Tag Archives: Carburetors

Inside A Motorcycle Carburetor – Needle-Height Tuning Adjustment

Do you know what part in your carburetor controls the majority of the fueling? What does raising or lowering the needle mean? Today on MC Garage we talk about the carburetor needle.

This is it guys—the last installment of our how to tune a carburetor series. We’ve got four other videos that preface this video; if you haven’t watched them yet, make sure you jump back and check those out. Each builds on the knowledge of the one before. We’ve covered how to set your float height, how to tune the idle circuit, and how to jet for rich and lean on the main jet. Now we get into the piece that transitions the fueling from the pilot to the main jet: the needle jet and needle.

The needle jet—or nozzle as it’s sometimes called—is located in between the main jet and the carburetors venturi. Fuel comes through the main jet and into the needle jet. So the main jet does affect the needle, especially as the throttle opening increases.

The needle is attached to the slide and slides up and down within the needle jet. The needle is tapered, like a needle, and has a smaller diameter at the bottom than at the top. As the needle’s position changes within the jet up or down, the opening increases or decreases because of the diameter of the taper changes within the fixed diameter of the nozzle. This is how the fuel is metered.

So if the slide is all the way up at wide open, the smallest diameter is inside the needle jet making for the largest opening. Vice versa when the slide is all of the way down. Some needles have adjustable height positions while sitting in the slide. An E-clip sets the position, and that position is often referred to as “the clip.” Moving the clip up will lower the needle, making it leaner. When there are no clip positions on the needle, you can move the needle up by placing small washers (a move called shimming) in between the needle and slide. To lower the needle? You will need a longer needle.

When someone says drop or lower the needle, that means a leaner setting. Raise the needle for a richer mixture. This is fine-tuning your carburetion in the areas you ride the most often. Honestly, how often are you wide open? Just a fraction of the time, but how often are you at 70 percent throttle? Often.

As always, your manual will have the recommended clip position, but a good rule of thumb is to start in the middle position. Then just as you did with the main jet, feel the response. If it’s sluggish and sputtery, it’s rich. A bog means lean. Also hold the throttle at the midpoint; if the revs climb or run away and will not settle into an rpm, it’s lean. When the rpms dip or drop, it’s rich. Adjust the clip position accordingly.

For even further fine-tuning, try optional needles and nozzles with different tapers and diameters. These will help to get that last little bit of perfection into your jetting and tuning, but in reality you should be able to get 95 percent there with the needle you have. That last 5 percent is usually for the mechanics who are tuning for expert racers.

That’s the gist of the needle. Get the idle and wide-open-throttle jetting set first, then pay attention to the mid. Take it step by step; think about the effects each change will make before you do it. Test and repeat. In a short time you will have a crisp, properly jetted motorcycle.

Source: MotorCyclistOnline.com

Inside A Motorcycle Carburetor – Main Jet Tuning

“I’m on the main jet!” Have you ever heard that expression while bench racing with your riding buddies? What the heck does that even mean? Well, after this episode of MC Garage, you’ll know. Today we talk about main jet.

For the past few weeks we’ve been going through a carburetor piece by piece. If you haven’t watched the first videos in this series, jump back, and check them out. We will have the links in the description below. We’ve covered the basic layout of the carb, the float bowl, and the pilot jet. The next piece in the properly running carbureted-motorcycle equation is the main jet.

The main jet is responsible for supplying the fuel that mixes with the air as it makes its way through the intake tract after the pilot jet is done doing its job. Starting at about 20 percent throttle the fuel flows through the main jet. (There is another part that meters that fuel until 80 percent throttle, but we will cover that next week.) When you are wide open, you’re on the main jet.

The main jet is located at the bottom of the carburetor inside the float bowl. It’s the larger of the two jets, and it is installed in the needle jet—that part we will be covering next week. If this guy is plugged up, you’re in for a full carb cleaning ritual. Everything will usually be clogged if the main jet is plugged up. Check out Ari’s video on how to clean a carb.

So we are going to assume you have a clean carburetor so we can get into knowing if your main jet is too rich or lean. First, start with the manufacturer’s recommendation in the service manual. Just like we discussed in last week’s video, there will be recommendations according to temp and altitude. This is where you want to begin.

