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Old Jul 20, 2007, 11:35 AM
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Carb inline with tank centerline??

Why is it important to keep the carb centered with the fuel line??

In my model, the fuel line is raised slightly by about 1/2 centimeter higher then the carb. Would that affect anything or do I need to keep the tank levelled??
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Old Jul 20, 2007, 11:43 AM
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For good fuel flow in non-pump setups the tank center line should be in line with the spraybar in the carb. It can vary up or down up to 3/8" (10mm).

being only 5mm off, I'd say your setup is OK.

Ivan
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Old Jul 20, 2007, 10:12 PM
Zor
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Quote:
Originally Posted by gpang788
Why is it important to keep the carb centered with the fuel line??

In my model, the fuel line is raised slightly by about 1/2 centimeter higher then the carb. Would that affect anything or do I need to keep the tank levelled??
Hi gpang788,

I might be accused of splitting hairs.

It is not important to keep the carb centered with the fuel line.
What is important is to have the fuel level from about one inch above to about one inch below the carb intake line when the airplane is in normal attitude.

What you may visualize and understand is that at idle and low rpm the airplane attude is normal or near normal. The conditions under which we set the mixture. We have a reduced suction in the carb and the engine must suck its fuel to keep idling.

You probably saw flyers preparing to go and take off put their plane in a nose straight up position. They do that at full throttle (or nearly so) to check that the engine will not quit. In that attitude the fuel pick up in the tank is much more than one inch below the carb line connection or the spray bar.

With the airplane in that nose up attitude the mixture becomes leaner. Another reason to enrich the high speed needle on the rich side in normal airplane attitude.

Many believe that the high speed needle adjustment to drop the rpm by about 300 rpm is to have the engine run cooler and to provide better lubrication. That could be a highly debatable subject which I would rather keep away from considering the different amount of fuel flow involved by going from 10,000 to 9,700 rpm or from 15,000 to 14,700 rpm.

All are entitled to their own opinion. I do not question it.

Regards de Zor.
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Old Jul 20, 2007, 10:59 PM
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Everett Wa.
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Zor,
Are you questioning the need to richen the fuel mixture on a non-regulated fuel system from that point which finds the peak power with the aircraft held static? Or are you questioning the arbitrary numbers attached to such a need?

300 to 500 rpm is usually stated because it is that we can differentiate with our ear and it really is about twice the resolution of most tachs. It the (rpm value) has nothing to do with the stoichiometric mixture strength.

Friends donít let friends fly nickel,
Konrad
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Old Jul 21, 2007, 09:07 AM
Zor
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Quote:
Originally Posted by Konrad
Konrad
Konrad posting below

Zor,
Are you questioning the need to richen the fuel mixture on a non-regulated fuel system from that point which finds the peak power with the aircraft held static?
No I am not. I do not see why you ask that. Below the general reader will read why we do that. Or are you questioning the arbitrary numbers attached to such a need? No I am not. The drop in rpm IS arbitrary because it is done most of the time by sound of the engine. Not everyone has a good tachometer or bother using it. It is not necessary anyway.

300 to 500 rpm is usually stated because it is that we can differentiate with our ear and it really is about twice the resolution of most tachs.
Correct.It the (rpm value) has nothing to do with the stoichiometric mixture strength. I assume that you are referring here to the DROP in rpm, not the actual rpm but if you are referring to the actual rpm, it makes no difference. The stoichiometric (see definition below) proportions have no relation to the rpm. The relation is based on the carb design.

Friends donít let friends fly nickel,
Konrad

Hello Konrad,

Please read my posting again. I am not questioning anything and my posting is not implying any such thing as you are mentioning.

Gpang788 asked . . . . .

"Why is it important to keep the carb centered with the fuel line??"

His question implies that we locate the carb (consequently the engine).

I splitted hair in the sense that the engine is located by design of the aircraft

What we do is try to locate the fuel tank, not the carb (engine) to meet the referred to relationship.

