How does a Carburetor Work?

how's it going eliminators today I'm gonna be showing you how a carburetor works so let's get right into it so over the years I've done countless numbers of videos on how to clean a carburetor but I've never really explained how a carburetor works so today we're gonna be doing that so I have in front of me a basic Honda carburetor for Honda engines a lot of koehlers use this type of carburetor but this one's perfect because it has a pilot jet on it and it also has an air fuel mixture screw which I'm going to get into that shortly but this is your average carburetor for a lawnmower basically all carburetors are essentially the same the job of a carburetor is to mix air with fuel into an air fuel mixture and then put that into the engine so on this side here you have your choke lever so when you want to start your engine this little lever right there closes so as the piston comes down on the intake stroke with this choke plate closed the carburetor has a little bit more suction into it and what that extra little bit of suction does is it just helps pull a little bit more fuel through the main jet or the distribution tube which is that little brass piece in the middle there so the choke is only used for starting your engine and it will run rough if you leave the choke on while your engine is running because again it's creating more suction you're getting less air and more fuel into the mixture which is known as a rich mixture we'll get into mixtures in a little bit but essentially it just creates too much fuel and not enough air inside of your carburetor which will bog down your engine moving over to our throttle plate here or our throttle butterfly valve this will limit the amount of air going through your engine so more air higher rpm less air lower rpm and this up here is normally hooked up to a lever or a cable that goes back to a governor in the case of a lawnmower with a governor linkage now if this is on a go-kart or a minibike this will just be hooked up to a cable and then when you get on the throttle it opens up the butterfly valve and that's full throttle also known as wide open throttle that's about 3/4 throttle and then that's closed where it normally idles now you're going to be wondering if that throttle plate is closed there's no air going through here how does your engine still run if that is in fact on the idle now this is where we come to what's known as the pilot jet but it's also called the idle jet and that's right in here so I'm gonna pull that out and show you guys what that is now to get that pilot jet out we're gonna have to remove the screw there and that's also known as your throttle back stop what that does is it sets where your throttle is going to be set at the closed position so by moving that farther in your throttle plate won't close as far and your engine will idle at a higher rpm so that is known as an idle screw or a throttle backstop screw so I've now removed the pilot jet and you guys can see that there's a couple little o-ring zhh on it and there is a hole going through this side here and there is a little brass jet and you guys can see that there is a hole in there and like I said this is your pilot jet but it's also known as your idle jet so this is what regulates the amount of fuel going into your engine when your machine is idling and not under load now pilot jets on different machines are going to be different on a lot of these Kohler and Honda carburetors they look like this I've done a video on how to bore one of these out on I believe it was a lct engine on a snow blower the engine is surging it's just not getting enough fuel and it's just getting a little bit too much air and the thing to do is just to bore that hole out so that your idle circuit as it's called gets a little bit more fuel and it will smooth out that rpm so you can check out that video in the top right of your screen if you'd like and I also did a video on how to fix surging on a Nicki carburetor the pilot jet on that carburetor was not removable however it works just the same you just find the idle jet and you're able to bore it out slightly oversize which just allows a little bit more fuel through and fixes the surging moving on to the bowl of the carburetor and this is what holds the fuel so that you can always have fuel for what's known as on-demand throttle user on-demand load so whenever you get on the throttle this has a constant supply of fuel in here and I'm going to remove this bowl by removing this bolt right here that's your drain bolt so in the fall time if you ever want to drain out the bowl of your carburetor all you do is shut off your inline fuel shutoff valve and then you can go ahead and remove that bolt and drain your ball so that there's not a bunch of fuel sitting in there because it can get gummed up so I'm going to go ahead and remove this and then I'll bring you back and show you the float and the needle valve okay so we now have the ball off of the carburetor it exposes the float and the needle valve which I'll get into in a moment this here is your Bowl gasket and that just helps make a seal so that you don't leak any fuel out of the ball and normally on these bowls there's going to be a sediment reservoir which is just essentially a dip in the bottom of the ball and that will catch all of the sediment that goes into the bottom there because gravity is going to pull that down and if you're not running a in-line fuel filter it'll go into your carburetor and hopefully it won't get sucked up into your main jet and we'll talk about the main jet in a moment but first I'm going to be talking about the float and the needle valve so here we have what's known as a float and they call it a float because it floats on the fuel so we have to think about this as the carburetor would normally be bolted which is in this position here and you guys can see that the float is not floating and that's because there's no fuel or liquid inside of the bowl of the carburetor so just imagine that there's a clear bowl on the bottom of this carburetor and we could see through this so what happens is when you turn your fuel valve on or you pour gasoline into your fuel tank and you don't have an inline fuel shutoff valve fuel is going to go into this inlet here and that Inlet goes all the way back down that little Valley right there into your fuel Inlet right there and that is known as your needle valve seat that