# Help me understand how a carburetor works



## FrostyTheBeerMan (Jan 13, 2017)

Hi all,

I've begun taking apart the carburetor on my Ariens snowblower model 921005 ( ST927LE ),
anyway, I'm a beginner, and would like to know how gas from the carb is supplied to the engine. 
Once the engine starts, is it the engine itself that causes a suction and this in effect causes or creates this venturi effect in the carburetor ?

That's just something I'd like to learn about... why small engines continue to run without the aid of a fuel pump like you would see on a car. 

2nd question, once gas enters the bowl of the carb, I'm guessing this venturi effect is how that gas now is removed from the bowl and goes into the
throat of the carb..... from there... which orifice does it leave the carb from ? I've taken part of my carb apart, and can't figure it out.... from what pipe does
the gas leave the carb and get sprayed into that pipe or "manifold" that enters the piston area ?

good question ? hope somebody wants to give a little classroom lesson... I would appreciate it, and many other beginners.

thank you !


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## tpenfield (Feb 24, 2015)

Yes the suction of the air into the engine draws the fuel into the intake

The main jet / venturi draws the fuel from the bowl.

There is also an emulsion tube that mixes some air into the flow.

Also, most of the fuel tanks are gravity flow into the carb bowl, so no fuel pump needed. It holds the cost down.


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## cranman (Jan 23, 2016)

You don't need a fuel pump, because the gas tank is higher then the carburetor.......carburetors work...no one knows why......


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## tpenfield (Feb 24, 2015)

FWIW - when I fog my boat engines, I stick a small tube into a bottle of oil and place the other end of the tube at the air intake of the engine . . . the air flow draws the oil through the tube and empties the bottle in a few seconds.


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## jsup (Nov 19, 2017)

First, look at the physical characteristics of the carb. It's essentially a straw with a bowl in the middle.

Vacuum going through the straw, being pulled from the downward stroke of the piston, pulls the gas from the bowl, into the vacuum stream of the "straw" above it. 

The amount of gas that is pulled is determined by the amount of vacuum created by the engine, which is regulated by the throttle plate, and governed by the size of the jets. When you move the plate open, more vacuum is released into the vacuum stream of the "straw" and pulls more gas into the stream, making the engine go faster. 

That is VERY short version.


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## vinnycom (Nov 6, 2017)

there are vids on the internet that explain visually how carbs work. in short...its magic, physics and chemistry etc rolled up into one

https://www.youtube.com/results?search_query=how+carburetor+works+animation+small+engine

.


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## jsup (Nov 19, 2017)

tpenfield said:


> FWIW - when I fog my boat engines, I stick a small tube into a bottle of oil and place the other end of the tube at the air intake of the engine . . . the air flow draws the oil through the tube and empties the bottle in a few seconds.


Air is treated as a fluid in physics, and the reason for that is the air pressure is changed (dropped) by the vacuum in the intake stream, and the higher air pressure pushes the liquid to the area of low pressure. Brunoulli's Principle if I remember correctly.


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## tpenfield (Feb 24, 2015)

Bernoulli's principle . . . :smile2:


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## paulm12 (May 22, 2015)

tpenfield said:


> Bernoulli's principle . . . :smile2:


the pasta company ??
.


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## cranman (Jan 23, 2016)

Now you've taken the magic out of it.......


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## tpenfield (Feb 24, 2015)

paulm12 said:


> the pasta company ??
> .


The Bernoulli's . . they live up the street . . . nice people :smile_big:


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## nwcove (Mar 2, 2015)

paulm12 said:


> the pasta company ??
> .


isnt pasta high in carbs ?:wink2:


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## jsup (Nov 19, 2017)

nwcove said:


> isnt pasta high in carbs ?:wink2:


Rimshot!


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## FrostyTheBeerMan (Jan 13, 2017)

Hi guys, really interesting stuff.... enjoyed all those responses. Good animation videos - some really good ones, thank you !
When the weather is a bit warmer ( right now we have -24 degrees C --- insanely cold right now ) I want to get more into the carb
on my dad's blower. It was brand new..... he used it about 5 times my mom said... then it sat in his garage for about 5 years ( left full of gas ) ( my dad is now 85, he could
no longer operate the machine ) so there it sat... brand spanking new in his heated garage for all those years. 

I've cleaned out a lot of the carb, but did not touch that inner tube ( emulsion tube ) from what I'm reading here, this not only mixes air with the gas, but it is
this tube that is the final area of the carb that holds that air/gas mixture ( ? ) is that right ? ... so if that emulsion tube is clogged, nothing at all will enter the engine....
seems like that is what I may have learned here. So that emulsion tube is actually where the engine gets its supply of gas / air mixture..... 