I am reproducing my posting again below and inserting
in red additional text to help your understanding of what I originally posted.

Please also remember that my postings always keep in mind the general readers.

This posting now is not aimed at explaining things to you. You personally know all this. I know why you have posted concerning my own posting but I fear that your posting is making it difficult for the general reader to properly understand the subject.

For the benefit of readers not so familiar with the English language . . . .

Definition of Stoichiometric (word used by Konrad)

Having exact proportions for chemical combination.



Originally Posted by gpang788

Why is it important to keep the carb centered with the fuel line??

In my model, the fuel line is raised slightly by about 1/2 centimeter higher then the carb. Would that affect anything or do I need to keep the tank levelled??




End of gpang788 posting

My original reply follows with additional text in red . . . .


Hi gpang788,

I might be accused of splitting hairs.

It is not important to keep the carb centered with the fuel line.
What is important is to have the fuel level from about one inch above to about one inch below the carb intake line when the airplane is in normal attitude.
Most fuel tanks have a level variation from full to near empty of about two (2) inches (50.8 mm).

What you may visualize and understand is that at idle and low rpm the airplane attitude is normal or near normal.
(A tail dragger on the ground or any airplane in a steep climb may have its fuel tank lower than in straight and level flight) The conditions under which we set the mixture on the ground.(adjust the needle valve(s). We have a reduced average suction in the carb at low rpm and the engine must suck its fuel to keep idling.

You probably saw flyers preparing to go and take off put their plane in a nose straight up position. They do that at full throttle (or nearly so) to check that the engine will not quit. In that attitude the fuel pick up in the tank is much more than one inch below the carb line connection or the spray bar.

With the airplane in that nose up attitude the mixture becomes leaner. Another reason to enrich the high speed needle on the rich side in normal airplane attitude.

Many believe that the high speed needle adjustment to drop the rpm by about 300 rpm is to have the engine run cooler and to provide better lubrication. That could be a highly debatable subject which I would rather keep away from considering the different amount of fuel flow involved by going from 10,000 to 9,700 rpm
(typical 4 stroke) or from 15,000 to 14,700 rpm. (typical two stroke) The DIFFERENCE in fuel flow is very small but it allows for the leaning that takes place when the airplane attitude goes nose high and / or the fuel level goes down. In other words, when the engine has to suck fuel from a lower point. This remains true wether the fuel tank is vented to atmosphere or pressurized by the muffler line.

The reason for the drop in rpm by enriching the mixture near maximum rpm may be the subject of another posting.


All are entitled to their own opinion. I do not question it.

Regards de Zor.

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Old Jul 21, 2007, 09:52 AM
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Most engines have more fuel draw at part throttle and reduced muffler pressure, at full throttle there is less fuel draw but more muffler pressure. It's a balancing act. I set the engine up to run perfectly on my PSP test stand where I have the tank mounted lower than it will be in most planes. Setting the needles with the test stand tank half full will seldom require any change when the engine is mounted on a plane.
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Old Jul 21, 2007, 11:51 AM
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Zor,
Your posting was not clear as to the need to richen the mixture. As I read your statement to richen the mixture in a nose high attitude to compensate for the different head pressure I saw no mention as to the need to set the mixture strength aprox. 300 rpm richer still.

The next statement seemed to question the need to richen the mixture for the 300 rpm drop at all.

The need to richen the mixture is true for our simple fuel system as there is no feedback to insure consistent fuel pressure at the fuel inlet. This is why I mentioned a regulated fuel supply. Atmospheric, muffler or pump pressure still needs this hedge against leaning out of the mixture as the engine unloads (speeds up). This is because the air is much less viscous than fuel and hence flows much easier for any given pressure. This is why large throat area carbs are more sensitive to proper fuel system set-ups.

Regulated fuel supplies should have reserved inlet pressure to insure consistent fuel pressure. The variable fuel jet of an auto mixture carb would control mixture strength.