A “lean” condition means the ratio of air to fuel is too high. This can cause issues from not making full power at wide-open throttle and rpm to detonation and pinging that could damage your engine over time. Lean jetting is the kiss of death for two-strokes with a possibility of a quick meltdown and seizure. Damage can take longer in a four-stroke.

It’s fairly easy to hear pinging on a two-stroke. Think of a metallic tick that sounds like a rattling ball bearing in a tin can as the fuel charge detonates before it should, smashing into the top of the piston too early before it reaches top dead center (TDC). The same sound can be heard from a four-stroke, but the sound is masked by the sound of the valve train. If you hear this, you need to rejet before you do serious damage.

Also, a lean condition will have a crisp throttle response, but the top-end will feel weak and the bike will run hotter than it should. Many times I’ve heard two-stroke racers say it was running so crisp and snappy and then it just stopped. Classic case of a seizure from being too lean. To fix a lean position, go up a jet size and run the bike again; repeat until you find the right size. These numbers may not increase or decrease in increments of one; usually it’s by five or 10.

“Rich” means there is more than the optimal amount of fuel in the air charge. This leads to stumbling and poor overall running. It’s doesn’t lead to a premature death like a lean condition can, so if you must, err on the side of being rich. As the bike moves through the midrange and onto the top-end, the power will stumble and miss. It’s an easier condition to feel and hear for most people. It’s clear it is running bad. Just drop a jet size and retest.

On a two-stroke you can also do a plug chop to check for proper jetting, but this will be a separate video in the future. For now, that’s it for checking the main jet. It’s one of the simpler jets to get dialed in, but if it’s not right, it can cause issues with next week’s topic—the needle jet and needle.

Source: MotorCyclistOnline.com

Inside A Motorcycle Carburetor – Idle Circuit

Does your motorcycle idle funky or not at all? There can be several causes for this, but the most common and first thing you should troubleshoot or tune is your carburetor. Today on MC Garage we are going to talk about the idle circuit of a carburetor.

With modern fuel injection a stable idle, no matter the conditions, is a given, but if you have a carburetor in your ride, things could be less consistent. Environmental factors such as altitude and temperature can require you to make some adjustments to your carb.

And of course the darn thing could just be full of crud from sitting too long or from not being serviced for a long time. Ari did a killer video on how to clean a carb a few years ago, we’ve included the link to learn all the ins and outs of cleaning below. For this video we are continuing on proper adjustments of all the systems of the carburetor.

The pilot jet along with the mixture screw is responsible for how your bike idles. It also provides the fuel for about the first 15 to 20 percent throttle. The pilot jet is the smaller of the two jets under the main body of the carb in which fuel flows to mix and atomize with the incoming air on its way to the cylinder.

In your service manual the manufacturer will list the standard size pilot for your bike as well as give alternatives for ranges of altitudes and temperatures. However you can’t just change the pilot and be ready to rip. You might be close, but there are a few steps for tuning. This is where you are going to need to listen to your bike.

First your bike should be warmed up. When the bike is cold you are going to need to use the choke or enrichment circuit. A cold engine needs a richer (more lower air to fuel ratio) to run. Most flat slides will use a seperate enrichment circuit to supply additional fuel. It’s basically a plunger that opens and closes the circuit. Other carbs use a secondary butterfly to limit the airflow while the pilot provides the same metering of fuel, thereby creating that richer mixture. Once the engine is up to temp, turn off the choke.

We also need to be sure your air or fuel mixture screw is in the middle of the range to start this process. If the mixture screw is on the airbox side it’s an air screw. This one meters air into the pilot circuit. Turning it clockwise closes the opening, richening the mixture. Counterclockwise adds air and leans the mixture. Check your service manual, but a proper baseline setting should be around 2.5 turns out (clockwise) from fully closed or seated.

If the mixture screw is on the engine side, it meters fuel instead of air. Clockwise is less fuel and a leaner mixture, whereas counterclockwise is more fuel for a richer mixture. Just like the air screw, check your manual for the standard position or starting point, but a good rule of thumb is this should be around 1.5 turns out. So once you have those in a correct starting position we get down to tuning or jetting.

Let’s discuss what the idle behavior will be like when a warmed-up bike is properly jetted, too rich, and too lean. A proper idle should be consistent without any input from the throttle. It should also settle quickly after revving the engine.

