STUPID QUESTION! What is "choke" ?

[ScottishBiker]

I have read it on here loads of times but what is it?

Sorry for the thick question but im not from a biker family, and I have only just passed my CBT so I dont have very much knowledge on bikes.

[blurredman]

LOL

It doesn't just apply to bikes, but any engine from a car, tractor, motorbike.

It's just a lever attached to the carburretta which controls the flow of fuel and air into the engine. You would use it on a cold engine, and when the engine is warmed up sufficiently, then you turn it off.


To be honest though, I am not supprised a youngun' doesn't know what a choke is, being that it has more or less been replaced with electrical systems making it 'auto choke' and no need to bother.

However, many learner/beginer bikes these days are quite out of date in their design even until the 90's and 2000's. For example, you'd be hard pressed to find a manual choke car made after 1992/3 because of emition laws. However, a bike can still have them until 2005-6 because the same laws don't apply to them.
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CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 - 40k, 1989 MZ ETZ251 - 52k.

[G]

Yea, it basically restricts (chokes) the air flow - this means there's relatively more fuel for the same amount of air, which helps it running from cold. You should take the choke off when the bike with idle without it.

As above; cars used to have it, but most newly designed vehicles with FI don't; they have a servo to remotely control the throttle instead.

[ScottishBiker]

ah, cheers guys.

Im guessing my bike doesnt have one since its a 10 plate but I thought I should ask in case I was screwing up my engine.

[lihp]

[G]

Ah, <shrugs>, I've had several cars with 'idle control' units, which directly affect the the throttle cable/lever.

[lihp]

IAC (Idle air control valves) are small servos, that allow air to bypass the butterfly via a small valve, this allows the ecu to idle the engine far more accurately, especially in varying conditions, without having to mess about with screws and idle speed adjusters. Although they do still have them, but are rarely touched.

Ones that directly affect the throttle cable I've not heard of, but would love to know which cars so I can have a read up, as I've not seen it implemented like that before.

[Polarbear]

My FI busa has a choke lever. Its actually a fast idle but Suzuki still call it a choke for some reason, and use the same symbol on the lever.
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[lihp]

[blurredman]

Personally, If I didn't have a choke in my car, and I've never had a vehicle that has FI in my own name, I don't think I would know what to do with my hands while starting. I would also miss having a part of the car that connects us into one being taken away from me to be tragic.




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CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 - 40k, 1989 MZ ETZ251 - 52k.

[Skudd]

My FI varadero has a fast idle lever that they call a choke too. Just easy to explain and people tend to understand the principle.
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[G]

My apologies, may be wrong as thinking about it, pretty sure that's what it was called on the Laguna at least.
Thought it was that way on my beemer too, but may well be wrong there too.

[Howling TerrorOutOfOffice]

Fast idle on my duc. Opens throttle body upto 25%...almost usuable cruise control
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[Teflon-Mike]

Just for elaboration really as I think the original questions been pretty well answered.

Technically a 'choke' is any deliberate 'constriction' in the profile of the induction tract, but most often used in a carburettor.

This is the confusing bit. becouse the 'choke' you use on start up, may not actually strictly be a 'choke', but I'll come back to that.

When talking carburettors, they will often be rated by thier 'Choke' diameter, so when they talk of a bike having a pair of 26mm carbs, or a car having a 1 1/2 inch carb, that dimension is reffering to the diameter of the 'choke'.... Diameter of the hole at either end, will tend to be a bit bigger, but thats what helps make the carburettor work.

Now, the conventional carburettor works on a thing called teh 'pitot effect'. Its the same principle as lets a flute make a noise or why you can get a note by blowing over the top of a beer bottle. As air passes over a hole at 90 degrees to the direction of flow, it causes a vacuum over the hole. So, if you have air flowing along a bit of pipe in an inlet tract, you can drill a hole in the pipe, and solder a smaller pipe to it and dip that in a jam-jar of petrol, and as the air passed over the smaller pipe, the vacuum created can suck fuel up the smaller pipe and into the airstream.... THAT is a rudimentary carburettor.