Thank you guys for your time, and for going easy on me. I am a beginner and have already learned a lot on this forum. Thank you.


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## jonnied12 (Jan 14, 2017)

paulm12 said:


> the pasta company ??
> .


 12 year old Bernoulli over ice. Smooth drink!:wink2:


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## LouC (Dec 5, 2016)

Bernoulli's law is what explains not only how carbs work but also how an airplane wing provides lift. Basically when you speed up the velocity of a fluidic medium such as air you lower its pressure. So in the case of an airplane wing, the top surface in cross section is curved, the bottom is flat. Air rushing across the curved top has to speed up and its pressure is reduced. The air pressure on the bottom of the wing is higher and therefore lifts the airplane. 

In a carb, it is Bernoulli's law and the venturi principle. When the air gets sucked into the carb throat by the downward movement of the piston, it creates a partial vaccum in the carb throat under the throttle plate. The narrowing of the throat (carb venturi) causes the air to speed up as it passes through. This increase in the velocity of the air in the venturi lowers the pressure in that area. Since the air pressure in the venturi is lower than in the carb bowl (which is at atmospheric pressure), gas is forced into the venturi via the pressure difference. There are also emulsion tubes that introduce air into the stream of gas to better atomize it. So if any of the jets are clogged or the emulsion tube holes are clogged, the engine will run lean or not run at all. Small engine carbs have small jets and small passages and this is where the problem usually is. 95% of rebuilding a carb is properly cleaning it. This is the same whether we are talking about a small engine carb, or a 4bbl Rochester Quadrajet or Holley both of which were used on some of the largest displacement engines ever built.


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## FrostyTheBeerMan (Jan 13, 2017)

So that "straw" that jsup was talking about, is the middle tube ( emulsion tube ) is really the last part of the carb that actually holds that gas/air mixture..... 

so in theory, if that center tube was blocked, then zero gas would ever get to the carb, is that correct ? ( it's the one and only exit for the gas to reach the engine )


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## LouC (Dec 5, 2016)

It's not so much that gas would not enter the Venturi of the carb; its that not enough air would be introduced into the stream of liquid fuel to atomize it well. So the fuel mixture might actually be too rich for the engine to run well. Engibes don't run well on liquid fuel it must be atomized into a spray; think of a cologne bottle that atomizes liquid cologne into a mist...


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## jsup (Nov 19, 2017)

FrostyTheBeerMan said:


> So that "straw" that jsup was talking about, is the middle tube ( emulsion tube ) is really the last part of the carb that actually holds that gas/air mixture.....
> 
> so in theory, if that center tube was blocked, then zero gas would ever get to the carb, is that correct ? ( it's the one and only exit for the gas to reach the engine )


The "straw" I refer to is the path of air from the choke to the throttle plate, the bowl is underneath that and the fuel is drawn from the bowl into the "straw". When you open the throttle plate, more vacuum, more fuel gets pulled up. As explained above. 

I don't know if you ever used one of those vacuum attachments to drain brakes, you hook it to your compressor, like this:










But it's the same principle, although the compressor is BLOWING, it's creating a vacuum and pulling the fluid through.


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## jsup (Nov 19, 2017)

LouC said:


> It's not so much that gas would not enter the Venturi of the carb; its that not enough air would be introduced into the stream of liquid fuel to atomize it well. So the fuel mixture might actually be too rich for the engine to run well. Engibes don't run well on liquid fuel it must be atomized into a spray; think of a cologne bottle that atomizes liquid cologne into a mist...


A lot of the atomization happens when the cold gas hits the hot valve. But you need enough air to make it happen. 

Gas, as a liquid doesn't ignite. You can put a match into liquid gas, and it would go out. It's the fumes that burn.


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## Shryp (Jan 1, 2011)

This is a bit off topic, but I found this video interesting recently.


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## FrostyTheBeerMan (Jan 13, 2017)

Very interesting indeed LouC, and jsup.... nope never used one of these, it almost looks like the paint sprayers I've seen at Home Depot.
I'm really liking this topic ( thread ) I started.

Ok so here's another question.... when you look at the carb... where is the final area or tube that supplies the engine that gas/air mixture ?
I've looked at the carb on my snowblower and without taking it 100% off the engine it's hard to see..... so what tube or exit area supplies the engine ?
That's the thing I'm not really understanding. The huge center of the carb... that's where that venturi effect happens, so it builds up pressure, but it must then spray that
unpressure-mixture into one of those outlets directly into the manifold or large tube that enters the piston area... which tube or exit is it ?


