Motorcycle and ATV Carburetors 101

This is Steve with Rocky Mountain ATV/MC and this is carburetor 101. Many people are intimidated by

carburetors

, so today I'm going to explain some types of

carburetors

, the basic functions, and how the internal parts work. We'll also cover some of the most common ones. We asked questions about carburetors, whether you're making changes or have a running problem with your engine. First, we need to understand the fundamental basics of a carburetor, the different types of carburetor, and how the internal parts work. The carburetor essentially controls the fuel. and the air that enters an engine by adding more speed to the engine or by removing it and making the engine idle now does so using the venturi principle now the venturi principle is essentially a large diameter tube that is gradually reduced to a smaller tube diameter now as air flows through it, the air will accelerate once it reaches this restriction, there are small passages that will allow fuel to flow into the carburetor bore.

Now, because high velocity air passes through it, it will suck the fuel from the carburetor into the orifice, causing it to atomize giving you a precise mixture of air and fuel so that the internal combustion can function. Now there is an ideal air-fuel mixture that a carburetor needs to maintain and adjust based on load and engine speed, so we'll break it down for you to better understand how it works Now, there are three basic types of carburetors, but there are a thousand variations of Each type of carburetor is used in the power sports industry, so the first carburetor is the fixed venturi type.

Now you will see them as lawnmowers. or generators or electrical equipment, but their ideal purpose is to maintain certain rpm during engine operation. Now the second one is the mechanical slide carburetor. This is a very common carburetor on

motorcycle

s, ATVs, small bikes, and even performance bikes. I have two different variations of a mechanical slide here and that's because one is more four-stroke based and the other is two-stroke. They work very similar but slightly different. Now the third type is a constant velocity carburetor. There are two variations. There are two jets. and a three-speed constant speed carburetor or better known as a CV carburetor.

Now these are the most common carburetors in the powersports industry. They are used on ATVs, UTVs, Dual Sport Bikes, Adventure Bikes, Sport Bikes, Cruisers. I mean, it's the most common carburetor out there now. The reason for this is because it will only deliver the fuel and air mixture to the engine when it needs it and only when the engine needs it. Now the difference between a CV carburetor and a mechanical carburetor is in a mechanics, you are forcing the slide opening that allows air to enter the engine now in a CV the engine is using vacuum to open the slide and allow fuel to enter air in the engine function is basically the same between the three different types of carburetors, so let's talk about the three circuits that each carburetor uses the parts within that circuit and how they work.

Now let's first talk about the low speed circuit, also known as the idle circuit or pilot circuit. Now in this circuit you have a jet, if you don't know. What a jet is is a jet is generally a brass fitting with a small hole inside that will allow fuel to flow through it. Now on the side of the jet there will be a number stamped and this will reference the size of that hole now. if you have a larger number it's usually a larger hole and that would mean it would allow more flow or more fuel to flow and a smaller number would mean a smaller hole that would allow less fuel to flow so essentially when you listen to someone I need read jet my carburetor they are basically changing these jets so they can get the correct amount of fuel that the engine needs to run properly now on the low speed circuit we have a pilot jet and it is susceptible to clogging because it is a small passage so that if you have problems with that circuit, it's a good candidate to clean or replace and it needs to work in conjunction with an air screw or a fuel screw like on this particular carburetor, this would be the fuel screw that Basically, you can adjust it on clockwise in or out and the fine point needle closes or opens a passage allowing more fuel or air, in this case fuel.

Now I like this carburetor, it controls the air, so it allows you more air or less air, turning it clockwise or counterclockwise, this is very useful in a carburetor because you can tune that circuit when you're tuning the engine now with the pilot circuit because of those little passages and the fact that it can get gummed up or clogged very easily if you have running problems with that circuit those are the first two things I would check to clean or replace now to clarify the difference between a fuel screw on an air and air screw. The screw is usually located on the air boot side of the carburetor and on a fuel screw like on this FCR carburetor it would be on the engine side of the carburetor like for example this is where the fuel screw would go now the Another thing is more The fuel screws are accessible, some have a tamper proof plug over them so you can't even see it's there and to adjust that removal you have to drill it.