brass piece right there and once I get the float in the needle valve out you'll have a little better look of that but we're gonna imagine that this carburetor is bolted on to a machine so you have a constant supply of fuel going into your Inlet as fuel fills the bowl it's going to close the float because the plastic will float on the liquid what that does is it pushes the needle valve into your seat see that right there now this one has a spring and normally that is just to absorb a little bit of vibration so if you have an engine that vibrates a lot or let's say this was on a minibike what will happen is when your bowl is filled with fuel every time you hit a bump the float will move a little bit but the needle valve will still be seated you see that so only the spring is moving right now but the needle valve is going to remain closed and what that does is essentially it just keeps the fuel from going into your distribution tube in your main jet and it will prevent your engine from flooding while you're hitting bumps so basically if it didn't have that spring every time you hit a bump on let's say a minibike that float would open and it would put more fuel into your carburetor and your carburetor may not need that fuel and that's called flooding out your engine because it would be getting too much fuel or that's just simply running rich so that's how a float and a needle valve works and rate in the center of this post here that is what's known as your main jet so that meters the amount of fuel going into what's known as your distribution tube now in this particular carburetor the design has a bolt holding the bowl on and that goes into this long tube here now you're going to notice these holes on the sides of that tube what that does is it allows fuel from the bowl to go into the center tube and then that fuel supplies your main jet on other types of carburetors for the old Tecumseh's what you'll notice is they don't have these holes what they do is they have a hole in the actual brass bolt and that takes the fuel from the ball through the brass bolt and then that lets fuel into this little tube here so I have one of my two cups of snow blower carburetors here and that's essentially what it is there's just a bolt there and I'm gonna remove that and show you what it looks like so on this particular carburetor design your fuel would go from your Bowl in through the big hole on the top there and they have holes on both side and then it would let the fuel through that little hole there and then from there it would go back down into your distribution tube see that now this carburetor that I have in front of you this is also for a snowblower and the previous carburetor just had a bolt going in to hold the bowl in that's what's known as a fixed main jet so there's no adjustments that you can make to that carburetor whereas this is known as an adjustable main so this brass bolt or screw here also has the holes in it but on this one it has a needle that goes in so by putting this farther in you're limiting the fuel that's going in to the main jet and by unthreading this you're increasing the amount of fuel that's going into the main jet so these carburetors are a lot nicer especially the ones that have an air fuel mixture screw I'll get to that in a moment but what you do to make an adjustment on this carburetor is you adjust your main jet in until it starts sputtering you adjust it out until it starts sputtering and then you find the center point and then you go ahead and adjust your air fuel mixture screw which again I'll get to in a second now like I said adjustable carburetors are the best simply because if your engine starts running rough you can go ahead and use a small slotted screwdriver and it's quite simple pretty much everyone has one of them and you're able to make a quick adjustment on your machine and keep it running so that you don't have to go and take it into a shop for an expensive repair bill most of the times you'll just be able to make a small little adjustment what you're gonna find nowadays though is that they set these from the factory and then they break them off or they put some type of epoxy over top of them so that you cannot make adjustments and that's just simply because of government regulations so the EPA the Environmental Protection Agency they don't want you adjusting your fuel mixture because you can pollute the environment more so they come preset from the factory and if one of the little holes inside of this carburetor gets a little bit of dirt in it you can't go ahead and back that screw up to give it a little bit more fuel you have to take the carburetor off and clean it that's the only way to make them run right but now I'm going to remove what's known as the float garage so that's just a on the two posts here that hold your float into position sometimes they can get stuck and I have a video that I did showing you guys how to remove one of those if they are seized in position if you want to see that video you can click the link in the top right of your screen but by removing that float rod it allows us to pull up the float and the needle valve so the job of the needle valve is to make a seal in between your seat which is right there and the needle valve itself so again as the fuel goes into your carburetor it pushes up on that needle valve and just simply makes a seal you're going to have two different kinds of needle valves you're gonna have a rubber tipped needle valve like this one with a metal seat or you're going to have a rubber seat with a metal needle valve now I like the brass or metal seat with the rubber needle valve because even though they're a little bit more expensive they generally seal up a little better now it might not look like it's rubber because of the reflection but I can promise you it is a rubber tipped needle valve and again I explained the purpose of that spring there just to absorb a little bit of shock here is what the seat looks like so that hole has been drilled to a specific size and that will let a certain amount of fuel into the ball so getting a little bit more complex on higher performance vehicles like ATVs and dirt bikes you'll be able to customize your seats so a larger hole in your seat will give you a richer supply of fuel into the ball which means your bowl will feel faster and a leaner seat will have a smaller hole which will fill your bowl slower now the idea behind that is