The statement that we have reduced average suction is questionable, as at idle we reduced the choke area keeping the velocity high or higher. In fact the pressure drop (suction) is greater at idle. This is why very simple carbs need an air bleed to by-pass some of the airflow past the spray bar. If you will recall Bernoulliís theorem that as the velocity of a fluid increases the pressure drops. This pressure drop is what draws the fuel into the engine,

I use the term stoichiometric mixture to differentiate from the proper (real world) rich mixture we fly with

I do question everybodyís opinion. As Iím try to learn what assumptions they are under. I hope I donít discount these opinions unless they cannot be defended. Often time I can learn something when someone questions my assumptions.

My posts are also for the general reader. As I have little math, chemistry or physics in my in posts. My question may be directed to you but my responses are general so that anybody can (I hope) follow my thought process.

. There is more than one way to skin a cat. You like a pictorial approach to defining a problem and solution. I donít. I like to give excruciatingly detailed explanations. Your method of explanation is just as valid as mine.

Friends donít let friends fly nickel,
Konrad
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Old Jul 21, 2007, 12:58 PM
Zor
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Quote:
Originally Posted by I TOBOR
Most engines have more fuel draw at part throttle and reduced muffler pressure, at full throttle there is less fuel draw but more muffler pressure. It's a balancing act. I set the engine up to run perfectly on my PSP test stand where I have the tank mounted lower than it will be in most planes. Setting the needles with the test stand tank half full will seldom require any change when the engine is mounted on a plane.
I TOBOR my restpectful salutations,

I believe that by now the originator of this thread "gpang788" has pretty well all the information he asked for.

I did not see much advice to answer his question except for ivanc who addressed "gpang788" question.

Konrad took off on a tangent in respect of my posting and I tried to straighten things out for the average reader and for "gpang788".

Concerning your post above I am a little confused as to exactly what you are illustrating.

As you say . . . .
Quote "Most engines have more fuel draw at part throttle and reduced muffler pressure, at full throttle there is less fuel draw but more muffler pressure." Unquote

I wonder by "fuel draw" if you mean suction at the carb spray bar or if you mean fuel rate of flow.

It is evident to me that the fuel rate of flow follows the rpm and the power developped by the engine at different throttle settings.

You seem to indicate that at medium power (mid rpm range) the flow is due more to the carb suction and at high power (max rpm) th flow is mainly due to pressure of the exhaust in the fuel tank.

I am probably not as knwledgeable as you are but I feel that both venturi suction and tank pressure just keep increasing along with the power setting (the opening of the carb).

Perhaps additional explanation of your statement would help me (and readers) to see better what you are saying as well as understanding the logic of the situation.

The rest of your posting is clearly understood.

What say I TOBOR ?

We would appreciate furhter elaboration.

Best regards de Zor
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Old Jul 21, 2007, 02:42 PM
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Quote:
Originally Posted by Konrad
Zor,
Your posting was not clear as to etc . . .
Konrad
Konrad

My best salutation.

Due to the various aspects of your posting I am inserting my reply within your text
in red for easy distinction.

Zor,
Your posting was not clear as to the need to richen the mixture.
I thought it was clear as I said The DIFFERENCE in fuel flow is very small but it allows for the leaning that takes place when the airplane attitude goes nose high and / or the fuel level goes down. In other words, when the engine has to suck fuel from a lower point. This remains true wether the fuel tank is vented to atmosphere or pressurized by the muffler line.

As I read your statement to richen the mixture in a nose high attitude to compensate for the different head pressure I saw no mention as to the need to set the mixture strength aprox. 300 rpm richer still. So you are now 600 rpm (approx) from peak rpm. (you say "richer still").

The next statement seemed to question the need to richen the mixture for the 300 rpm drop at all.
I am trying to find what you refer as "the next statement". Why did you not quote it?

I cannot find for sure what you refer to as "the next statement"

I did write . . . .

"The reason for the drop in rpm by enriching the mixture near maximum rpm may be the subject of another posting."

I am referring here to an explanation why the rpm drops when we enrich the mixture from peak rpm as possibly being the subject of another posting.