Mucking about with the size of the pipe, poking a tapered needle into it to change it size being the most common, would then let you vary the resultant air/fuel mixture, or you could mess with how long the T piece tube was and the level of fuel in the jam-jar to fine tune it... and then you add more compliction to the thing to use it to not just meter the fuel into the air, but govern the engine, by adding a slide to control the amount of air going in and stuff like that.

ANYWAY, Pitot effect, on its own isn't ALL that effective.... it'll suck fuel, but you dont get a very big suck from it, AND tends to only be effective at a certain air-speed. All to do with harmonic tuning, like having the right level in your beer bottle to get a note when you blow over the top, and be blowing air at the right speed.

SO, to refine the effect, commonly a 'choke' is incorporated into the carburettor. The diameter of the tube is constricted a bit, and its normally nicley shaped and rounded before and after to aid smooth air-flow.

The choke causes an increase in air-speed as it passes through the constriction, and also creates a change in pressure, which can greatly enhance the pitot effect, and be used to make the pitot effect a lot more useful over a much wider range of air-speeds.

In some carburettors, the 'choke' is actually a removeable part and can actually be changed to alter the metering of the carburettor.

On Fuel Injected engines, fuel is possitively squirted into the air stream, rather than sucked into it, and the mechanism for metering the correct amount of fuel controlled by some external apparatus. Most often a micro-processor, deciding how much fuel should be added from variouse sensors around the engine. Needs to know how much air is flowing into the engine though. Depending on ths sophistication of the Injection system, it may be that it simply uses a throttle possition sensor, most crudely a pottentiometer, or a carbon track variable resistor like a volume control knob on an old radio, to tell it how far open the throttle is, and from that it can pretty much guess who much air is flowing and squirt the fuel into the engine. But more sophisticated systems will have Air-Mass-Flow units, to work out how much air is going into the engine, and since on a colder day, air will be denser than on a hotter one, simply measuring the volume of air flow may not be good enough, so as well as measuring air speed, it will often measure air temperature and air pressure..... and to so THAT there may be a pitot effect arrangement, complete with 'choke' placed in the induction tract, or throttle body.

So, onto the cold-start 'Choke'. At its most basic, the cold start choke is an 'enritchement device' to chuck some extra fuel into the engine on start up, to help get the engine to start more easily and run until its warm enough to take load on a weaker mixture.

It may NOT actually utilise ANY kind of constriction to the inlet tract to do that. Some choke mechanisms simply gob a load of extra fuel into the air-stream by opening up a big jet, but most do incorporate a constriction, though in the form of a flap-valve, more like the throttle butterfly than a 'choke', to restrict flow into the engine, and create a vaccum in the inlet tract from the engine's 'suck' to drag extra fuel up the jets, providing the desired enritchment.

Which is all completely irrelevent to answering the question 'Whats the Choke Knob for'... but the point is, the 'Choke Mechanism' more often ISN'T strictly a 'choke' but a throttle Valve and mixture enritchment device.

A 'choke' is more strictly a constriction in the inlet tract, most often in the carburettor, or fuel metering device, used to create a speed and pressure change in the air-flow, utilised to assist fuel metering.

BUT might help if you get further confused when people start talking about carburettor 'choke' sizes, and you are bamboozled as to what the size of the cold start mechanism has on helping a tuned engine make more power..... reason.... becouse they are different 'chokes'.
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[SirEdward]

Carbs suck ass.

And a choke is like deepthroat but not quite like it.
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[lihp]

Apologies if I mis-interpreted your post, but are you suggesting that the term choke, can and is used interchangeably with the venturi?

Your descriptions seem to be more of a Venturi and the venturi effect.

