Date: Tue, 10 Dec 1996 17:38:50 -0800 (PST)
From: Mike West <mwest@cdsnet.net>
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 wierd 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.
Definition:"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 wonderous word works too well.

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

 We'e 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 carbuetors 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.

Ratio:

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.

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.

 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:  "Awsome"! 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
two.

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 
vaporize.

 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 
operation.

 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"

 west