How To Adjust Air/fuel Mixture On A Motorcycle Carburetor

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How To Adjust Air/fuel Mixture On A Motorcycle Carburetor – A fixed jet carburetor mixes air and fuel through multiple nozzles. Nozzles are small holes that allow precisely controlled amounts of fuel or air to flow through them. The quantities are controlled by the diameter, length and shape of the nozzles.

The nozzles of these types of carburetors are not adjustable, so the air-fuel mixture is adjusted by directly regulating the flow of fuel or air.

How To Adjust Air/fuel Mixture On A Motorcycle Carburetor

How To Adjust Air/fuel Mixture On A Motorcycle Carburetor

The nozzles are activated as needed by changes in the vacuum inside the carburetor, caused by different engine speeds and throttle openings.

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Since the nozzles are fixed, not adjustable, the mixture is adjusted by directly regulating the flow of fuel or air to them.

On carburetors manufactured before 1974, you can adjust two settings. One is the volume of fuel (or some types of air) entering the engine. The other is the engine idle speed, which is set by adjusting the throttle stop screw.

These are called emission control carburetors and have sealed, tamper-proof adjustments that you normally can’t change (see Adjusting an Emission Control Carburetor).

Before adjusting the carburetor, make sure it needs adjusting by eliminating all other causes of malfunction (see Faultfinder).

Adjusting A Fixed Jet Carburettor

Some adjustments are made with the engine running, so while it’s cold, locate the adjustment screws and figure out how to get to them easily without burning yourself when working on the hot engine.

It can also help you get the job done without removing the air purifier. Removing the air cleaner increases airflow and disrupts mixing.

Get the engine up to normal operating temperature, then work quickly before it gets too hot, which will also affect the mixture.

How To Adjust Air/fuel Mixture On A Motorcycle Carburetor

On most carburetors, the volume screw regulates fuel flow; in this type, the position of the volume screw is low in the carburetor body.

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An air volume control screw is located above. Turning it clockwise enriches the mixture; turning it counterclockwise weakens the mixture.

Find the two points where the chase and the clumsy run begin. Place the screw about halfway between them, unless another setting clearly makes the motor run better. Note the screw setting.

Reset the throttle stop screw to normal idle speed and repeat the test with the mixture screw. This will probably result in the same final configuration.

On a Weber, the mixture adjustment screw is sealed on many later types. Idle speed is adjusted by turning the throttle stop screw.

Air/fuel Mixture Screws Or Pilot Screw Set

On a Solex, idle speed is adjusted by turning the bypass idle screw, which is sealed with a plastic cap on later models. It’s the simple elements that are often overlooked. Nobody talks much about how to set the idle mixture on a carbureted engine because, frankly, it’s not very sexy. While this may seem like an unimportant detail on the way to achieving prime horsepower, the truth is that a typical street engine will spend more running time at closed throttle than at any other engine speed. Therefore, it is important that this setting is adjusted correctly. The benefits will far outweigh the effort required to do it right.

We’ll approach this story using a typical 750cfm four-barrel vacuum secondary carburetor with two idle mixture screws. If your carburetor is equipped with four idle screws, the procedure will be exactly the same, except that the two additional screws will receive the same attention.

This is a 750 cfm carburetor that we will be using to tune idle quality on a small block Chevy on a dyno. This is a 4150 style secondary vacuum carburetor with two idler screws.

How To Adjust Air/fuel Mixture On A Motorcycle Carburetor

To start this process, we assume that the engine in question is in good physical condition, with no dead cylinders, no misfires, vacuum leaks, or other ailments that could affect idle quality. This also assumes a reasonable starting time of at least 8 degrees before top dead center (BTDC). Engines with even mild performance camshafts often benefit from an initial timing of 10 to 12 degrees BTDC.