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## tadawson (Jan 3, 2018)

The main jet is typically in the center of the venturi (the narrow part), often extending towards the center. For idle, there are a number of other ports (and another needle) that subsequently are uncovered by the throttle plate as it moves, and these are typically in the side of yhr carb throat, typically in line with the throttle plate. The choke simply blocks air intake, allowing more suction to build (and fuel to flow) to start a cold engine.

Oh, as as to fuel pumps, pumped and gravity feed carbs are ptetty much identical in that both use a float to rdgulate fuel level in the bowl. About the only difference is the force getting fuel there - IE gravity or a pump . . .

- Tim


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## jsup (Nov 19, 2017)

FrostyTheBeerMan said:


> Very interesting indeed LouC, and jsup.... nope never used one of these, it almost looks like the paint sprayers I've seen at Home Depot.
> I'm really liking this topic ( thread ) I started.


They all work the same way as a carb, but instead of sucking up gas, they suck up paint, or undercoating, or brake fluid. Same principle. The air pushing through is not done by a compressor, it's pulled through a carb, not pushed, however, the same physics apply.



> Ok so here's another question.... when you look at the carb... where is the final area or tube that supplies the engine that gas/air mixture ?
> I've looked at the carb on my snowblower and without taking it 100% off the engine it's hard to see..... so what tube or exit area supplies the engine ?
> That's the thing I'm not really understanding. The huge center of the carb... that's where that venturi effect happens, so it builds up pressure, but it must then spray that
> unpressure-mixture into one of those outlets directly into the manifold or large tube that enters the piston area... which tube or exit is it ?


As stated above, all the little ports, jets, etc.. result in the final AFR. These are pre determined and designed by people much smarter than me. 

However, just a note on the choke vs the throttle plate and how that works. When you close the choke, the amount of vacuum the piston pulls stays the same, so if the choke is closed, it's going to pull more fuel from the bowl. This happens because the choke is BEFORE the point where the fuel is pulled from the fuel bowl.

The throttle plate governs how much vacuum is allowed to be pulled through the "straw" past the fuel bowl, hence its location past the fuel bowl considering the direction of air flow. By opening the throttle plate, you're allowing more vacuum to be created in the "straw", which pulls more fuel, and more air, which makes the engine go faster.

The choke regulates the AFR by regulating whether the vacuum is drawing in air from outside, or from the fuel bowl. It's inversely proportional. If you have 100% the choke modifies that percentage to be X% air and Y% fuel. It changes those two percentages never to exceed 100% of the vacuum potential. It's going to pull so much based on the throttle plate position, the question is, at what ratio, and that's what the choke does. 

A carb is really an ingenious design if you think about it. I find EFI easier to understand. You had injector on time, off time, and slew rate. Once you know that, it's just math (kinda).


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## jsup (Nov 19, 2017)

FrostyTheBeerMan said:


> e huge center of the carb..


The bowl



> that's where that venturi effect happens, so it builds up pressure, but it must then spray that
> unpressure-mixture into one of those outlets directly into the manifold or large tube that enters the piston area... which tube or exit is it ?


It doesn't build pressure, it drops pressure. The bowl is at one atmosphere, 14 lbs/sq in. The piston creates a vacuum, or absence of pressure, the atmospheric pressure behind the bowl pushes the fuel to where there is no pressure. Just like when you drink a Coke through a straw. Suction is simply removing air pressure. The atmosphere does the work. 

If the bowl is 2 sq inches, that's 28 lbs pushing down on that little area forcing the gas up through the area less pressure, less than zero pressure or a vacuum. Picture a 28 lb weight on top of a two inch pipe. The pipe has little holes in the end, the 28 lb weight will push all the water out through those holes.

Air is treated as a fluid, if you think of it that way, it's easier to visualize.


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## FrostyTheBeerMan (Jan 13, 2017)

Hi everyone, THANK YOU for all the great info... really amazing thread here ! really enjoying it !

I have a strange question now...... I was just in the garage and looked into my emulsion tube... I see something that looks like plastic right in the middle of it....
can that plastic be removed ? and how do you go about doing this ( if you can ) without removing the whole carb from the engine ?

I've watched a bunch of videos in the last few days and I have not seen this inner piece that looks like a plastic tube ( the tube is closed at the end ) it almost looks like a cap 
inside the emulsion tube. Perhaps each carb has a different emulsion tube design and that's the reason I haven't seen it on any of the YouTube videos I have watched.

Thanks for any info.