let's say you're on the throttle a lot and you're using the fuel in the bowl very quickly what's going to happen is because the fuel supply goes through here if your bull can't fill as fast as the supply of fuel that's getting taken out of the ball then what you do is you just go ahead and run a richer seat and that normally solves that issue now you can also get into setting the float because on this type here there is no adjustment on it it's plastic if you try to bend it then it will break but on a lot of machines you're going to have a brass float and if we come down here you'll notice that the float is level so you always want to level your float guys with the carburetor right there even though the float rod isn't in it might not be sitting perfectly level but generally speaking you want to level your float so that when the fuel comes into your carburetor it closes the float at the correct time if you run your float higher it will close sooner and if you run your float lower it will close later and that is a little bit more complex and on these types of carburetors you really don't have to deal with that too much all you guys have to remember is you want to keep your float as level as possible now after I've reinstalled a float and a needle valve onto a carburetor before I go to put the carburetor back onto the machine I do what's known as a pressure test so pressure testing the needle valve essentially that just gives me an idea that the carburetor will not leak fuel because of a leaking needle valve before I go ahead and reassemble everything so the tool here is simply an air pump and you guys can see there's a gauge on top of it it goes up to about 30 psi normally I test carburetors up to about 5 psi and you simply pump the plunger there and it supplies air through the line now on this carburetor the fuel Inlet is a little bit smaller than the tube here but essentially what you're doing is you're just pressurizing that needle valve and if you hear air coming out when the float is in the down position then you'll know that your needle valve is leaking so if you lift your float and your needle valve is up you should hear air coming out and if you drop your float and your needle valve is closed you should not hear air coming up and this gauge here should read up to about 5 psi and it will hold there if your needle valve leaks you'll see the gauge slowly dropping down if you want to see that video I'll link it in the top right of your screen now so you guys can check that out so if you ever pulled the dipstick on your machine and you notice that you have what looks to be like oil way up on your dipstick and it looks like your engine is overfilled with oil what you want to do is go ahead and smell the dipstick if the oil smells like fuel then chances are you have a leaky needle valve on your carburetor so I'm just out on my patio and I have a rototiller here that has a carburetor that is located below the intake manifold so this is where the air comes in through your air filter and it's introduced into the carburetor from the bottom on this type of design and the fuel comes in through here and your float is in here now what happened on this one is it has a leaky needle valve so I'm going to have to go ahead and fix that but when this carburetor floods because the carburetor is located below the intake manifold our engine is not going to flood with fuel on other machines you're going to have the carburetor here and it's going to have an intake manifold that runs horizontally into your intake port and what's going to happen is the fuel will go from that carburetor down the intake manifold and then into your engine and I can show you guys what that looks like oh well that's no good that's no good at all that's gas in the bottom end of the engine folks what happens when you have a weak needle valve on your carburetor and you don't have a fuel shutoff valve it's still going so on this particular craftsman it just has a 12 and a half horsepower Briggs & Stratton the fuel line normally comes out of the tank and then comes through this hole in the frame here and then comes straight down to your carburetor without any fuel shutoff valves or inline fuel filters what I do on these particular machines because there's plenty of room is you come up under here you put a fuel line you run your inline fuel filter right there you come under your throttle choke cable and then you run it your shutoff valve right there so this carburetor has been rebuilt the needle valve has been replaced and I pressure tested it before I change this line so I know that the needle valve holds fuel and then it won't leak into the bottom end so what my customer can do is that's to run your machine and then in the fall time he can shut this off and he can run his carburetor dry now what that does is it prevents the fuel from having a constant pressure on the float which then puts a constant pressure on the needle valve so a constant pressure on let's say a rubber tip needle valve will actually cause the rubber to deform and you'll get a little ring around the rubber tip and what will happen is it will leak over time now if you compare that to a metal needle valve this won't deform however the rubber seat in which the metal needle valve seats on to right those little rubber things there that will actually deform as well because you're always pressing that steel needle valve into the rubber seats so it will end up getting enlarged and then that will leak as well so just running an inline fuel shutoff valve makes things so much easier you just shut your fuel off you can run your carburetor dry they're still going to be a little bit of fuel in the bottom of the ball but if you want to go a step further once your engine dies out you can crack the bull bolt they're loose and then drain the rest of the fuel into a little jar at the end of the season put it away for winter and your engine would be good to go and removing the spark plug and turning your engine over to get the rest of the fuel out of the cylinder is also a good idea should do that in a well-ventilated area now on an engine with one of these this is known as an inline fuel pump so the fuel comes in on one side and is going out to the carburetor on the other side it is run by crankcase pressure so as your piston comes down in the cylinder it creates a positive pressure in your crankcase which moves a diaphragm in here one way and then