Whats carbs do
Gasoline has best burn when a cylinder has a 14.7:1 ratio of air:gasoline, this is a WEIGHT RATIO
14.7 POUNDS of air to 1 POUND of gasoline
14.7 grams of air to 1 gram of gasoline
The air opening size on the top of the carb is known, so the flow of air thru this opening can be calculated, the Jet sizes used are setup to meter gasoline flow to this 14.7:1` ratio based on the air flow thru that size opening
As the air passes by the Jets it SUCKS gasoline out of them, its not a very accurate system but works fairly well
Problem with Carbs was that the WEIGHT of the air varys by temp(hot air rises, because it LIGHTER), and altitude, air is thinner/lighter at say 4,000ft elevation than at sea level
The displacement of the engine is also known, a 2 liter engine can suck in 2 liters of air, every 2 RPM, at wide open throttle(WOT), so its just number crunching to know how much air will be sucked in at say 2,000RPM
A 3.0l engine pulls in 3 liters of air, 5.0l engine 5 liters of air, ect.................
So the carb's air opening is setup for engine displacement at WOT, and then the Jets are setup for that air flow
Too small an opening and you can't get full power, full air flow
Too large an opening and you loose SUCTION, so lose power
There is a float bowl to feed the Jets and also feed the Accelerator Pump, this pump squirts gasoline into the engine anytime you press down on the gas pedal, this makes for a Richer mix because of the instant increase in air flow with throttle opening quickly, without this there would be a hesitation as the Jets don't instantly react to the increased air flow as suction drops a bit when throttle is opened
Now thats ABOVE the throttle plate
Below the throttle plate are the Idler Jets, needle jets
These much smaller jets are used to get the 14.7:1 ratio at low RPMs, the MAIN Jets are too big to fine tune the idle
Generally the idler jets are tuned Rich, to prevent overheating at idle, and also to keep Cats nice and hot at lower RPMs
Why a Choke
Liquid gasoline can't be ignited by a spark, yes the movie guys take liberties with that one, lol
Only gasoline VAPOR can be ignited by a spark.
You need 30% gasoline vapor in a cylinder to get a good ignition with a spark plug
Cold gasoline has less vapor, if you can smell gasoline that's the Vapor you are smelling, and in cold weather it has almost no smell so no vapor
The Choke plate blocks air flow into carb but the engine cranking still pulls in the same amount of air
So what happens is MORE gasoline is sucked out of the Jets than normal
Even cold gasoline has some vapor, especially when sucked out of the jets
The choke is adjust to get at least that 30% vapor into the COLD cylinders so spark can ignite it
Once engine starts the cylinders warm up pretty fast, but the cold air being pulled in prevents much vaporization until the whole engine starts to get warm, so choke plate stays partially closed and opens bit by bit as engine warms up
This is why most vehicles had the air warmer pieces on the exhaust manifold, when cold air was sucked in from around the exhaust manifold to warm it up a bit to help engine warm up faster and run better cold
The Choke adjust also resets idle cam, so engine idles higher when cold, you set the automatic choke and high idle by pressing gas pedal to the floor ONE time before cranking
Engine should then start and idle high without any other input from driver, if things are working as they should
Manual choke needs to be set for start up and then adjusted as engine warms up
Fuel injected engines need the same thing, Rich fuel mix for at least 30% vapor and high idle on cold start
The computer does these things for the driver on cold start
Computer can also calculate air weight using air temp sensor and MAF or Barometer sensors