The need to richen the mixture is true for our simple fuel system as there is no feedback to insure consistent fuel pressure at the fuel inlet. This is why I mentioned a regulated fuel supply. Atmospheric, muffler or pump pressure still needs this hedge against leaning out of the mixture as the engine unloads (speeds up). I do not intend to question your choice of words. The way I see it, when the engine revs up it loads up, it does not "unload". A different way of understanding the same phenomena.This is because the air is much less viscous than fuel and hence flows much easier for any given pressure. This is why large throat area carbs are more sensitive to proper fuel system set-ups.

Regulated fuel supplies should have reserved inlet pressure to insure consistent fuel pressure. The variable fuel jet of an auto mixture carb would control mixture strength.
I understand you as saying "air to fuel ratio by weight". The auto carb I have ever known do not have any variable fuel jet. I am not arguing with you nor saying you are wrong. I am just saying that any auto carb I have ever known and played with have an accelerator pump so that the mixute is less influenced by the sudden opening of the carb during which the low viscosity air can outrun the higher viscosity fuel and its restrictions in the venturi passages thus influencing the instantaneous mixture ratio.

The statement that we have reduced average suction is questionable, as at idle we reduced the choke area
(we do not really have a choke. In most carbs. The jet (spray bar) is located in the center of the throttle and the throttle acts as the venturi. At idle (or low rpm) less air is admitted and the venturi effect has less suction. That is the way I see and understand the situation. keeping the velocity high or higher. In fact the pressure drop (suchtion) is greater at idle. We must not confuse the suction created by the venturi with the effect of the throttle opening. It is two different things. The venturi increases the air velocity creating a lower pressure which pulls the fuel out of the spray bar (We may call it the "jet") The throttle opening controls the amount of air that passes the venturi. Of course with less air pass the venturi we have less air acceleration and consequently less venturi suction.

This is why very simple carbs need an air bleed to by-pass some of the airflow past the spray bar. Very simple carb do not have a low rpm needle valve. Instead of controlling fuel flow at low rpm they bypass air to keep an acceptable fuel mixture. If you will recall Bernoulliís theorem that as the velocity of a fluid increases the pressure drops. This pressure drop is what draws the fuel into the engine, You are correct. I thought I also knew that.

I use the term stoichiometric mixture to differentiate from the proper (real world) rich mixture we fly with
I put in the definition of this word to assure that all readers would know what it meant.

I do question everybodyís opinion.
You have Joined my club and some will disgrace you for doubting their opinions. As Iím try to learn what assumptions they are under. I hope I donít discount these opinions unless they cannot be defended. Two negatives . . . but I agree . . . I respect opinions only if they can be defended. Often time I can learn something when someone questions my assumptions. I try very hard to avoid "assumptions" in publications. I have lots of assumptions But I keep them for myself if I cannot defend (explain) them.

My posts are also for the general reader. As I have little math, chemistry or physics in my in posts. My question may be directed to you but my responses are general so that anybody can (I hope) follow my thought process.
Your postings are very valuable to the readers and you are very knowledgeable. Keep up the good work. I would not question your postings unless they are addressed obviously to me or you are questioning mine.

. There is more than one way to skin a cat. You like a pictorial approach to defining a problem and solution. I donít. I like to give excruciatingly detailed explanations. Your method of explanation is just as valid as mine.


We all have our own way. What is important is a clear understanding by the readers.


Friends donít let friends fly nickel,
Do not worry, I shall not.
Konrad

Have a good day Konrad.

De Zor

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Old Jul 21, 2007, 05:09 PM
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Unload refers to the relative motion of the incoming air in forward flight lessening the relative pitch of the prop. Less pitch equals less load hence the term unload. Depending on the shape of the power curve this usually results is less power at the crankshaft.