I highly suggest you follow your owner's manual specifically on that before doing this. The other thing is that they can be very difficult to access as we have created this fang fuel screw that actually works very well as they hang down far enough that you can reach them while they are on the bike. and it's easy to adjust, as you can see here, with this stock fuel screw you have to use a flat head screwdriver and when it's mounted on a bike it can be very difficult to get to. There are a couple of special tools that you can buy that basically come with. at an angle so you can adjust it, but it's something that you definitely need to adjust when you're tuning your carburetor and that's like if you changed, you know, 4000 feet of lift, that's something that you're going to have to adjust, otherwise your butt The bikes They're not going to work as well as they could work now.

The last thing to remember about the pilot circuit is that it will continue to flow as the throttle is applied and transition to the midrange circuit and so on to the main circuit, so if the pilot circuit is not flowing correctly, Your midrange circuit will be off and your main circuit will be off, so they all have to work together and all flow together for your engine to run properly, going into the midrange circuit. Now this is where 1/4 to 3/4 throttle would be applied and where most of the time spent operating the engine is on the mid range circuits, so I have three different examples here just to show you that there is a lot of variation in the working parts, so this is where the slide jet needle and needle jet come into play.

Now in a CV carburetor you are going to use a rubber diaphragm that uses the engine vacuum to suck the jet upwards and allow more air to enter the fuel in a mechanical system. the slide like these two is obviously mechanical, you use a cable or a different type of operation to open that slide and allow air into the engine. Now I have a jet needle and a needle seat here off of this FCR carburetor that I'm going to use. What I'm showing you is an example, so the needles on the jet are tapered and when you put them on the needle seat when it's fully closed, when the throttle is fully closed, it goes out of the pilot loop, so essentially neither It doesn't even allow fuel to rise through it now.

When you transition to the midrange circuit the needle rises and because of that drop, the higher you go the less restriction you have, the more fuel will flow, so once you open the throttle wide, eventually this isn't going to work. . Do anything and you will be running completely off the main jet and that would be from 3/4 throttle 2 to full throttle. Now the other thing about needles that you need to know is that most of them have a clip position or by You can adjust these and this as part of the tuning, especially when you change the elevation, where you will write it that way, so if You will lower the clip all the way, that will raise the needle allowing more fuel to flow and vice versa.

Conversely, if you were to lift the clip all the way to the top, the needle will sit lower on this kneeling seat and allow less fuel to flow. Now, if you want to learn more about that, we have a video about Justin, his injection needle, the last thing I want. What I want to highlight about the midrange circuit is this needle seat and they are also called the nozzle or emulsion tube. If they have small holes in the side you want to make sure you clean them or just go ahead and replace it, it's a pretty important part of the function of the midrange circuit, moving on to your main circuit.

Now this is where you would apply your 3/4 throttle to fully open the throttle or your pin to gain on the circuit. Now the functional part is basically the The main jet is now on the three jet CV carburetors, they will have two main jets, but all the other carburetors will have one. The main jet is now mounted on the needle seat. They now work together so that the main jet will supply the jet needle. with fuel now, but the needle is doing its restriction, so as you apply the throttle, your needle will eventually have no more restriction for your fuel and all the restriction will be done only by the main jet because the main jet supplies the fuel needle. fuel.

If your main jet is clogged, your main and midrange circuit will not function properly, so if you have a machine that runs fine at idle but shuts down when you rev ​​it, chances are the main jet is clogged, that's essentially how it works now. If so a lot of carburetors will have a splash guard which is essentially just a guard that keeps the fuel from spilling out so you can't suck in air or something and give you a better running condition now obviously the main jets are interchangeable, so if you end up rejecting a carburetor, its seal numbers will also be printed on it and you will most likely end up changing your main jet when you reject your carburetor.

Now let's talk about supplementary circuits. Now, the most common in various supplemental circuits is the choke system or enrichment system now, which is to allow fuel to enter the engine when the engines are cold to help it start much easier. Now there are several different ways to do this, like for example on a fixed venturi, you will have a choke plate that basically closes to not allow air to pass through the carburetor and because when you start the engine it will help suck the fuel out of the carburetor and create that condition, which makes it much easier to start now another type than What you will find is a plunger style, you will now see them primarily on flat side mechanical carburetors, which is essentially a lever or plunger that is pulled out by opening a step that allows fuel or fuel and air to enter the engine to create that condition now.

