carburetors and chokes

[rubiranch]

Engines and carburetors don't "suck" fuel because there is no vacuum. The word vacuum is used to describe a low or negative pressure. The only place a true vacuum exists is where there is no atmosphere, like in outer space.

When a piston travels down the cylinder it creates a low pressure, something has to take the space that the piston occupied. Now that a low pressure has been created the atmospheric pressure, 14.7 pounds per square in at sea level, forces air in to low-pressure area.

A low pressure is created in the intake manifold below the carburetor throttle plate from the pistons downward motion of the intake stroke. As the throttle plates are opened the low-pressure rises up into the venturi area of the carburetor.

The low-pressure in the primary venturi area causes the atmospheric pressure that is against the fuel inside the float bowl to push the fuel through the main metering jets, through the emulsion tubes (where metered air and liquid fuel are mixed and the fuel becomes a finely atomized liquid and then flows through the secondary booster venturi (the round booster nozzle that the fuel comes out of).

When the choke plate is closed it raises the area of low pressure even higher in the carburetor and closer to the fuel level of the float bowl causing even more fuel flow to through the main metering system.

The reason so much more fuel is needed when the engine is cold is due to the fact that the fuel does not vaporize as well in the cold and tends to puddle on the bottom of the intake manifold floor and it never reaches the cylinders.

As the engine warms and needs less fuel, the choke slowly opens lowering the low-pressure area and less fuel is delivered.

I’m sure this is as clear as mud.

[fitzwell]

uh.....ok

[Hawkrod]

As eloquent as that explanation is, it is not actually entirely correct. Atmospheric pressure does push some fuel, in some circuits, in some carbs, but other circuits in other designs use the vacuum (or technically suction) below the venturi. If you have an emulsification tube with an air bleed, vacuum is the only way it can work because air pressure on the fuel in the bowl and air pressure through the air bleed prevent fuel from being pushed through and thus it must be drawn by vacuum. Not all carburetors work exactly the same way so it gets very confusing when comparing different brands or even different models within a single brand. Just a big pile O' apples and oranges sometimes! You also mention that there is no vacuum and that is correct. In fact, even space is not truly considered a vacuum and I do not believe there has ever been a true vacuum as it is a philosophical concept not yet realized by science. We actually use the word vacuum to refer to what physicists call a partial vacuum and that is what you described as "a low or negative pressure". Space is also considered a partial vacuum. In common colloquial use we consider a vacuum to be any pressure less than atmospheric pressure but technically it is suction we are refering to and it is suction that does the work in carbs that are not using atmospheric pressure to push the fuel into the circuits. An engine is an air pump and as such can have negative or lower than atmosperic pressure in the intake tract thus creating suction and, as I noted, many carb designs use this low pressure (measured in inches of Mercury) to draw the fuel for internal combustion process. Hawkrod

[peanutman]

Well thats way over my head but i'll take ya'lls word for it.

[Hawkrod]

Well thats way over my head but i'll take ya'lls word for it.

Heck, I never even got into the rarified few carbs that use the siphon principle too! LOL Hawkrod

[rubiranch]

As eloquent as that explanation is, it is not actually entirely correct. Atmospheric pressure does push some fuel, in some circuits, in some carbs, but other circuits in other designs use the vacuum (or technically suction) below the venturi. If you have an emulsification tube with an air bleed, vacuum is the only way it can work because air pressure on the fuel in the bowl and air pressure through the air bleed prevent fuel from being pushed through and thus it must be drawn by vacuum. Not all carburetors work exactly the same way so it gets very confusing when comparing different brands or even different models within a single brand. Just a big pile O' apples and oranges sometimes! You also mention that there is no vacuum and that is correct. In fact, even space is not truly considered a vacuum and I do not believe there has ever been a true vacuum as it is a philosophical concept not yet realized by science. We actually use the word vacuum to refer to what physicists call a partial vacuum and that is what you described as "a low or negative pressure". Space is also considered a partial vacuum. In common colloquial use we consider a vacuum to be any pressure less than atmospheric pressure but technically it is suction we are refering to and it is suction that does the work in carbs that are not using atmospheric pressure to push the fuel into the circuits. An engine is an air pump and as such can have negative or lower than atmosperic pressure in the intake tract thus creating suction and, as I noted, many carb designs use this low pressure (measured in inches of Mercury) to draw the fuel for internal combustion process. Hawkrod

Thank you.

[SteveC]

Well thats way over my head but i'll take ya'lls word for it.

Heck, I never even got into the rarified few carbs that use the siphon principle too! LOL Hawkrod

those are the carbs that mount on the bottem of a inline intake?? ive only ever seen one of those and that was on a studebaker. least thats what i took it as.

and i accually understood most of that .... i did more research on this subject so i dont look foolish

[averagef250]

Thanks for the good read gentleman, very educational!

[SteveC]

i know there was some carbs that the gas went up into the intake, the common- gas goes down into the intake ,,, and dont qoute me wasnt there a one were the gas went sidways into a intake....

and yes this was informative

[cdherman]

When the choke plate is closed it raises the area of low pressure even higher in the carburetor and closer to the fuel level of the float bowl causing even more fuel flow to through the main metering system.

That may be true -- I cannot refute it. But closeing the choke plate also reduces the air flow through the carb. Now that in turn, makes the mixture "richer".

If closeing the choke plate ONLY richened the mixture and did not reduce the air flow, then we would play with running our engines with the choke partially closed. But we all know that the engine runs best, once its warm, with the choke wide open, assuming you have a decent carb on there.

To me, that means that the mixture has been made richer by reducing air flow and not by increasing the flow of gas.......

BTW -- just how did this thread come into existence?????

[rubiranch]

cdherman, you are right, the choke does restric air flow + it increases the pressure differential in the veturi which increases the amount of fuel beng delivered.

[rubiranch]

BTW -- just how did this thread come into existence?????

I was trying to answer a question for another member and didn't want to post it in another thread.

[FORDification]

Good topic! I'm learning a few new things here myself!

[Hawkrod]

Just for the heck of it, carbs only work because of Bernoulli's principle which is also what makes airplanes fly:

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

This principle is yet another way fuel is drawn into the engine in a carb (sorry, I missed listing this in my previous response). The venturi effect causes its own pressure differential independent of atmospheric pressure and some carb circuits use that to deliver fuel as well. Who woulda' thunk that all those engineering classes would be usefull in fixing up old trucks! LOL Hawkrod.

[rubiranch]

Thanks Hawkrod, appreciate your input, great info.

Just about everybody here has a carburetor on their truck. I believe that if you really understand how they work it will be a lot easier for you to both diagnose them and repair them.

Besides that, I hope we all learn something new.