Carburetor #1:Function Basics

well i haven’t written about this topic yet, so since i have time why not. most of my experience with carburetion is mostly for street/strip applications. i don’t really mess with a whole lot of carb’s for demo derby simply because so many people claim to be the cat’s ass at doing it….let em run with it. For the sake of arguement i am going to write according to performance oriented brands or styles: Holley, Rochester, and AFB/Carter/Edelbrock(which i will refer to as AFB for the rest of this as that was the original designation). i will probably write separate sections on each and my dealings with all of them.

The basic purposes of a carburetor are fuel atomization, air/fuel mixing, and air flow regulation, which all in turn govern engine speed and power output. in order for gasoline to burn properly in the cylinders/combustion chamber, it must be present in an atomized state and mixed with some amount of air. ideally for what most performance guys are going for, you would want 14.7 parts air to 1 part of fuel(as stated in other parts of the blog). obviously this is damn near impossible to accomplish in all driving condition. under heavy load/acceleration you would want more like a 11-12 to 1 ratio, and cruising or decelleration may yield 15-17 to 1 ratio’s of air to fuel. the carburetor must compensate as best as possible to most all driving conditions. have i lost you yet? stick with it i will get there…..

A carburetor basically has the job for atomizing the fuel going into the air intake charge of the engine as best as possible. it does this via carburetion with help of the venturi effect. for basic carburetion, think of how an old time perfume bottle works. as air passes over a tube, it draws liquid out  into an atomized state. to give you an even better idea you can try this experiment: take a glass full of water and stick a straw in it. then grab and air hose and hold it across the tip of the straw. it will draw the water out and spray it. it’s also the same principle as a paint gun. 

 the venturi effect is basically the same priciple they use for the cooling towers on a nuclear plant. it is basically a tube that narrows at the center and is referred to as a Venturi. at it’s narrowest point air flow will have a tendency to speed up creating a low pressure area with high air speed. by introducing fuel for carburetion at the narrow portion of the venturi for atomization, it is both drawn into the air stream  and atomized kind of like a double whammy. that is the basic principle of operation of a carburetor.  

Next we need a primary way of regulating engine speed. so we then put a throttle blade down the line to regulate engine speed  by controlling air/fuel amount entering the engine. you guys get the point  by now what is going on. if we had a governor and always wanted to run a constant speed(like most new push mowers)this would be pretty much all there is to the carb function. a fixed venturi and a throttle blade.  However, we aren’t going that direction and are from here on it going to talk about performance automotive carbs. we need to be able to vary the amount of fuel at different times for different driving conditions. this is where the carb starts to confound some people but really it isn’t that bad.  basically we are going to divide the carb into 5 main parts at this point: idle/low speed circuit, main metering circuit, choke, float bowl, and accelerator pump.

lets start with the main metering circuit as it is the one that does the bulk of the work.  obviously you have heard me talk about the venturi’s of the carb already. most carbs use 2 venturi’s. the first is the large outer venturi that makes up the bulk of the main body of the carburetor is there to simply regulate bulk air flow and usually. Air flow is measured  in cubic feet per minute of flow…..also known as CFM.  at wide open throttle the size of this venturi and the number of venturi’s(barrels) dictate air flow into the engine. if it is too large you will  lose performance overall as you will lose that low psi area i referred to earlier – too small and it will act like a restrictor plate. the CFM capacity of a carburetor must match the capacity of the engine for peak performance. i will get into that more toward the end.

the second venturi is a smaller venturi that hangs to the center of the first larger barrel. it is often referred to as a boost venturi and is where most of you fuel for operation flows through. if you are following along you probably figured out that by having two venturi’s you double that low psi effect for optimum atomization. fuel to the boost venturi is regulated by two things: Jet size and a main metering valve of some kind. the size of the jet regulates the amount of fuel entering the air charge going through the carb. it is the sole thing regulating fuel flow at wide open throttle: hence changing jet sizes at the race track.  however there is still a need to regulate this a bit  further based on engine load for average driving conditions. a metering valve with additional fuel flow regulation based off engine vacuum signal is also used. these are either done by use of something like a power valve or  metering rods.

next is the float circuit. it is basically like a toilet tank on the side of the carb. it’s job is to keep a small regulated amount of fuel on hand for all the circuits of the carb to draw on. there is a needle and seat valve inside the fuel inlet  of the car that is attached to a  float. the primary metering jets, metering valve, and accelerator pump all draw from this.

under sudden acceleration/movement of the throttle, the carb does not have enough time to shit fuel out of the main metering circuit to keep the car from stalling. to get around this we inject fuel via a small pump in the float bowl that shoots fuel directly into the large venturi area of the carb, and is referred to as an accelerator pump. this is usually adjustable via the linkage being manipulated somehow.

at idle or just off idle , we need minimal fuel and the throttle is almost completely closed. to create the same atomization, we introduce fuel at the edge of the throttle blades(point of peak vacuum at idle) which creates the same perfume bottle effect without the use of a venturi. the amount of fuel to this point varies as the idle point of the throttle blade can vary from one engine to the other, so the idle mixture is adjustable. to cite another section of the blog, your engine timing directly effects your idle mixture as it effects idle rpm and fuel demand. first timing-then idle rpm-then mess with the idle mixture screws.  just above the idle mixture ports are two small slots. these allow for a transition between main metering and idle mixture….and are hooked in with the idle mixture circuit.

and lastly when we have cold start conditions, an engine will need extra fuel to function until it reaches operating temperature. so we need to enrich overall mixture for a short time by”choking” the amount of air going into the engine, hence the blade on the top of the carb is called a choke. if a choke comes off too fast an engine can stall- if it sticks it will lack power and smoke black- run like shit. most of the bullshit that hangs to the outside of a carb is all related to the choke and cold run operation. in performance application we just rip it off and pump the accelerator quite a bit to make up for it so it will fire when you cold start.

now that you have some idea on how a carb works in about 5 paragraphs…LOL!….i will get into other things. for what most of the guys reading this blog are doing you pretty much have to fuck with 4critical adjustments: float level, jet size, metering  valves/springs and accelerator pump adjustment. these are all different per manufacturer. the overall most critical thing with a carburetor is have the right carb for the right job. this is where the 2g carbs work well for derby- and holley 4150’s work well for drag racing- and edelbrock carbs work great on the street rods.

ideally when buying a carb you want to have it at or just below the CFM capacity of the engine. this is done through use of a very simple formula that can be applied to most engine builds;

                          MAXIMUM  RPM X DISPLACEMENT

CFM= ————————————————-


so as an example if you run a 7000rpm set up with a 468 big block, you would need around 950cfm capacity carb. whereas with a 3000 stall 355 derby motor you would need more like 300 cfm carb, and would be a shining example of why the 2G carbs work well for derby use. always remember, you don’t make horsepower without feeding the horse.

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