You'll find this particular choke more on two-stroke carburetors as it serves two purposes: one as a choke and the other as an idler, so instead of having an idler screw on the side of the carburetor, this will serve as an idler for you to use. you can turn. do it clockwise or counterclockwise to adjust it so it's nice it works pretty well now another type you'll come across is a plunger style or a primer style that It's essentially before you turn it, you should go ahead and press this a couple of times. moments that would force raw fuel into the engine creating that rich condition to help start easier now one of the last common ones you will come across is a servo style that essentially opens a passage to allow fuel to pass through electronically, so once the engine starts and warms up, it will automatically close that circuit, creating the correct air-fuel ratio that the engine needs to run, so the next supplemental circuit that I want to talk to you about and that is mainly just seen on high performance four stroke carburetors it's hot start now because high performance engines run hotter, the fuel in the carburetor will atomize sooner once the engines warm up, so if you're sitting typing and doing it you turn off, it will actually createa rich condition, so to correct this some of the manufacturers We have created a hot start which is essentially a plunger that can be operated by a lever like this or a cable operated one that is on the handlebars, but essentially opens a passage that allow the air to just correct that rich condition so it's easier to start now.

The last circuit I want to talk about is the accelerator pump. You'll now see them more commonly on your four-stroke carburetors. You may see two-stroke Monsuns, but for the most part your mechanical four-stroke carburetors are light now because you can mechanically force the slide open and introduce air to the engine immediately to correct that it needs fuel immediately, so it They made it using a diaphragm and a small passage through the carburetor for the fuel, so when you apply the throttle, it will push. in the diaphragm, forcing raw fuel straight into the engine, now you could run into this and a swamp, you step on the gas and the engine just won't start, it falls flat on its face most of the time, this is why You're done done and needs to be corrected, so there are several different ways to do it.

Many times there is a jet in that circuit that you can change. There is also a nozzle in the circuit that sprays fuel directly downwards. The carburetor hole, this nozzle also has a small passage that can become clogged. Also, there are some aftermarket companies that made this very simple and easy to do and it basically involves replacing a part of your carburetor that has several different fixtures. holes or a screw that you can adjust like on these guys and the ones that are Indies come out with a version that works similarly, which is a quick adjust screw and that is very convenient and nice so you don't have to take the float bowl or the carburetor part to make those adjustments, you can do it on the fly, so that's something you'll be able to do.

I definitely want to look into whether you have any kind of jamming issue. The last thing I want to point out with the choke circuit is that if you have a plunger style that works with the cable, what happens is that water gets in through corrosion. there and the plunger gets stuck in the hallway. Now, if this happens, you'll definitely want to take it out and replace it, but a quick tip on how to avoid this is if you take it out and put some grease inside. that will help keep it running smoothly and obviously if you lose cables frequently that will help fix that problem and prevent it altogether so keep that in mind.

Now let's talk about the float valve system. Now the float valve system is essentially a power on. /shutoff valve to control the fuel entering your carburetor. The way it works is that fuel is filling into the float container. The float will float in it and, in turn, close the fuel supply with a valve so that the working parts in it are a float. a real float valve with pivot pin and float seat now come in different sizes and shapes some have filter screens some have small retaining clips some have a rubber tip some have metal tip some of them press into the carburetor body some of them bolt on but now they all serve the same function if the floats and you know hanging them will allow fuel to come in and as it fills it will close the needle on the seat and close the fuel now. this flow needs to maintain a certain level and that is very crucial as it is its most important job so here in this electronic carburetor I have to illustrate it so you can see where this level needs to maintain a certain level now if your float is out of adjustment if it is too high and the fuel level is too high, that will cause a rich condition in all circuits and then vice versa, if the float level is too low it will create a lean condition in all circuits, so that is something you definitely want check every time you get into a carburetor, now to adjust that level, you can measure it.

There are a couple of tools like this where you can basically stick it to the float and move this little arm and that will give you you measure others, you can use a tube from the bottom of the carburetor and you have some lines that you can refer to and you fill it with fuel and measures it that way. Now how to adjust it is most of them, most floats have a little bit of metal in them. Hook it where the float valve attaches or rests, now you can bend it up or down and that's how you adjust it, so it may take you a few times to get it accurate, but it's pretty easy to do now with some of they.

