How carbs work
Mike West
Date: Tue, 10 Dec 1996 17:38:50 -0800 (PST)
From: Mike West <>
Subject: Carburetor Stuff (no numbers)

There are no doubt better carb people here on the list, but they didn't volunteer.
If any of this is off, it's your own fault. :-)

This is about the weird looking device sitting on top the engine with a big hole in the top and hoses running to it and 
from it and it has screws and knobs and levers.  The thing that makes all the sucking noise and has gasoline 
dribbling down the outside.

It's kind of grey with a brown patina and is under the air- cleaner. 

Note the words: "carburetor" and "air-cleaner".
Carburetor is a big nonsense word.
"an apparatus for adding hydrocarbons to nonluminous or poor gases, or air for the purpose of producing an
illuminating or explosive gas. 

 It's a British word it says and they have every reason to be proud.

 If that don't keep that kid from messing with it nothing will.

"Don't mess with it Luke, suck you right in there and if you fool with them screws, you'll likely blow us all up".

Air-cleaner on the other hand is expected to be changed or cleaned so the word is functional and basic.

Unfortunately the intimidating and wondrous word works too well.

 It's about to pass into history and we still are afraid of it.

 We're not farm-hands any more so I think we can handle it.

 Big, small, simple or complex, all carburetors share the same basic operating principles.
 If you understand those principles, you'll be able to cope with any you happen to encounter even if it's unfamiliar to you.

The Basics:

First, some simple physics.

To begin with, only gasoline vapor will burn, so fuel must change its state from a liquid to a gas at some point or the 
engine won't run.

To do this it must absorb enough heat to boil.

" Boil the gasoline, now I know this guy's a dufus"!

"For the sake of argument how the hell can I boil gasoline when it's below freezing and icicles hanging off my nose?" 

"Them carburetors may be magic but they ain't that magic!"
The answer is reduced pressure. What we call "vacuum".

Water boils at a lower temperature on a mountain top than it does at sea level because there's less atmospheric pressure
bearing down on it, right?

So the vacuum in the carburetor's venturi and intake manifold do the same thing only better, causing the gasoline to boil
and fuel vaporization to occur when very little heat is present.

The boiling point is reduced so that the fine atomized droplets that are sprayed into the intake stream vaporize from the \
latent heat in the air, no matter how little that might be. Within reason.

We do know about carb icing, right?

Of course, until the engine has warmed itself up, only a small part of the available gasoline actually turns to vapor, which 
is why we need the choke to cut down on the air.


Air/fuel ratio is expressed in terms of weight, so a 15:1 ratio means 15 pounds of air to one pound of gasoline.

That is maybe 2000 gallons of air to a gallon of gas.
That's why the big hole in the top of that thing.

The ideal "stoichiometric" ratio in which exactly the right amount of air is present to burn the fuel is actually 14.7:1.

Maintaining such perfection requires the use of an oxygen or lambda (the Greek letter that has come to represent the 
ideal blend) sensor and electronically-controlled feedback or closed loop mixture adjustment, which is another subject.
(and no longer accepted as gospel truth, magic, pretty much not anymore - ed)  

Any ratio from about 8:1 to 18:1 will fire dependably.

In the 8:1 ratio, there'll be more gasoline present than is needed, so all the air will be used up while much of the fuel 
will find nothing to combine with. It will be pumped raw into the exhaust system.

In the 18:1 ratio, the lean mixture, there's more air present than is necessary to burn the fuel, so all the gasoline will 
be used up.

 It's important to realize that the actual reaction (the rapid oxidation of the fuel) always occurs at the 14.7:1 ratio
regardless of the mixture that's actually supplied by the carburetor.
(Note: the proper air / fuel ratio, due to imperfect internal engine factors, such as non-lab optimum from laboratory, (where that 14.7:1
ratio comes from), various nooks and crannies in the combustion chamber, too far advanced or retarded ignition timing, cam overlap
during blowdown period, which distorts from the laboratory conditions, the exact amount of fuel that is required to make best power.
Actually, running an engine at a real, measured air and fuel ratio (not a lambda sensor derived value) of 14.7:1 rarely runs best, 
except by accident!
At full throttle for instance, a high compression engine requires more cooling than a low compression engine - and that required 
cooling is supplied by a bit more fuel. Bang! There goes the 14.7:1 "theory" to heck - along with the 13.1 theory and also the 12.5:1 
theory..... Best mixture, IS the mixture that makes best power. When the engine is making best power, even at cruise operation, 
it will also run the smoothest.....
Additionally, the amount of residual oxygen (what a lambda sensor reports) is affected by all the engine imperfections mentioned
So..... Even if you actually KNEW what the A/F Ratio SHOULD be to make best power - the amount of oxygen isn't a reliable
 enough indicator of the A/F ratio......)   

 Venturi action: What the hell is a "venturi"?

In our case, it's a necked down place in the carby.

The engine wants more air but the venturi restricts it causing a partial vacuum.

Whenever air passes through a tube rapidly, a pressure drop occurs, and this is the principle that moves fuel into the 
throat of a carburetor.

 Since the strength of the vacuum is directly proportional to the speed of the air column, some kind of boost 
is needed at low rpm, and that's the reason for the venturi.

 By placing a restriction in the throat, the air is forced to move faster and an extra pressure drop is created allowing 
atmospheric pressure on the fuel in the bowl to "push" fuel through the jets into the throat of the carb.
Damn! that sure cleared things up huh? 

