Carburetor Theory and Tuning

For some reason everyone seems to think tuning a carb is simple. Change a jet or two and you’re done. There are quite literally millions and millions of jet combinations. A rough check on Bing carbs shows there are at least 13,860,000 different possible jetting combinations.

If you look at a carburetor, you will notice a rather large hole going from front to rear. This is called the Venturi. Air passes into the engine through the Venturi. As the velocity (speed) of the air entering the carb increases, it's pressure decreases creating a low pressure or vacuum in the Venturi. This vacuum moves around in the Venturi, as the throttle is opened, and sucks gasoline through the different jets in the carb. The gas then mixes with the air going through the Venturi. The way the jets are made causes the fuel to vaporize as it goes into the Venturi. Where the jets are placed in the carb and where the jet's outlet is located in the Venturi, determines what part of the throttle opening that jet controls. The idle jet system (comprised of pilot air jet, pilot fuel jet and pilot fuel screw) controls from 0% to about 25% of the throttle opening. The throttle valve controls 0% to 35% of the throttle opening. The needle jet and jet needle control from 15% to 80% of the throttle opening and the main jet controls 60% to 100%. This means that when you open the throttle about one eighth of the way open, all of the gas/air mixture going into your engine is controlled by the idle jet. As you can see the different jets over lap the operating range of each other. That is, the jet needle starts to effect things before the effect of the idle jet ends. This is something to remember when working on carbs... everything is interconnected. Change one thing and it will affect other things.

OK, let's go over the different systems in the carb and see what they do.

Fuel Level: The fuel level is controlled by the fuel floats and the fuel float valve. The floats are hollow or made of something that will float on gasoline, such as cork, metal, or plastic. Part of the float presses against the float valve, sometimes called a needle and seat. Most times the part of the float that touches the float valve needle is bendable so you can adjust the level of the fuel in the float bowl. Some floats are not adjustable. If this level is way too high, gas can leak out the carb overflow tube or into the engine and/or air box. If fuel gets into the engine it will thin out the engine oil, ruining its ability to lubricate. This will cause serious engine failure. If a full tank of gas in the evening turns into a half tank by morning, check your oil. If it's thin and smells like gas, change it and replace your float valve and/or check your fuel level. If the oil is OK, check under the overflow tube. If it's OK, then check where you are parking your bike because someone is walking away with your gas!If your fuel level is just a bit high, the mixture will tend to be a bit rich. If it's low, the mixture will tend to be a bit lean. This is because a higher fuel level requires less vacuum to draw fuel into the Venturi. This is called Service fuel level. You can measure this level with special tools with the carbs on the motorcycle.

Pilot Circuit: The pilot jet controls the idle and its working range is from idle to ¼ throttle opening. On some carbs there is an air jet, too. In conjunction with the pilot jet there is a pilot jet air screw. This screw leans or richens the fuel mixture for a smooth idle and up to 1/4 throttle opening. From the pilot jet there are little passages cast into the carb body that leads to holes just in front of the throttle valve or plate. There can be one or more holes depending on carb design. They affect the air/fuel mixture as long as vacuum in the Venturi is over them. As the throttle opens wider vacuum moves to the needle jet and jet needle.

Throttle Valve/Slide: The throttle slide looks cylindrical & moves in a vertical pattern. The slide has a bevel angle cut on the intake side. This angle comes in several angles and helps control the fuel mixture from idle to ¼+ throttle opening. CV or Constant Vacuum (diaphragm) type carburetor slides have flat Venturi bottoms. The throttle plate acts as the slide angle in CV type carbs.

Jet Needle: The jet needle looks like a metal tooth pick. It meters fuel/air mixture from ¼ to ¾ throttle opening. This is the needle that rides in the throttle slide and goes into the needle jet. It can come in many different sized tapers. Sometimes, one needle can have several tapers on it. The top end of the needle has grooves cut in it, usually five, and you can move the little clip on the end up or down to lean (down) or richen (up) the mixture. Most late model bikes have needles with only one groove cut in them. This is so you can't richen the mixture, thereby keeping the EPA happy.

Needle Jet: The needle jet is brass or bronze and is located directly below the throttle slide. The needle protrudes up into the Venturi. The leading edge of the protrusion creates a low pressure, or choke, on the trailing edge which aids in drawing fuel/air mixture up through the emulsion tube, and into the Venturi. The main jet and emulsion tube are directly below the needle jet. The jet needle and the needle jet work together to meter fuel/air.

Main Jet: This jet controls the fuel mixture from ¾ to wide full open throttle position.

We want nice clean acceleration from idle to full throttle with no stumbling or flat spots. This can be quite a tall order if we are starting with a new carb. Actually, it can be a real challenge to get the air/fuel mixture right after something as simple as an exhaust pipe change. If you are working at home, you may not have the jet selections the local dealer’s service department has on hand. You may make several trips to the dealer before the job is done.

Read the spark plugs to determine changes to be in jetting. The following is an actual means of performing a plug reading test: Find an open highway or roadway that you can lawfully and safely perform this test. Run the engine at the throttle opening of the jetting range you want to test, for the distance of about a mile. While riding, pull in the clutch lever and kill the engine, then find a safe location to park. Remove and examine the spark plugs. Is the spark plug reading lean or rich? When you get back to your shop or garage, you can look for the jet that controls that particular throttle opening and exchange it for a richer or leaner one.

The throttle transition from one jet to the next must be smooth. Each fuel circuit overlaps another. Make sure you have a good selection of jets. Most carburetor manufacturers have tables of specifications on the jet needles and needle jets, and other jets. You will find these charts very useful. With these specs you can make a better guess as to what jet will work best to start tuning. Some places use motorcycle dynamometers for testing. These can be a big help to get real close to the best jet setting. Working out the best main jet for a 170 MPH bike can be quite unhealthy if you only have a freeway to perform tests.