They just aren't adjustable like this plastic one. If you have a fault here, you just need to replace it now. The reason I stress so much about the float system is that it's usually one of the first things to fail in Italy, they can't do it. turn off the fuel and it will leak out of the carburetor overflow or even leak directly into the engine and fill the crankcase or fuel either way, it's not good and that's the cause, right now there's this float valve. One of the biggest things that people miss is that they will replace just the float valve, but now they won't do the seat on some carburetors, you can't replace it, like on this one, it's pressed in there, you just can't. you just have to replace the whole thing which sucks but its always better if you can like on this one which has an o ring and that order will be messed up big time if a guy replaces just this will only seal the inside and not the outside so i They'll put everything together and still leak and be stumped, so any time you can replace the entire assembly is always the best way to do it.

The last thing I want to talk about is the flow pivot pin. Now most of them are free floating like this one where it just floats. You can remove it either way to remove the float from the actual mold or carburetor body. There are quite a few out there that have a slightly press fit, where you put it on and you have to press it and it stays there stationary and then the float just spins on that, there are a number of other items that you can potentially crash into carburetors like this sensor TPS. There are many carburetors that have devices that simply tell the bike or machine's ECU that you are in the position that the throttle actually is.

I always pay attention to the hoses and any fittings you push in there fit inside the carburetor mold, you could run into air shut off valves, you always have to make sure the idle adjustment screw runs freely and if there's basically something on a carburetor, make it rubber if it has a tear. There is a net kink in it, chances are it is giving you problems or is already giving you problems, so go ahead and replace them now. We are going to cover some frequently asked questions about carburetors. My first question would be: should I properly reject my machine?

There are a number of factors you need to take into account when doing this such as altitude, humidity, temperature, driving conditions and if you have recently modified your machine, now the jet does not change during the night, so if you have a sudden change in the way your bike runs, there's probably something clogged in your carburetor or you may have another problem with your machine, so don't go ahead and start changing jets thinking that'll fix the problem you're having. having. We recommend giving your carburetor a thorough check and making sure all passages are clean and working properly now.

If you need help with that, we have a really good video on how to clean your carburetor. We offer a variety of different carburetor rebuild kits so that when you check your carburetor to make sure it is clean and working properly, that will definitely help you. Now you need to turn off your carburetor. We offer a number of aftermarket kits for that, as well as detailed instructions on how to read. Turn on your carburetor now, the last thing you can do is look at our OEM parts finder. Now you can go in there and look at a diagram specific to your carburetor on your machine and you'll be able to see all the individual parts so if you just need to do that. replace a part, you can do it now.

My next question is how do you know if your machine is rich or lean? A quick way to tell is to take out the spark plug and see if it is black, darker. it would be a rich condition if it is very dry white that would be a lean condition now other ways you can tell is if it surges if it stalls or has a hard time idling which is usually a rich condition on the other hand if you get A suspended idle means that it wants to idle much higher than it's supposed to or it becomes counterproductive, those are indicators that it's running poorly, but ultimately what you'll want to do is look in the OEM service manual.

See the carburetor section for specific specifications. Now that will give you an indicator of what jet is in your carburetor or you can just pull it apart and look and see exactly where it is now. You can also make incremental changes to see if your bike starts to perform better. Now keep in mind that any time you go lean, you have the potential to block or seize an engine, especially if it's a two-stroke, so be aware of that. My next question is if your machine won't boot or is having difficulty. As of now the most common problem is on the low speed circuit your pilot jet or your fuel screw or air screw is dirty or clogged so I would go there first and make sure they are clean, clear and running correctly, hopefully that will be resolved. your problem now my last question is if your carburetor has a fuel leak.

Now there are really two problems you need to consider. Obviously, there are gaskets that seal the carburetor. Then you can check them first. The other thing you should check is the float valve. The most common problem with carburetors is a failing float valve. Now I would first check to make sure it was clean and clear and working properly. I mean, a lot of times the debris would just hang it up and it wouldn't close the valve. off completely, so if you do that you can seal it first, but if it doesn't seal it, you'll probably want to go ahead and put the stream file in.

Now, hopefully, you have a better understanding of carburetors, basic functions and parts. inside, visit us at Rocky Mountain atv/mc comm for all your carburetor needs. Now remember that all orders over $75 ship for free and if you have any questions please comment below but you should make sure to subscribe to our YouTube channel. It's Steve, we'll see you next time.