise we would not have any fuel available for the engine to idle with.

The term "influences" means, of course, "to adjust". Keep in mind that the taper on the LS
needle keeps the mixture correct, just like the taper on the main needle does, but it only
does is in the idle range. When the LS needle is pulled out, or clear of, the spraybar....it no
longer does anything. The main needle is metering the fuel....all the fuel. The LS needle is
now just along for the ride.

This is why Webra states; "the idle needle only affects the "idle range". It is true also that
the high speed needle only affects the high speed range. It is also true, as the Webra manual
says: "it is important to grasp these principles" if you want to understand how the carb works.

FBD.

The document that Dar supplied the link to is curious. If you bothered to read the WHOLE THING, you would see that only the TN instructions are worded that way. Why? Well the individual putting together the pretty PDF document might not know anything about carbs and didn't question the text given. It could be old text that nobody bothered to review and there was a issue in translation years ago. Rather than wasting your time here, one might write to Webra and question their manual.

Both of you are correct. The low range certainly is influenced by the high speed needle. Make a drastic change to the main needle and it will change the "mixture control"(idle) needle. But it should be noted that Dar is very correct that you would not use the main needle to adjust the midrange (part throttle) running characteristics. Very few of our carbs have a midrange adjustment so we are stuck with what the manufacturer supplies as a tapered needle to control midrange and hope they did things correctly. If the the taper and inward movement of the barrel is correct will properly meter midrange, in conjunction with providing a good idle mixture. The taper on the idle needle does two things.

The taper on the low speed needle is chosen so that it:
A. Allows adjustment of the low speed mixture strictly by means of turning the needle in and out like you do when you adjust it.
B. Meters fuel in the mid range by the action of the barrel or slide moving the entire low speed needle toward the spray bar orifice.

Now, it should be apparent that adjusting the low speed needle influences the midrange mixture. It should also be apparent that the taper on the low speed needle influences the midrange mixture and is dependent on throttle setting. No manufacturer offers different tapers for the low speed needle, because good enough is just that. If you don't understand what I mean maybe you could look at [link=http://www.mikuni.com/fs-manuals.html]Mikuni HSR Tuning Manual[/link]. Here it is obvious that there is a separate high speed, midrange, and idle circuit. You can adjust the midrange independently by changing the metering needle or adjusting its position. Our carbs don't have this.

I recently installed a Rossi slide carb on a diesel conversion I'm working on. It's a two needle setup and works just like I described above. The high speed needle is easy to set. The idle to off-idle is set nice and lean. The midrange is obviously rich and smoky. Why? Simply because the taper on the low speed needle is incorrect for use with fuel at nearly 15:1 air fuel ratio compared to the 4-5:1 used in most glow car engines at 30% nitro. There is no adjustment on the carb that will correct for this. I need to make a new needle with a appropriate taper to properly meter midrange.

Now, the TN carb I have seems to run on the main needle until about half of the area of the throat is closed. That seems to be more than others I have but similar to a couple car slide carbs I have. Keep in mind though that a reduction of half of the carb area is still a lot of power, probably 75%, not half RPM.

I'll have to agree that the wording for the TN models appear poor, but do we really know that this isn't how it is designed to work? It's been a while since I ran my Webra T4-40, but I had no trouble tuning it when I did. I used regular procedure tuning the main needle first, then adjusting the low speed until I had a good transition. Looking at the carb I knew this method would work and it did.

I was going to bring up Saito carbs last week. If you don't know that the low speed needle influences the main, you will have fits tuning a Saito. I noticed it when I was tuning the the FA-80 I own. Funny how the manual makes no mention of it.

Dave,


I understand perfectly how a two-needle carburettor works.

What I don't understand is your claim in post #39 that the main needle controls 90% of the carburettor's function...

Would you care to explain how does it, for example, tailor the fuel curve for the engine's decreased fuel needs at 1/4, 1/3, 1/2 throttle?...
Oh, there is no explanation for this?

That is because it doesn't!

The low-speed needle's taper, in conjunction with the fuel nozzle's tip, is what controls this function.
This is done, as Webra did not care to explain, as the diagonal locator slot in the carburettor barrel causes it to move into the carburettor body, together with the low-speed needle, as the throttle is closed.
The needle's taper moves about 2 mm into the nozzle, as the carburettor barrel is rotated from fully open to idle.

Contrary to what you wrote, the low-speed needle moves much more than does the high-speed needle. And its movement is directly controlled by the throttle.


Maybe you, like Webra, chose the wrong wording... The 'main needle' (for which I prefer the name 'high-speed' needle) does control the fuel mixture at 90%-100% open throttle; not over 90% of its range.

Please read what I wrote in post #38 again, as to the interaction of the low-speed needle with the high-speed needle.

This is not meant as an insult.
Understanding this should clear up many misconceptions.