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## FrostyTheBeerMan (Jan 13, 2017)

After asking my question, I just Googled and found this video:
https://youtu.be/mQJVNfEiNkY?t=1m50s ..... DonyBoy73 removes that plastic inside the emulsion tube ( or is that actually the tube ? )
and you can see that it is open on the end... the end of it that you would see if you looked down the carb tube.....

on my carb ( I was just out in the garage ) that tube is CLOSED on that end.... ( perhaps that's not normal ) ? ... maybe a video is in order......

thanks for any help !


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## SayItAintSnow (Dec 15, 2017)

Frosty,

As Jsup correctly pointed out, it is atmospheric pressure that actually pushes fuel through a carburated fuel system, not a vacuum sucking it through. That's the true scientific explanation. :wink2: Gravity also helps, but gravity alone cannot do the job.

When we casually talk about a carb, we all by habit, (myself included) talk about suction created by the downward movement of the piston "pulling" the fuel in. But what's really happening is the piston moving down in the cylinder is creating a lower pressure environment. When the intake valve opens, it exposes that low pressure to the carb and the fuel tank which are still at normal atmospheric pressure, (which is higher by comparison). So, the higher pressure moves towards the lower pressure trying to achieve an equilibrium.

*I once had this discussion with an old time mechanic, whose response was that I was full of crap, and he insisted that it was all suction.

I convinced him that I was right, when I asked him: What happens when the vented cap on a fuel tank on a piece of power equipment gets plugged? He said correctly, it will run just so long, then the engine will quit. I asked him why. He had no explanation. I pointed out if it was all vacuum dependent it shouldn't matter, since the piston is still creating a vacuum. So why did the engine stop? He still didn't have an answer.*

_The answer is_: That because the air in the top of the tank is trapped, and no more air can get in, as the tank empties of fuel, the same amount of air will now occupy a greater space, dropping its pressure. At a certain point, the pressure becomes so low that there isn't enough of a pressure differential between the air in the tank and the cylinder. Fuel can no longer move through the system because atmospheric pressure can't get in the tank to push on the fuel. Unscrew the cap and let atmospheric pressure back in, and everything works like magic again-- or at least until the pressure drops in the tank again, _*OR*_ you finally figure out that the cap's vent needs to be cleaned..lol.


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## FrostyTheBeerMan (Jan 13, 2017)

oh oh...... so I thought... I'll shoot a video for you guys...... but just before I started the camera, I took my little screwdriver and went into the emulsion tube and wiggle it around,
and pop, the end piece broke off ( this piece I described as closed ) .... here's the video I just took after this happened....


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## jsup (Nov 19, 2017)

FrostyTheBeerMan said:


> oh oh...... so I thought... I'll shoot a video for you guys...... but just before I started the camera, I took my little screwdriver and went into the emulsion tube and wiggle it around,
> and pop, the end piece broke off ( this piece I described as closed ) .... here's the video I just took after this happened....
> 
> https://youtu.be/oQYnZW98jOs


If you're going to dig into a carb, do it on the bench, not on the engine. Just sayin'.


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## skutflut (Oct 16, 2015)

FrostyTheBeerMan said:


> oh oh...... so I thought... I'll shoot a video for you guys...... but just before I started the camera, I took my little screwdriver and went into the emulsion tube and wiggle it around,
> and pop, the end piece broke off ( this piece I described as closed ) .... here's the video I just took after this happened....
> 
> https://youtu.be/oQYnZW98jOs


Worst case scenario, you replace the carb if you cannot get any bits and fragments out. You can only spend so much time fiddling with tiny bits before some other part gets damaged or a thread gets damaged and causes a new problem. 

Plenty of carbs available on Ebay, from OEM units at ridiculous prices, to Chinese clones for 20.00 or so. 

Remember to get replacement gaskets as well, whether you fix the original, (off the machine) or order a replacement. New gasket is a good thing to have handy since if you don't have one handy then the original is absolutely guaranteed to break when you remove the carb.

Also remember to take pictures of the linkages from the governor rod and make sure to mark which holes in the throttle plate linkage the rod and spring are installed in. It has to go back together exactly the way it was.


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## Old Gal (Dec 5, 2019)

My MTD snowblower didn't start this season. The carburetor was leaking some, so I removed it and it was loaded with maple seeds, the flaps were literally stuck open! I replaced it with a new carburetor. It still won't start. What do I have to clean out? How did the seeds get in? Help, snow is on the way.


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## Wazoo (Nov 21, 2019)

Look at the air filter. Sounds like it's somehow become contaminated with seeds. Is the air filter housing rusted thru?


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