How a carburetor works: Carbs explained

The Article Formerly Known As Carbs 101

THE BASICS - Mixing and Delivering

While most of the following information applies to all CV carbs, and to some extent all carb types, this article is specific to the Keihin CVK40 found on the KLR650. Variants are on the KLX650, KLR600 and some Harleys. (The carbs on the Ninja 250 are Keihin CVK 30, or CVK 32 on the 86-87's. Also remember that the Ninja has two carbs; the bike this article was written for only has one.)

The CVK40 is technically a bleed type carb, with a variable venturi that's controlled by constant velocity (CV). It's also known as Constant Depression or Constant Vacuum. The 40 in the name represents a 40 mm venturi exit diameter. Wonder why 'K'awasaki added the K...?

BLEED TYPE CARBS - Let it Bleed

Most carbs have a one piece needle jet. On the CVK40 it consists of a collar and what Kawasaki calls a holder. Most 4-strokes have pin holes along the length of this brass jet. These pin holes allow air from the air jet to premix with fuel from the fuel jet to start the mixture emulsification process before it enters the venturi. Primary type carbs use a solid jet, (no pin holes).

VARIABLE VENTURI - Getting Sucked In

A venturi is a tube with a convex taper, (one end wider than the other). As air enters the wider end it's squeezed into the narrower section of the tube, lowering the air's pressure. The area of lowest pressure is just past the narrowest point and is called the depression. This has always seemed counterintuitive, but Bernoulli's Principle outlines the fluid dynamics involved in this effect. This lowered pressure, or comparative vacuum, is separate from the engine vacuum. A variable venturi varies the venturi diameter at the depression by raising or lowering an obstruction. This obstruction is called a slide. On a CV the slide is called a piston or diaphragm valve.

CV or SLIDE CARB - The Great Compromise

Both the CV and conventional slide carbs are classified as variable venturi carbs. The slide on a conventional carb is directly connected to the throttle cable. Twist the throttle grip and the slide is raised in the venturi. On a CV carb the throttle cable is connected to a butterfly valve that varies the volume through the venturi. It's not the throttle, it's the pressure difference from the venturi to the outside atmosphere that moves the slide.

So which type is better? That depends on what you want to do. The manufacturers will tell you the CV is the next best thing to electronic fuel injection. It does feed a precise amount of mixture to smooth out throttle response, reduce pollution, and stretch your fuel budget and gas tank range.

This is great for tarmac cruising and feeling warm and fuzzy about doing your part to reduce global warming while pocketing some spare change. It sure makes it easier for the manufacturers to get the EPA approval stamp on the bike.

But what if you feel that no matter how much you hop up your little beast, you're never going to match the belching of that cager in the gas guzzling V-12 ? What if you don't mind spending more for gas, and when you go off-road you want a burst of power to blip over obstacles or steer through a wash without fanning your clutch while waiting for the vacuum to build in the venturi? If you can discipline yourself to control the throttle so that you don't bog your engine, then you want a conventional slide carb.

40 MM - Size Matters

With the same engine and carb design, a 38mm diameter venturi will more accurately meter the mixture on the low end, while a 41mm diameter will do a better job of supplying mixture at higher engine speeds. It's another factor to consider if you replace your carb.

MIXING AIR AND FUEL - All In the Pressure

It all starts in your engine. Think of your thumper as a big pump, happily sucking and blowing down the road. During the intake stroke the engine's piston descends in the cylinder. This creates a void that sucks in whatever is on the other side of the open engine intake valves. Unless the engine intake valves are open, the carb is not being sucked on by the engine. This vacuum action powers the carb.

Look at the 4-cycles of our 4-stroke thumperpumper.

1) Intake - piston pulls down, sucking mixture in.

Intake valves open, exhaust closed.

2) Compression - piston goes up.

Intake and exhaust valves closed.

3) Ignition/Power - piston forced down.

Intake and exhaust valves closed.

4) Exhaust - piston goes up blowing out burned gas.

Exhaust valves open, intake closed.

Each time the piston goes up and down, (two of the four strokes), one revolution has occurred. It takes two revolutions to complete the four strokes, or one-half of a revolution to complete one of the four strokes. At redline, your engine is sucking on the carb at a rate of over 60 times per second. At idle it's around 11 times a second. The math below shows that with a 651cc displacement, a theoretical maximum of between 7 and 40 liters of mixture are sucked in each second.

7500 (engine RPM)/ 2... (2 revs per intake stroke) = 3750 (intake strokes per minute)

/ 60 (seconds per minute) = 62 (intake strokes per second) x .651 (engine displacement in liters) = 40.69 (liters per second)

These numbers assume 100% efficiency. Mister_T calculates about 38% efficiency at idle. So these numbers are only to illustrate the relative difference in volume from idle to redline.

Remember Bernoulli's effect in the venturi? Sitting below the venturi is the fuel float chamber which is vented to the outside of the carb to match atmospheric air pressure. The fuel in the float bowl is also 'vented' to the low pressure of the venturi through the starter, pilot and main fuel jets.