All auto mixture carbs have some sort of variable fuel orifice. This can be the tapered slot on the OS barrel and tube type spray bar or the tapered needle of the classic Webra TN carb, or the tapered fuel slot of some Enya GM of Fox easy adjust carb. It is this variable fuel orifice that makes a carb an auto-adjust now some carbs have adjustable features that can vary this profile of this jet outside of the action of the variable choke (air valve or throttle).

The only real issue I have is the following:
The jet is not the spray bar. A jet is a metered orifice and may or may not be in or on the spray bar. A spray bar is the device that senses the air flow (pressure drop)
An air valve, throttle and choke are all terms for the same thing. Some people also use the term choke for the secondary air valve that limits airflow to richen the fuel mixture but not control engine speed. There are also enriching schemes that don't inhibit airflow. The ability of the venture to create a pressure drop is dependant on velocity only

The by-passing of air in an air-bleed carb is done so that the velocity of the bypassed air does not add to the pressure drop at the spray bar. This is why air-bleeds discharge the bypassed air under the spray bar. The relative higher pressure at the spray bar (as result of the bypassed air) is what slows down (impedes not restricts) the fuel flow this impeded fuel flow with the smaller choke area of an engine at idle maintains the proper fuel mixture.

An auto mixture carb restricts the fuel flow (smaller fuel orfice) while maintaining strong fuel draw (low pressure at the spray bar)

An air bleed carb impedes (spoils) the ability of the spray bar (venturi) to create this needed low pressure hence reducing fuel flow, at the cost of the engine being able to draw fuel at idle.

To OP this is why tank placement is so critical for air-bleed engines compared to auto mixture engines. The raised fuel line will have little inpact as long as both end come to the same level. Think of how a siphon works

Friends donít let friends fly nickel,
Konrad
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Old Jul 21, 2007, 07:25 PM
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I think Zor is incorrect or difficult to understand.
Konrad & Ivan Seem to have the best scope on this
subject.
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Old Jul 21, 2007, 07:43 PM
Zor
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Quote:
Originally Posted by Konrad
Konrad
Konrad

You have demonstrated once more your extensive knowledge of engine and carb performance.

I am noticing that semantics (basically the meaning of words) is becoming important as you may see in my comments
in red in your text.

Quote:
Originally Posted by Konrad
Unload refers to the relative motion of the incoming air in forward flight lessening the relative pitch of the prop.
Quote:
Originally Posted by Konrad
Right on, I could not guess the circumstances you were thining about out of your text. Less pitch equals less load hence the term unload. Depending on the shape of the power curve this usually results is less power at the crankshaft. In those circumstances yes of course.

All auto mixture carbs have some sort of variable fuel orifice. This can be the tapered slot (the slot has to move or something has to move inside the slot to change fuel flow) on the OS barrel and tube type spray bar or the tapered needle of the classic Webra TN carb, or the tapered fuel slot of some Enya GM of Fox easy adjust carb. It is this variable fuel orifice that makes a carb an auto-adjust (now I know what you were expressing as an "auto-adjust" carb. I always considered this as part of the carb design and did not consider it as automatic-adjust) now some carbs have adjustable features that can vary this profile of this jet outside of the action of the variable choke (air valve or throttle). I like to consider Choke, air valve and throttle as having different functions. The spray bar being in the air flow I consider part of the venturi restriction (increasing the air velocity). When the throttle opening is a bore (drilled hole) inside which is the spray bar then the throttle does not only control the air admission but also affects the venturi size and the direction of the air flow pass the spray bar opening (holes, cat's eye or whatever was designed to let the fuel in). Some carb have adjustents to change that relationship to improve mixture in the mid range of rpm.

The only real issue I have is the following:

The jet is not the spray bar
I consider that wherever a liquid is ejected into a mist is a jet (matter of semantics). A jet is a metered orifice and may or may not be in or on the spray bar. If I read you properly then you consider that the needle valves are jets. A spray bar is the device that senses the air flow (pressure drop) and let the fuel mix into the air stream as the mixture needles control the fuel flow rate.

