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How a choke in a carburettor works....explanation!

6K views 23 replies 11 participants last post by  ccmhunt 
#1 ·
Actually i'm looking for one?

After that thread...cam't find it, I owed an explanation, as to how it works.

Have done some reading and am abit confused.

Read in one tuturiol that when it's cold, fuel sticks to the cylinder walls and therefore requires MORE fuel to start the bike....didn't say how it provides the extra fuel?!?!?!

In another tutorial, it says that the choke opens an air circuit and runs the engine at a higher rpm, which is totally contradictary to what was said above? :scratch

Whats a throttle valve?....that's the slide, right?
 
#2 ·
Throttle valve is the butterfly. The slide metres the fuel. The choke I think moves a butterfly that changes the air pressure difference across the carb and enrichens the air/fuel mix. How it does it I don't know
 
#3 ·
Throttle plate is below the jets. It doesn't change any pressures, only air velociy at the jet which is the venturi effect vacuum. The choke is above the jet. Closing it applies a direct vacuum at the jet increasing the fuel.

This is in the simplest terms. It can get much more complicated.
 
#4 ·
In your Mikuni CV'S this>> In another tutorial, it says that the choke opens an air circuit and runs the engine at a higher rpm,
Remember, I told you that the choke plunger opened an air passage from above the piston diaphragm to the upper venturi on the engine side of the butterfly . This was the direct effect.
Now, what is the indirect effect?
 
#5 ·
The idirect effect is it lifts the slide more increasing the amount of fuel while keeping the amount of air the same enrichening the mix
 
#8 ·
I know what the venturi is.
If the choke plunger opens an passage above the piston diaphram, this will in turn lift the needle, allowing more fuel in,no?
I mean the slide, slides up the needle, allowing more fuel.

Can someone put up a correct diagram of whats going on?
 
#10 ·
The real confusion comes in that choke enrichener and fast idle are all incorrectly used as if they are interchangeable only because they are used to achieve the same desired effect.

There are tons of things that are like this too.

Backfire/after fire
Flooding/gas fouling
Etc.
 
#12 ·
cold fuel doesnt atomize well..

wish this showed a air choke like on the srad. instead the choke valve actually opens a port to let more fuel where in our carbs the choke valves lets vacuum travel to the diaphram and lift the slide and needle to let more fuel.



when you look at srad carbs and many others you will see the choke valve on the top of them. basically the direct opposite position then the video. because you want vacuum to be directed to the top side of the diaphram. when you pull the choke you open the port in the top of the bore on the engine side of the butterfly to let vacuum up to the top of the diaphram lifting the slide and needle.
 
#17 ·
cold fuel doesnt atomize well..

wish this showed a air choke like on the srad. instead the choke valve actually opens a port to let more fuel where in our carbs the choke valves lets vacuum travel to the diaphram and lift the slide and needle to let more fuel.

http://www.youtube.com/watch?v=wyspAHrMbb8

when you look at srad carbs and many others you will see the choke valve on the top of them. basically the direct opposite position then the video. because you want vacuum to be directed to the top side of the diaphram. when you pull the choke you open the port in the top of the bore on the engine side of the butterfly to let vacuum up to the top of the diaphram lifting the slide and needle.

Ohhhhhhh...pretty colors!
See pretty...
 
#14 ·
So after all the hints, we have a conclusion, the choke is an air mechanism, whose secondary function is to raise rpm and meter more fuel.
That's the distinction I was driving for, because all too often there is a misconception that there is a choke jet, or that the choke itself is metering fuel. So my questions in the other thread, that asked you all the ponder it, and gave you all the information that the mechanism opened the passage from venturi to chamber above diaphragm, was merely an exercise to make you arrive at a forgone conclusion and to remove the misconception of a choke fuel jet mechanism in our carbs.
 
#19 ·
In another tutorial, it says that the choke opens an air circuit and runs the engine at a higher rpm, which is totally contradictory to what was said above? :scratch

he said, it adds more air, he also said that in another thread a while back when I was going through my carbs when I thought fuel may be leaking out the choke o-rings..

he said "remember the choke adds more air"

I fear you blonds aren't going to do so well on the test... :dunno
 
#23 · (Edited)
LESS air. It's a valve (butterfly valves, like one the ones that open and close on giant superchargers mounted on drag race cars) that leaks when it's closed; on most carbs, either there is a valve that only controls the amount of fuel/air mix, but doesn't close all the way unless the choke is activated, or there is a completely separate choke valve that is closed when the choke is applied.

The airflow is significantly reduced, and like putting your...hand...over the end of a vacuum cleaner hose, any other openings that are present (like leaks in a vacuum cleaner hose) increase their flow to help offset the low pressure. Inside a carb, this usually includes the fuel atomizer (where the venturi is- a venturi sounds much more technical than it is; it's basically just a part of the air passage which has been narrowed in a mathematically designed way). More suction results from the lower volume of air being allowed into the motor, which draws more fuel out of the atomizer.

The other stuff, such as the idle speed adjustors, are secondary mechanisms that are turned on at the same time as the choke to make it more effective/prevent stalling. They are not the same mechanisms, but linked. It's a similar setup to acceleration pumps: on large car carburators, when you stomp the throttle there is a little piston that also jumps; it blasts a bunch of fuel into the carb to cover the slack as the normal jets are still adjusting to the instant change in fuel demand caused by you opening the butterfly valves in a hurry. Secondary fuel jets are often used to help with the choke, too. They work in concert with the throttle, but are not actually the throttle. The idle adjustors and secondary jets work in concert with the choke if they are designed to, but are not actually the choke.

Best high school class ever: Small engine repair

EDIT: More about venturis- imagine the top of an airplane wing. The low pressure area at the top of the wing is responsible for a lot of the lift, drawing the wing up into the low pressure area. A venturi is basically the same idea, but in a circle with the wing wrapped up like a ring with the top facing in. In the middle of the circle of low pressure is the fuel jet/atomizer- instead of drawing a wing up to provide lift, it draws gas out to provide fuel.
 
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