So you can think of the float chamber as being pressurized in comparison to the partial vacuum of the venturi. The fuel naturally tries to fill in the low pressure of the venturi's depression by injecting itself through one, or a combination of the three fuel jets.

THE ODDS - Skinny or Fat

Any carb problem boils down to either it's too rich, (too much fuel or too little air), or it's too lean, (too much air or too little fuel). The 'mixture', (a mixture of fuel and air, your engine's 'food'), is theoretically ideal at around 15 parts of air to one part of gasoline by weight, (not volume). The outer limits are 12:1 and 18:1 .

Too rich and you're wasting fuel, spewing more pollutants, diluting your oil, fouling engine parts, and performance suffers. Too lean and you run the risk of detonation, engine operating at higher temperatures, and performance suffers.

An air-cooled engine needs to run richer (more fuel) to aid in engine cooling. This is another plus of our water-cooled system, to balance out its disadvantages.

PARTS - Is Parts

For images of Ninja 250 carbs, see Carburetor photos.

Supplementing the factory manuals, the following text and photos should make it easier to understand how the carb works. The parts are broken into the following groups:

FLOAT SYSTEM - You're Floating In It

The float system acts as a fuel reservoir to meet engine demand. The float is hinged on a pin in the float boss. It rises and falls with the fuel level in the float bowl. The small metal tang integrated in the plastic float supports the float valve, aka float needle. As the fuel in the float bowl rises, the float valve is pushed into the valve seat, until it's high enough to shut off the fuel flow to the bowl. The level in the bowl drops lowering the float which pulls the float valve from its seat, and fills again.

STARTER ENRICHER - Ninja's Morning Coffee

This system is referred to as the choke. But that's a misnomer. When you apply the choke lever, what you're doing is retracting a plunger that opens a tube connected to the starter jet, allowing additional fuel to enter the venturi just below the vacuum hose nipple. It supplements the pilot system at start up.

PILOT SYSTEM - Steering You to Better Lows

The primary purpose of the pilot system is to supply the mixture at idle. It continues to supply fuel throughout the entire throttle range, but after about 1/8 throttle is reached the MAIN SYSTEM starts to put out an increasing percentage of the total mixture up to full throttle.

When you set the idle with that big screw on the right side of the carb, what you're doing is covering or uncovering one or more of the four small holes that are drilled into the venturi, (leading to the pilot jet) just under the butterfly valve, and letting more or less air pass the butterfly. When you adjust the pilot screw that's under the carb, you are varying the amount of air premixing with the gas before it enters the venturi.

MAIN SYSTEM - The Meat

As you advance the throttle cable that's connected to the butterfly valve, the butterfly opens to allow more air through the venturi. This increases the vacuum effect that is transferred up through the vacuum drilling (the hole you bored out for the Dynojet kit), on the diaphragm valve (slide), that leads to the TOP diaphragm chamber. (Note: Drilling your slides is not recommended for the Ninja 250.)

The top chamber is separated from the BOTTOM chamber by the rubber diaphragm. The bottom chamber is open to atmospheric pressure from the airbox by the crescent shaped casting on the top of the venturi. When the vacuum in the top chamber rises sufficiently, the constant ambient pressure of the lower chamber helps the diaphragm valve overcome the downward force of the diaphragm spring, and it rises from the venturi.

As the diaphragm valve is raised from the venturi depression (lowest pressure area), the needle is pulled further out of the needle jet, exposing a thinner portion of the needle taper, which allows even more fuel to rise into the venturi to meet the increased engine demand.

COASTING ENRICHER - Feeding a Closed Mouth

You're blasting down the road and you see a hairpin coming up, so you close the throttle. This closes the butterfly valve. You and your 400 lb. baby (the bike, not the other love of your life), are still rolling with inertia, keeping the engine revs high. Now it can't get the mixture it's trying to suck in because the butterfly valve is closed, blocking the air flow.

To compensate for this there's an air jet in the lower diaphragm chamber that transfers ambient pressure to one side of the coasting enricher's spring-loaded cut off valve. The excess vacuum in front of the butterfly valve is transferred to a drilling that leads to the other side of the cut off valve. This sucks it open, allowing the pilot jet to feed more fuel to the engine, preventing an overly lean condition. (Pop. Pop. Pop.)

WARNING!

DO NOT tamper with your carb. Unless it's time to clean or overhaul it, you've changed the exhaust or airfilter, you need to troubleshoot it, or the Kid in you just has to know, "how does it go?", and is willing to figure out how to put it back together.

When troubleshooting the carb, remember its job is to mix fuel and air to a specific ratio for the engine's consumption. There are many things that can cause an imbalance in your mixture besides the carb itself. Things like a leaking or pinched hose, valve clearance, a dirty air cleaner, removing the airbox lid, even running out of fuel. Anything that affects your big pump as it sucks and blows affects your mixture. Before blaming the carb, eliminate as many other variables as you can.

Please see the rest of the FAQ for more information on working on your carbs.