when your piston goes up in the cylinder it creates a negative pressure in your crankcase which pulls a little diaphragm the other way and with the continuous running of your engine there is always a pulse inside of this fuel pump and that's what will pump the fuel through your fuel line so on a machine that has a fuel line going from the fuel tank directly to the carburetor that's what's known as gravity-fed whereas if the engine has one of these this is a fuel pump then it's a fuel pump operated machine so with that being said if you have an engine with a fuel pump and you notice there's a bunch of gasoline in your oil chances are you just have a pin hole in your diaphragm so every time the piston goes up in your cylinder and creates a negative pressure in your crankcase what's going to happen is a little bit of fuel is going to go through that pinhole and it's going to go through the tube that goes from your fuel pump back in your crankcase and that's how the fuel is introduced into your oil on a machine with one of these guys but moving on to the main jet you can see there on the brass main jet there is a slot and you can simply use a slotted screwdriver so this is just your basic average slotted screwdriver that I've taken to the bench grinder to grind down on the width and also just a little bit taken off of the profile and the thickness of the screwdriver itself and what that does is it just allows me to go inside of that tube and remove the main jet here so I'll get that out and I'll show you what that looks like so here we have our main jet and you guys can see that there is a hole that goes through there so the number one cause of engines that don't start is simply a clogged main jet so what happens is little bit of debris and sediment gets in there a little bit of old gasoline gums up so that's normally what limits your amount of fuel so you're gonna go and pull the pull cord on your lawnmower and it's not going to start and it's simply because it's not getting enough fuel but getting back to the main jet itself if you want a Richert mixture so more fuel and less air what you're gonna do is you're gonna run a bigger main jet so it's gonna be a larger diameter hole if you want a leaner fuel mixture more air and less fuel you're gonna run a smaller diameter hole now underneath your main jet is going to be what's known as your distribution - now I've oiled this up just so that it came out a little easier sometimes they can just get a little seized in there you're gonna notice that it's simply a tube with a long hole in it and it has a bunch of smaller holes all the way down the tube here so this is known as your distribution tube but also your emulsion tube so as the fuel comes from your bowl through your main jet it's going to bypass this emulsion tube and all of these tiny little holes are going to turn the liquid fuel into more of a mist because we have to remember that in this part of the carburetor here the bottom of your carburetor there's only going to be fuel the fuel isn't introduced until it gets into this chamber here which runs horizontally this way so that emulsion tube basically takes the liquid fuel and turns it into a mist so by the time it's introduced to the air but then it can mix easily and it makes burning more efficiently because if you just sprayed some liquid fuel into your engine it's not going to burn as efficiently whereas if you spray in a mist it's going to burn more efficiently finally we move on to our air fuel mixture screw so on a lot of these carburetors you're going to notice that there's all kinds of holes we can see at the front here little brass jets and those are fixed jets so they've been drilled to a specific size and the job of those is to let air past so again going back to how does your machine idle if the throttle plate here is closed well these little holes here let air past so these holes are what supply your engine with air when your machine is idling and your pilot jet here is what supplies it with the fuel when it's idling as soon as you open up your throttle and you put your engine under a load it starts pulling fuel through the main jet the big thing here about this fuel mixture screw is you guys can see that the hole is right here and it runs along a valley but the air fuel mixture screw here is the same type of principle as the other main jet adjuster needle that I showed you on the carburetor in that case though the needle adjusted the amount of fuel going into the main jet the air fuel mixture screw adjusts the amount of air going into your engine so if we think back to our mixtures more air and less fuel means a lean mixture but also less fuel and the same amount of air can mean a lean mixture whereas more fuel and less air can be a rich mixture but less air and the same amount of fuel can also be a rich mixture so essentially if your carburetor has an air tube going through there the farther in this screw is the richer it's going to be because you're restricting air and the farther out this screw is the leaner your machine is going to run because you're introducing more air into your air fuel mixture but that's pretty much it for explaining how a carburetor works it can seem a little complex especially on v-twin engines and it has what's known as a dual barrel carburetor and taking one of these things that part guys is pretty complex you can see just how many parts there is on my workbench there whereas these carburetors here it's a little simpler because there's a little bit less parts but essentially every carburetor works the same like I said the job of a carburetor is to simply mix a metered amount of fuel with air and then supply that to your engine so that's it for today's video guys I hope you enjoyed learning about how a carburetor works if you did think about leaving me a thumbs up you know it really helps me out you can click here to subscribe and click over here to watch one of my previous videos I upload every single week so be sure to stop on by next week check the channel up for new content and as always guys thanks for watching [Music] [Applause]

Channel: Eliminator Performance

Views: 308,390

Rating: 4.9307246 out of 5

Keywords: How does a carburetor work?, Carburetor, How to clean a carburetor, Carb clean, Understanding a carburetor, Riding lawn mower, Lawn mower, Eliminator Performance

Id: 3ri2_bKiuMo

Length: 23min 59sec (1439 seconds)

Published: Sun Jun 16 2019