Although my knowledge of carb systems on vehicles is low as most of my tuning experience is with ecu's and electronic fuel injection systems. I am studying for a Private Pilots License, and carbs are studied in a lot of detail as most small aircraft still use them, and carb icing and the effects of different parts of the carb are very very important to avoid turning your aircraft into a giant rock in the sky.

Pitot's are generally used on aircraft to measure stuff like air speed, and works on air pressure. As it moves through the air, the air going through the Pitot tube pushes on a sprung mechanism that forces an airspeed indiactor to move.

However, your description of the carb, seems to match the Venturi and the Venturi effect. The restriction in the carb, The Venturi, causes a high vacuum in this area, if you place a jet in the middle of this, you get it sucking through fuel. Now, if you increase the airflow, the vacuum increases pulling in more fuel, this is the venturi effect.

The important thing to know about the Venturi effect, is the vacuum brings air temperatures down, a LOT. This can cause ice to form inside the Venturi and carb in general, thus reducing airflow through it. Even on a warm day, carb icing can occur so long as the atmosphere is moist enough (relative humidity, but weather systems and geology is something for another thread).

Now, a choke can open up a bigger jet, or make the venturi narrower, thus increasing the vacuum effect (venturi effect), simply restricting the general flow of air through the carb, won't have the desired affect as it will also reduce the venturi effect.

[Teflon-Mike]

[mistergixer]

A friend of mine had an 'old' mini, his mum needed transport so he lent it to her.

It had been running just fine when he had, as soon as he lent it to the old dear the problems started.

She complained that it ran roughly, frequently stalled and drank petrol.

He checked it over, ran fine, mother still complained.

He asked me to look at it, no problem, mother still complained.

After much hair pulling, we decided we needed her to recreate the problem for us.

So, we all climbed into the car and the old dear got into the drivers seat.

Her first action once she was in the car was to pull out the choke lever and hang her handbag on it! The handbag stayed hanging on the choke lever all journey, the car ran like a 3-legged dog once warm.

Problem solved.


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[lihp]

Flight computers ftw, though we're not allowed to use electronic flight computers for test and training purposes. And for any flight, manual flight computer wheels must still be carried as a backup if using an electronic one.

The most awkward part with nav is the usage of speed, ground speed, air speed and indicated air speed are 3 very different things, and can result in 3 very different calculations.

You can be doing 55mph on the ground for take off, but if you have a 10mph head wind, that's actually 65mph indicated airspeed, but 45mph true airspeed.

Solo navigation is kind of terrifying at first, but then it's bliss. Until unexpected fog covers the aerodrome, and you're asked to land somewhere you've not been before, and have to call up the instructor to get them driven over to fly the plain back under IFR (y). I even had to pay landing and take off fees

As for triangulation, you're supposed to be able to do that while flying. You're asked to put the aircraft into a slow cruise and then triangulate you're location and navigate to a set point. Quite fun to be honest. Got me out of the habit of using a sat nav on a road (y). Makes british road signs look like they make sense!

[stinkwheel]

[lihp]

[Pete.]

As Teflon Mike says, though it's universally referred-to as a 'choke', very few carbed bikes have chokes, they have 'fuel enrichment circuits'.
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[stinkwheel]

[Walloper]

The choke/fuel enrichment device is used to start a cold engine solely because of condensation/evaporation effects of temperature.
The fuel droplets drawn in/injected into the engine as air-fuel mixture must be at a precise stoichiometric ratio/rate.
When warm this is simple to achieve when cold the cold surfaces of the engine allow the fuel droplets to condense against the cold surface effectively 'dropping out' of suspension.
If no enrichment/choke device was used at start-up then the mixture would be too lean to be effective and the engine would not start.

FI systems can also have cold start valves which help to enrich the initial charge of fuel in the engine at start-up.

Diesel engine can use the same sort of thing but they normally use heater plugs to help heat and ignite cold fuel or they use ether which ignites at a lower temperature than diesel fuel.
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[pinkyfloyd]

Choke

https://blogs.e-rockford.com/applesauce/files/2008/06/choke.jpg
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