If it’s too lean, you’ll find a hesitation or bog right off idle when you crack the throttle. Also, when revving the engine, the rpm will hang at a higher level than normal idle or will not settle into a consistent idle quickly.

A too-rich pilot setting will give you a sputter when the throttle is cracked. Response will just feel a bit heavy or sluggish. When the engine is revved, the rpm will drop quickly and dip below ideal idle before recovering; sometimes it will just die.

Depending on that behavior you should have a pretty good idea if you need more or less fuel. Is it rich or lean? From there begin with the mixture screws. Lean or richen the pilot circuit one-quarter turn at a time. Rev the engine, listen, feel, and repeat. If you get a nice, stable idle, you’re good to go. If you can’t get that nice idle without going to the extreme edges of the mixture screw adjustments, say one turn to a turn and a half from the starting point, you are going to have to go up or down a pilot jet size. When you change that jet make sure you put the mixture screws back to the standard 1.5 or 2.5 turns out before beginning the process again.

This should give you a great base to start with on your idle or pilot circuit. Some modern four-stroke carbs have an additional accelerator pump and jet. That’s an additional wrinkle that we will tackle in a later video. But just be sure the jet and pump circuit is clean before trying to set your idle and cracking that throttle. Later we will also jump into the art of syncing a bank of carbs on a multi-cylinder engine.

But that’s the basics and will get most of you with a single carburetor well on your way to fine-tuning your motorcycle idle circuit. In the next video we will move on to the needle jet and needle.

Source: MotorCyclistOnline.com

Inside A Motorcycle Carburetor – Float Bowl Height

There are a number of things that must be adjusted properly on your carburetor in order for your motorcycle to run properly. And each of those things works in conjunction with each other, but in order for a proper air and fuel mixture, you must have fuel. Today on MC Garage we talk about the carburetor float.

Last week we ran through all the basics of a carburetor, touching quickly on what everything does. If you haven’t watched that video yet, stop right here and jump back to that vid. It will help with the entire picture of what is where.

The carburetor in the simplest of terms has just one function: Mixing air and fuel in the proper ratio. And to do that you need fuel. Fuel is delivered to all the carburetor’s circuits via the float bowl. It’s a pretty simple system, but if it’s not right, it can mess with everything.

How it works is fuel enters the float bowl via the fuel inlet fitting. From there it flows through a needle valve. That valve is then actuated by the float itself. When the level is insufficient, like when you are using fuel or the bowl is empty, the float hangs down and opens the valve. When the level is reached at full, the valve closes. Super-simple system, but there are some things that can go wrong.

First is the issue of a stuck needle valve. Sometimes, the needle can get stuck, whether that is a piece of crud holding it open or it’s not sliding smoothly. When this happens fuel will continue to flow and overfill the bowl. When this happens the fuel will flow out of the overflow tube. A quick fix is to tap on the side of the bowl with something like a screwdriver handle to shake the crud loose. If that doesn’t remedy the situation, the carb is going to have to come apart. Which you should do anyway if the needle is sticking.

The next issue is the needle might be worn out, also leading to overflowing or incorrect metering. When you pull the needle out, the sealing surface should be smooth. Run your fingernail down the needle; if you can feel a ridge, it’s toast. Replace the seat at the same time; the needles usually come in a seat and only run about $15 to $20 per body.

Once you know the needle valve is good, you need to make sure the float is good. First thing, make sure the float, well, floats! Do this in gasoline, as it has a different specific gravity than water or some other fluid. After that make sure it moves freely and doesn’t bind up. Once those checks are complete. It’s time to check the level.

To check the level, you will need the proper spec from your manual. This measurement will be the point at which the float just closes the needle valve. You can use a clear external tube attached to the overflow that will show the level but that is a pain. You’ve already got the bowl off, might as well measure it manually.

You want to measure the height just as the float touches the needle. The easiest way to do this is to hold the carb at a 45-degree angle. Then watch the small metal tab on the float as you move the float up toward the body. Just as the float touches, that’s your level. If you hold it straight up and down, you will have an incorrect height. The float will impart too much pressure on the small spring under the pin in the needle. That is the biggest mistake people make when measuring float height. If you need to adjust the height, up or down, just slightly bend the tab that contacts the need in the correct direction.

That’s it. Once you have the float height correct you can move on to the next step, the idle circuit. Which we will cover next time on MC Garage.

Source: MotorCyclistOnline.com