I stand by my claim that this part of the Webra manual is incorrectly written, uses incorrect terms and does not explicitly illustrate the function of this excellent carburettor.


In the mean time you can practice 75% throttle landings (it is within your definition of Webra's 'idle range'. No?)...

I have examined and tested nearly a dozen different carbs for .60 and larger engines. The only carb that I have found that is almost perfectly tunable at any range, and on any engine from .60-1.8 cu.in. was the old OS 7M-7H. No two adjustment carburetor can be tuned to work best over a multitude of engines and exhaust combinations, many are designed specific to its engine and application. Adding a pump changes everything and will nullify a lot of your previous tuning knowlege. Its not easy but all issues and mysteries can be solved with enough time, effort, and selection of carburetors.

Darzeelon, I will answer your questions.

I am not certain you "understand perfectly how a two-needle carburettor works". In order to
understand how this carb works, you must take into account how the main needle and the
nozzle/spraybar work. I have explaianed this already, but perhaps you did not read the
information.

The needle/nozzle combination controls all the mid-range fuel metering except
the idle setting, and the wide open/full speed setting. Without understanding what the taper
on the needle does, it is not possible to understand how the carburator works. 1/8, 1/4,
1/2, 3/4, and 7/8 throttle settings are controled by the high speed needle.

As the carb barrel opens, and slides sideways, it pulls the nozzle away from the HS needle.
As the fat part of the needle moves out, the thinner part of the needle permits more fuel
to flow past itself, and through the spraybar, and out the end of the spraybar. The fuel is
pulled through by the vacuum of the engine, and is helped by the fuel pressure fron the fuel
tank.

Like I said, the taper on the main needle controls 90% of the function of fuel metering.

You said:
You might want to have another look at a carb or read up on them to gain the understanding
on how they actually work.

FBD.

No! The main needle is not doing all the metering at part throttle. It has some influence but it is not the primary control.

Both of you have to realize that both needles influence the part throttle mixture.

The question is which one you adjust to get an appropriate part throttle mixture? If you peak the engine and back it off 300RPM you can't adjust the mid range with the main needle and expect the WOT mixture to be the same as it was, obviously. If the part throttle mixture is bad after you have set the main needle at WOT, you use the idle mixture screw to make adjustments to the part throttle mixture. Plain and simple. Dar questioning of the wording in that document is correct in that an inexperienced user doesn't realize that you would not use the main needle to correct a bad part throttle mixture.

If the carb is designed properly, (thankfully methanol burns at a pretty wide range of fuel air ratios) the part throttle mixture and idle mixture will provide acceptable operation.

Our carbs are very simple, comparable to turn of the century automotive technology. It works as long as you understand how to tune it properly. The fuel is tolerant of a wide range of mixtures ratios, so engines run just fine even if metering is less than ideal.

Yes they certainly do "influence" partial throttle settings.

But consider this...."If" you have the carb adjusted properly, top and bottom, the engine
will run crisp and clean from idle to full blast. This is because the carefully manufactured tapered
needle see to that chore. Sure, if the top end is set too rich, it will throw off the whole range.

If the low speed is set too rich, the engine will "load up" and will be running too rich as you
advance the throttle to a point where it will clear out....providing the high speed is set correctly.
If the high speed is set too rich as well, the engine will be sloppy everywhere.

The key is the taper on the needle takes care or everything between idle and full speed, and will
do a really good job of it....as long as the low and high speed are set correctly, the needle takes
care of the rest. Trust me, that's how it works.

FBD.

Dave,


You clearly don't understand this...

The taper of what you call the 'main needle' only controls the maximum amount of fuel the carburettor will pass.
It is a fixed control for all practical purposes.
In the Webra sketch is is marked as '1' and if you extend arrow '5' to the center-line of the carburettor, it points approximately to the taper of this same needle and to the fixed seat, against which it seals when this needle is fully closed.

The position of this control is only changed by the user, on the ground.
Neither the taper of this needle, nor its seat, move as the throttle is closed and opened. With no relative movement, there is no change in this fuel orifice size and clearly no metering is done.


Further to the right of this needle is the fuel nozzle ('A'), also fixed and stationary, relatively to which the low-speed needle needle's taper ('B') moves (along with the carburettor's barrel).

When the barrel is closed it moves to the left ('B' moves further into 'A'), effectively decreasing the fuel-orifice size.
When the barrel is opened it moves to the right ('B' moves further out of 'A'), effectively increasing the fuel-orifice size.

This is the way the low-speed needle achieves fuel metering and the high-speed (your 'main') needle, or its taper, have no effect on this whatsoever.

Greg,


While both needles do have an influence on the flow of fuel all across the RPM range, the high-speed needle has absolutely no effect on the part-throttle fuel metering.

Its influence is fixed; not variable, which it needs to be to affect the metering.

Sure "If" things are set correctly we wouldn't be having this discussion and everyone buying an engine wouldn't have trouble. The point was to clarify how the carb works.

