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Questions about how fuel mixture works

1K views 15 replies 9 participants last post by  THE EVIL TW1N 
#1 · (Edited)
I know the basics of how a carb work just from messing with them on my cars. I understand that the choke can be adjusted to make the car run more rich or lean during start/cold engine operations. What happens when the car is warm and the choke is fully open...at idle? Is the position of the throttle plate the only thing that determines the mixture at this point? Or, at idle, is it the mixture screws that now determine most of it? Once off idle and under throttle, what determines the mixture? Float? I read that the mixture screws really only affect the idle circuit, true?
 
#2 ·
Very complicated question. Hmm where to begin... this could turn out to be a big post.

The throttle plate does not control mixture. It does control air flow coming into the engine. More air = higher rpm, assuming correct a/f mixture.

The idle mixture screw controls the a/f ratio at idle. It does affect everything else too, even if only a pinch.

Off idle and part throttle can be both controlled by the low speed and high speed circuit. The two circuits overlap. Also, there's the accelerator pumps that help with part throttle.

It's much more complicated than that. I don't know if I have the energy at this time to write everything out.
 
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#3 ·
There are many books written on this, and WEB pages as well. Different carburetor manufacturers use different methods. I'll talk about a Holley carb here, but only touching on things. Idle screws for the idle circuit, transition circuits (slots) for off idle and into the partially open butterfly's. Wide Open Throttle (WOT) circuits. As you know there are idle mixture screws, but also accelerator pumps, accelerator pump cams and squirter nozzles, main jets, power valves and air bleeds. How do you know what is for what and how to adjust all this? I will suggest you do some searching here on the forum as this has been talked about many times. As I mentioned, there are also several good books if you want to dive deep into this, and some more simpler WEB pages that talk about the particular carb you are interested in.
 
#4 ·
A carburetor is a mechanical computer. The venturing is the point much like the expansion valve in a refrigeration circuit that divides the system and the needed point of pressure drop to make things work. This is what pulls the fuel through the circuit. In addition there is a parallel air circuit that fine tunes it.
 
#5 ·
what every one is dancing around is, carbs work on vacuum. the more vacuum you generate in the fuel passages, the more fuel that flows through them.

this vacuum is generated by airflow through the venturies. they do this by speeding up the air flowing through them, that reduces the pressure, and fuel flows. the throttle plates only control the amount of air that is available for the engine to consume.

as for the idle mixture screws, they actually do affect the air/fuel ratio up to about 1500-2000 rpm.
 
G
#6 ·
The idle mixtures screw do nothing above about 25-30 MPH. And even at a level slightly below that, they begin to taper off as the venturi begins to operate. Once out of the transition circuit, the mixture is entirely controlled by the main circuit, which contains the main jet. Your choke richens, never leans the mixture. You can "go" leaner, but actually is just a lesser rich condition. In reality, you have a lot to learn about the function of a carburetor because it is fairly complicated. It has many systems which must transition from one to another. It isn't easy with a mechanical device and why EFI took over. If you want to learn more, read more, I can point you to some reading on the web that will really jump start your learning curve . If you want to, just say so.
 
#9 ·
Sure, would like to stick to modern Holley and 4100 style, if that helps narrow it down.
 
#7 ·
Best off reading up on how they work. I started withthe first edition of the HP holly carb book it explained how all their carb circuits worked not just the 2300 and 4 bbls. Some circuits in carbs work the opposite of the most common holleys one thinks of. The first edition relay got into fine tuning more than the second.
EFI , SEFI are programmed to work like carbs do using mother natures rules.
 
#8 ·
Would those cover their modern stuff? I'm currently running a 570 street avenger.
 
#10 ·
The carb is a constant state of approximately delivering what you need with zip feedback. There are many conditions including throttle position, air temp, engine temp, timing, etc. etc. that affect what the A/F ratio should be. The carb is designed to give the best is can do at all these various conditions. EFI overcomes all these problems by measuring the actual ratio and adjusting on the fly as needed.
 
#15 ·
Set up a carb perfectly and EFIs only real advantage is programmable ignition built in unless one goes with the indivgual TB (webber style)

Which of these systems did best on the same engine same car. One got 20 MPG the other 24 highway. Nether has EGR capabilities, Both had spot on AF ratios. Nether prob would pass emissions, one would reach higher RPMs.
 

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#11 ·
My shade tree understanding of this situation is;

a cold engine has a harder time cranking and needs to run a bit richer because the fuel does not atomize/vaporize as well in a cold engine as a warmed up engine. The choke reduces the amount of air getting into the carb which proportionally richens up the air/fuel mixture getting. It is also needed to run the idle a bit higher when the engine is cold for the same reason which is why the carb has a fast idle cam associated with the choke. Once the engine is warmed up and the choke opens up, the air/fuel ratio is a bit leaner but the warmer engine vaporizes the fuel more efficiently so it gets a better burn off the charge. When the choke comes off the fast idle cam also lets go so the idle can drop back down to normal.

Technically speaking, gasoline doesn't burn. It is the atomization/vaporization of gasoline which combines oxygen with the gas which is what burns. The gas vapor flashes pretty quick too if you have ever messed around with it burning stuff.
 
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