Let's go back to "don't mess with it Luke"  :-)

Screw the venturi, there's pressure on the outside and a vacuum inside.

Now that you've all got your degrees in Thermodynamics, we can get into the heart of the matter.

There are six systems in a carburetor:  "Awesome"! she cried.

I can only find five on my Solex. I'm sure you'll all let me know if I'm wrong. :-)

Reservoir: The expensive version of "float bowl"

All the gasoline an engine uses, makes an intermediate stop in the bowl, which supplies the idle, cruising, and power circuits,
and the accelerator pump.

A needle-and-seat valve and float arrangement keeps the bowl from over-filling when the engine is not using all the fuel the 
pump supplies.

It may not seem that a bit of plastic or a tiny metal pontoon would have the buoyancy necessary to shut down the pump's 
considerable pressure, but it's hanging off a lever in such a way that it has plenty of mechanical advantage and can push 
the needle into its seat hard enough to do the job.

The Idle circuit:

Since there's very little air passing through the venturi at idle, not enough vacuum is generated to move the fuel through 
the Main jets or "cruising system", so the idle circuit has to take over.

 This is a port below the throttle plate where there's plenty of vacuum,a passage from the bowl, and an adjustment screw or

Remember those two screws on the pict 34 or "name your poison"?

Then there's the "air correction chingaso".
The velocity of the incoming air is low with the throttle almost closed, so the passage to the port usually has air bleed 
or emulsion holes in it to aid in atomizing the gasoline. 

 Some carburetors have idle air jets so that the throttle plates can close completely, somewhat similar to the idle air bypass 
of a typical fuel injection system.

Hot air is relatively thin, so when the temperature is very high not enough air molecules are getting around the throttle plates
at idle to make the proper blend.

To eliminate the excessive richness this would cause, some carburetors have a hot idle compensator,which is simply an air 
passage that bypasses the throttle plates whenever a temperature-
sensitive bimetal valve opens.

Maybe on a Weber or Dellorto.

Solex is done without, to my knowledge. 

Other fuel passages are needed to provide a smooth transition from idle to moderate rpm, and these are called transfer or 
off-idle ports or slots. They are positioned higher up in the barrel than the idle port and are progressively uncovered and 
exposed to vacuum as the throttle plate opens.

They generally get fuel from the same tube as the idle port, but are not affected by the mixture screw.

This stuff above is for the "fancies" too, not on my "Solex"!

What do we care about flat spots? We just passing thru anyway. 

"Cruisin'" and "Stomp" Circuit: 

The "main" action is the "cruising" system.

This is a nozzle that sprays gasoline into the part of the venturi or venturis where the highest vacuum is present.

It gets its supply of fuel in an amount controlled by the diameter of the main jet.

This circuit works constantly at steady speeds and is calibrated for good gasoline mileage.

"Stomp" circuit:

Whenever the driver asks the engine to provide all the power possible by pushing the accelerator pedal to the floor, thus 
opening the throttle plate all the way, too much air enters the engine for the cruising circuit to handle.

The mixture would lean out and output would be severely limited.  So, the aptly-named power circuit is activated.

(Pass if you have a Solex)

 This can be either a separate fuel valve or a metering or step-up
rod that normally blocks some of the main jet's flow, but is 
pulled up out of the way by linkage or a spring when extra fuel is 
needed (the rod may be held down against a spring by a vacuum 
piston, then rise when the vacuum drop that occurs at wide open 
throttle reduces the force on the piston).

 (Solex readers resume)

Lag eliminator: expensive version of accelerator pump

The circuits mentioned so far would be sufficient if the engine were only asked to run at a constant speed or to 
accelerate very gradually.

That's not the way we do things tho, is it? 

Stomp!!! "Out of the way old lady, you taken up too much room"!!

"Did you see that?!! She gave me the finger!!"

The throttle is opened too rapidly for the above systems to be able to keep up.

The sudden blast of air would cause the engine to stumble or stall before enough fuel could be moved into the intake stream to 
provide a burnable mixture.

The accelerator pump is what adapts the carburetor to the realities of the highway.

It squirts an extra charge of gasoline into the intake stream whenever the throttle is opened, and,
since it works mechanically, does so before the engine gets a chance to choke on too much plain air.

It's an ordinary pump with one-way inlet and outlet valves and an air bleed, weight, or spring set-up to eliminate the 
possibility of fuel escaping from its nozzle because of vacuum.

Super rich circuit: expensive choke

Finally, there's the choke, a device that gets the engine started even when it's too cold for the proper amount of gasoline to 

By closing off the mouth of the carburetor so the manifold vacuum present during cranking causes a great deal of fuel to 
flow out of the bowl into the throat, enough vapor is available to allow the engine to fire.

Once running, the powerplant starts producing sufficient vacuum to act on the diaphragm or piston of the choke pull-off 
mechanism, opening the choke enough to permit an adequate amount of air to enter the manifold for fast idle and cruising 

That's those expensive jobs, Solex just spring loads the flap and it comes open by itself.

As the engine warms up, a calibrated coil of flat metal that's connected to the choke plate expands from the heat of exhaust 
or an electrical element.

We Solexers have a electric element. Calibration is up to you. 

Gradually, this expansion opens the choke until air flow is no longer restricted.

And thus it ends, boys and girls, now when you go out and "kick the tires and light the fires". . . .
 Some of the "magic" is no longer there.  "well that sucks"