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We will also assume that the engine has reached maximum operating temperature and that the throttle is fully open. This is the only way to get a proper idle mix. Do not attempt to set up an idle blend if the shutter is partially engaged. Make sure the choke idle screw does not touch the shutter fast idle cam. A vacuum gauge must also be connected to the manifold vacuum.

Before we continue, it’s also a good idea to lay out the basics of idle quality measurement. All internal combustion engines operate with a certain amount of idle manifold vacuum. This is created as a result of the demand on the cylinders working against the enclosed gas. Engine vacuum can be measured in several different ways, but the most common is a vacuum (negative pressure) expressed in inches of mercury (“Hg). Engine vacuum and pressure gauges are usually displayed in “Hg or centimeters of mercury (cmHg). .

Production and mild performance engines will generally idle somewhere in the 12 to 18″ Hg range and the gauge needle will generally be stable and not bouncing around. A very erratic idle vacuum reading often indicates a problem such as a leaking intake valve or a failed exhaust lobe. when cylinder pressure is pushed into the manifold causing erratic needle movement, this is one of the main reasons why it is important to ensure make sure the engine is mechanically sound before attempting to adjust the idle mixture.

Before starting the engine, use a small screwdriver to check the position of each idle mixture screw. Turn each setscrew slowly clockwise (in) and count the number of turns until it seats easily. Typically this will be 1-2 laps from a fully seated position. When installing these bolts, be careful to avoid damaging the gauge block seats. Do this for both (or all four) idle mix screws and place them in the same position. It is important that all the idle mixture screws are adjusted the same so that the output of the carburetor idle circuit is balanced across the two idle mixture outlets. One to 1½ turns is a typical place to start.

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With the engine idling in park with the emergency brake engaged, the first step is to adjust the idle speed to the desired speed. We choose 850 rpm for this application. Idle is set with the idle adjustment screw on the primary throttle linkage on the driver’s side of the carburetor.

Start by tightening the idle mixture screw on the driver’s side (clockwise) about 1/8 turn while monitoring the vacuum. If the vacuum and/or engine speed increases, adjust the passenger side idle screw by the same amount and monitor the vacuum reading. Repeat this process and evaluate the result. If the engine vacuum drops after adjusting the first idle screw, go back to the basic settings and turn the mixture screw about 1/8 of a turn.

This first adjustment sequence is to position the idle mixture screws on both sides of the carburetor to achieve the highest possible idle vacuum setting. If after an initial adjustment the idle speed increases beyond the desired speed, turn the idle screw counterclockwise to return to the original engine speed. This will likely lower the idle vacuum reading, which is normal.

How To Adjust Air/fuel Mixture On A Motorcycle Carburetor

If the initial adjustment of the idle mixture screws resulted in a higher vacuum reading, make another slight movement of the idle mixture screw no more than 1/8 turn. Do this on both sides of the carburetor and evaluate the result. If the vacuum drops, go back to the higher setting and see if the vacuum reading increases. This means you are close to your final configuration. Continue making very small changes to the idle mix screws on both sides.

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The inactive screw (arrow 1) is located in the primary joint. The idle indicator adjustment screws (arrow 2) are located on the primary indicator block on most carburetors. A typical starting point for these idle mixture screws is 1 to 1 ½ turns from fully seated.

We have found it beneficial to sneak in the adjustment in very small increments towards the final adjustment. These adjustments will be literally no more than the width of the adjustment screw slot. These are very small changes, but they have an effect on the quality of idleness.

Back and forth adjusting the two idle mixture screws equally for each shift will eventually create the highest idle vacuum level. This should be achieved with the idle mixture screws adjusted with an emphasis on the lean setting. If removing the idle mixture bolts (richer) does not improve idle vacuum or engine RPM, the leaner setting is always the preferred option.

At this point, have a friend sit behind the wheel and place the transmission in Drive with your foot firmly on the brakes. Monitor the vacuum gauge and make sure the idle is stable. Running the engine will give a slightly lower vacuum reading. This is

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