An air valve, throttle and choke are all terms for the same thing. Not in the way I see these things. Some people also use the term choke for the secondary air valve that limits airflow to richen the fuel mixture but not control engine speed. I do not question their point of view but it appears to me a lttle misleading. The air going through the secondary air valve contrinutes to the engine speed. There are also enriching schemes that don't inhibit airflow. The ability of the venture(i) to create a pressure drop is dependant on velocity only which velosity increase is proportional to the cross section area reduction.

The by-passing of air in an air-bleed carb is done so that the mixture is proper at lower rpm. The velocity of the bypassed air does not add to the pressure drop at the spray bar (at the venturi). This is why air-bleeds discharge the bypassed air under the spray bar. The relative higher pressure at the spray bar (as result of the bypassed air) is what slows down (impedes not restricts) the fuel flow (in other words there is less vacuum at the spray bar and less fuel is drawn into the main air flow. The by-passed air joins the main air flow downstream of the venturi (spray bar) this impeded fuel flow with the smaller choke area (smaller throttle opening) of an engine at idle maintains the proper fuel mixture.

I am NOT re-writing your text. I am showing that I understand what you wrote. These are things that I understood 50 years ago.

An auto mixture carb restricts the fuel flow (smaller fuel orfice) while maintaining strong fuel draw (low pressure at the spray bar) Normal venturi action without air bypass.

An air bleed carb impedes (spoils) the ability of the spray bar (venturi) to create this needed low pressure hence reducing fuel flow, at the cost of the engine being able to draw fuel at idle.

To OP
(to OP ? ? ? the meaning does not come to my mind right away) this is why tank placement is so critical for air-bleed engines compared to auto mixture engines. Ther raised fuel line will have little inpact as long as both end come to the same level. Think of how a siphon works The routing of the fuel line from tank to carb has been discussed at length in another thread in the last couple of months.

Friends donít let friends fly nickel,

Konrad


Thanks for your posting and explanations. I am sure many readers have profited by our discussions.

As far as I am concerned we have reached the limit of the subject. We see some points of view in a different way but it all boils down to the same end results.

Have a great day Konrad

de Zor

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Old Jul 21, 2007, 07:52 PM
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Quote:
Originally Posted by Dennis157
I think Zor is incorrect or difficult to understand.
Konrad & Ivan Seem to have the best scope on this
subject.
Of course Konrad and Ivan have. They are experts.

Where was your contrinution to help "gpang788" ?

Or what was the purpose of this posting from you?

Zor
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Old Jul 21, 2007, 10:39 PM
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Hold on guys. This is not a contest. Zor's explanations are for the most part conceptually correct. I suspect that Zor is a lot like me in that English (and definitely not American) is not our first language.

Zor,
My apparent word smithing may seem trivial to you, as you know what you mean to say. I'm trying to define the concept for the audience. By no means am I trying to discredit your explanation. If I have a real issue with what is stated (that I think may be misleading or is in error) I will state it as such In this thread the only issue I had was with the way an air bleed type carb functioned and if there was a need for the over richening of the high speed needle to get an rpm drop.

Again your explanations for the most part are conceptually correct and may be easier for some to follow than mine. My excruciating detail is done to clarify some esoteric concept that may help someone understand some subtle nuance in the design and operation of the system.

Same end results??
Only a contest can prove this. I fly F3D pylon. Do you compete with these toys? I hope to see you on the field of honor some day.

By the way OP stands for Original Poster.
What is an expert?

Friends don't let friends fly nickel,
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Old Jul 22, 2007, 06:27 AM
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X==unknown quantity
Spurt=a drip under pressure.

The airbleed is merely a trim to compensate for changes in fuel, prop size, plug heat range and to a lesser degree ambient temperature. It only functions over a very few degrees of low throttle. The airbleed increases or reduces (fuel draw) by admitting air below the spray bar. A very simple device. I have about 10 Fox', 3 LA's, and a few Enyas with AB carbs. Every glow engine has the airbleed screw set halfway across the inlet hole, the Diesels have it about 2/3s closed.
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