Please correct the following statements if you believe them to be wrong.

1. The first carb adjustment is wide open throttle mixture. Run the engine wide open, peak it with the main needle and back off as much/little as you like. For most engines with a given prop and atmospheric conditions, this setting will not change(essentially fixed as Dar says).

2. Second, throttle back and idle for a while, open throttle and check transition. If transition is poor, adjust low speed needle to correct.

3. Some carbs will required you repeat steps 1 and 2 a few times to dial in the carb because the idle needle influences the high speed needle, Saito comes to mind.

4. If you are really picky about the running mid range mixture (not transition) this can be adjusted via the idle needle, but you must check transition afterwards to make sure it is acceptable.

All of this changes depending on fuel supply pressure, nitro content, even oil content in the fuel. It's all one huge compromise and relies mainly on proper carb design to achieve a good running midrange mixture. Tuning it wil standard fuel mixes is simple as long as you follow the above steps.


Dar, maybe you haven't ever tried as it's not normal adjusting procedure, but if you open the main needle at part throttle it does have some influence on the mixture. It is still a restriction(not as much as the low speed circuit at part throttle) and there is a small pressure drop in the fuel flow past the main needle. It is in series as you say, so it is not left out of the equation, but the influence is smaller. I experience it starting my four stroke diesel conversions. To start, I open the main needle 1/2-1 turn to richen the mixture. I start at 1/4 throttle. If I close the main needle to the running setting (still at 1/4 throttle) before it warms up, it quits. I have seen this with air bleed and OS and Magnum two needle carbs.

Ok, I've completely lost track of what is being argued here (forget the fact that my thread was about pumps with non-pump carbs).

The majority of two-needle r/c carbs that I own work in a linear fashion. That means that the fuel exits the tank along a single line and enters the carburetor through a nipple located along the tapered portion of the high speed needle. The fuel then travels past the HSNV, through the spraybar and exits into the barrel either through a slit or hole of some sort. This opening in the spraybar where the fuel exits is regulated by a second needle. Sometimes this needle is called an Idle Needle, Mixture Control Valve or Low Speed Needle (among other things).

The point is that fuel travels in a linear path. It passes the HSNV first and then travels past the Mixture or Idle needle.

Every single molecule of fuel that exits the spraybar is first regulated by the HSNV because that's where the fuel tubing attaches to the carburetor!!!

Now, this regulated amount of fuel - which has been regulated by the HSNV - then gets regulated a second time by the Idle or Mixture needle. However, this needle's setting is dynamic, not fixed like the HSNV. As the throttle is rotated the amount of regulating this needle performs is increased or decreased. But, it is always regulating fuel that has already been regulated because the fuel travels in a linear path.

To put all this in the simplest terms I can:

The HSNV controls the amount of fuel that enters the spraybar.

The Idle or Mixture needle controls the amount of fuel that exits the spraybar. However, it alters the amount of fuel that exits the spraybar in relation to the amount of air the barrel allows into the carburetor. It's purpose it to maintain a constant ration of fuel to air. It can never let more fuel exit the spraybar than the HSNV has previously allowed to enter the spraybar!

To sum up:

The HSNV is the "master valve" that determines how much fuel can enter the carburetor from the tank in the first place.

The Idle or Mixture needle controls how much of this pre-regulated fuel the engine will receive. And, this amount is increased or decreased depending on the amount of air that is being allowed into the mix.

Here's a good analogy: Think of a transmitter and servo operating a control surface. If you program your transmitter's EPA to allow 100% travel then the servo can rotate fully. If you set it to 75% then the servo can only rotate 75% of the way. This is the HSNV controlling the overall amount of fuel entering the carburetor.

Now, imagine moving the stick on your tx. This is the throttle/barrel mechanism of the carburetor. The stick controls the actual amount that the servo will travel - up to the maximum amount that your EPA allows, but not more. Even if you move the stick all the way, it can't move the servo more than your EPA (HSNV) will allow.

The stick moves the servo within the range of travel permitted by your EPA setting. Likewise, the Idle/Mixture needle in conjunction with the throttle/barrel determines the amount of fuel exiting the spraybar, but it can't exceed the amount of fuel the HSNV is permitting to enter the spraybar in the first place.

David

Greg,


Sure there is a 'crossed effect', of the high-speed needle over the part-throttle fuel flow.


This effect is virtually nil at idle and at off-idle settings, but becomes more noticeable at higher throttle settings.

But since the low-speed needle is the smaller orifice at 0-~75% throttle, its effect is much greater than that of high-speed needle, at these settings.


And since the high-speed needle's position cannot be changed in flight, is is a fixed obstruction that must only be adjusted for once.

Changes made subsequently to the high-speed needle's setting, will probably be small enough not to affect the part-throttle range, but this must be checked as a matter of course.