Adjusting Air-Fuel Mixture On A 1975 Cb550 Carburetor

The Honda CB550 is a vintage motorcycle with a carburettor assembly that may require maintenance or adjustments to the air-fuel mixture. Online resources, such as forums and video tutorials, provide step-by-step guides and tips for rebuilding and tuning the CB550's carburettors. These sources offer insights into common issues, such as vacuum leaks, jet sizing, and fuel starvation, which can affect the air-fuel mixture and engine performance.

Identifying the right carburettor kit for your bike

Understand your motorcycle's specifications

First, you need to understand the specifications of your bike to find the right carburettor kit. Here are some key factors to consider:

• Engine size: The size of your engine, measured in cubic centimeters (cc), will determine the size of the carburettor you need. Larger engines typically require carburettors with larger diameters.
• Engine type: The type of engine you have, whether it is a two-stroke or four-stroke, will impact the carburettor you choose due to differences in their air-fuel mixture needs.
• Intended use: Think about how you use your motorcycle. Is it for racing, cruising, or off-road adventures? Different uses may call for different types of carburettors.

Determine the correct carburettor size

You'll want to match the carburettor size to your engine's displacement. The venturi size, or the narrowest part of the carburettor where air passes through, affects the airflow velocity and the air-fuel mixture.

Measure the inner diameter of the intake manifold

Ensure that the carburettor can be mounted on your bike's intake manifold by comparing the inner diameter of the intake manifold to the outer diameter of the carburettor. Some carburettors may require adapters or modifications for a proper fit.

Check the fuel delivery method

Find out whether your engine uses a gravity fuel supply or a pump. Typically, if the fuel tank is located above the carburettor, it uses a gravity fuel supply.

Confirm throttle and choke types

Check the type of throttle cable your bike has. Some carburettors, like those from Nibbi Racing, are only compatible with pull-type cables. Also, confirm the choke type. For instance, Nibbi Racing carburettors are designed for manual choke types only.

Consider additional features

When choosing a carburettor kit, look for features that will make your life easier in the long run. For example, go for carburettors with easily adjustable needles, jets, and air screws so you can fine-tune the settings. Also, ensure that spare parts and support are readily available from the manufacturer or supplier to make maintenance and repairs more convenient.

Getting the right tools for the job

The right tools for the job are essential when it comes to changing the air-fuel mixture carbs on your 1975 CB550. While the project may seem daunting at first, it can be accomplished with relative ease if you have the correct tools on hand. Here is a comprehensive list of tools you will need to get started:

• A #2 flat blade screwdriver
• A new #2, good quality Phillips-head screwdriver
• A #3 Phillips-head screwdriver
• 3mm Hex Key (Allen wrench)
• Small needle-nose pliers (4-3/4”)
• Small needle-nose Vice-Grip
• Small metal file (about 6” long)
• Small awl kit (available online or at hardware stores)
• Small plastic hammer
• 1-gallon Pine-Sol
• Q-Tips (about 20)
• 7mm, 8mm, and 10mm combination wrenches

Most of these tools are readily available and affordable, so be sure to have them on hand before beginning the project. In addition to the tools mentioned above, you will also need a few common household items, such as:

• Deep aluminium pots with lids for cleaning parts
• Ice tongs for handling parts
• Muffin pans or trays for sorting and organising parts

By gathering all the necessary tools and creating a clean and organised workspace, you will be well on your way to successfully changing the air-fuel mixture carbs on your 1975 CB550. Remember to take your time, follow instructions carefully, and don't be afraid to seek additional resources if needed.

Disassembling the carburettor bank

Before starting to disassemble the carburettor bank, it is important to get prepared for the job. This project will take several hours to complete, so make sure you have a clean workspace and organise your parts so they won't be lost. It is also important to note that to do the job properly, you will need to fully disassemble the carburettor set.

The first step is to remove the staked choke plate screws. This is one of the most difficult tasks of the job. Each of the valves on the choke is held in place by two very small Phillips-head screws. These screws have a hollow shaft which allows the factory to "stake" or punch them once they are installed. This swells the ends of the screws to keep them from backing out and being sucked into the engine. You will need a brand new, good-quality Phillips-head screwdriver to remove these screws.

The next step is to separate the carburettors. Remove the brackets that hold the carburettor set together. There are two on the choke side and a large cast assembly on the manifold side. Be very careful how you handle the carburettors once these brackets are off, as you can bend the choke shaft and damage the bushes on the shaft.

Now, remove the throttle slide shaft and outboard carburettors. Loosen the screws that secure the throttle slide assembly to the throttle rod. Start with the first carburettor and remove the screw securing the slide assembly. There is a tiny split lock washer under the screw head, so be sure not to lose it. You can now gently slide the carburettor from the assembly. Be sure not to loosen the slotted screws attached to the throttle slides. These are for syncing the slides, so it's best to leave them alone for now.

With the outboard carburettors removed, you can now remove the throttle shaft and the inboard carburettors. Loosen the locknut on the throttle assembly bell crank and then use a 3mm hex key (Allen wrench) to back off the set screw holding the bell crank in place. Once removed, you will notice a dimple in the throttle shaft where the Allen screw resides. This keeps the assembly located in the proper position and makes reassembly much easier. Before you pull anything apart, take note of the small return spring on the choke rod. Note where it is attached and take a picture or make a simple drawing to help you remember. You can now carefully slide the throttle shaft out of the inboard carburettors. The shaft will exit from the side of the third carburettor.

With the carburettors removed from the shafts, you can now begin to disassemble and clean the individual carburettors. Start with the first carburettor. With the throttle rod removed, the throttle slide will pull out. Be sure to set the slide on a soft cloth so you don't scratch it. Remove the three Phillips-head screws securing the float bowl. There is a small pin that holds the float in place. Using a tiny pick, drift or nail (smaller than the hole), gently tap the pin until it protrudes from the opposite side of the other float tower. Once it protrudes, the pin can be pulled out with a pair of small needle-nose pliers. Be sure to clean it with a Scotch-Brite pad before reinstalling.

With the pin removed, the float and needle valve should easily lift out with the float. Take care not to lose the needle valve as you remove it. Use extreme care when handling the float, as it is very delicate and can be easily damaged. If your carbs have worked well in the past, the float tabs are probably still in proper adjustment, so be careful not to bend the float tab. You can test your float by placing it in a bowl of water. If, after an hour or so, both sides are floating evenly, you're good to go.

As gasoline ages, it changes from a liquid into a sludgy, varnish-like material that can block carburettor circuits and jets. If your carburettors are corroded or varnished, removing any part can be difficult. Jets can snap off, so you might need to soak the carburettor in a 50/50 mix of water and Pine-Sol overnight or longer if it's badly blocked. After soaking, you'll be able to disassemble the carburettor more easily.

Now, use a flat-blade screwdriver to remove the main jet and the mixture screw. Be sure not to lose the spring under the mixture screw. Under the spring, there is a tiny washer and O-ring. To remove the emulsion tube, you will need a 7mm wrench. The pilot jet is pressed in, so you will need a very small pair of needle-nose vice grips to pull it out. Grasp the jet with the vice grips and then move it side to side as you pull it straight out. Be careful not to wallow out the hole for the jet. You can normally use your fingers to pull out the aluminium fuel tube, but if the O-rings are dried out and hard, be careful not to crush the end of the tube as they are very thin.

With all the internal parts removed, you now only need to tap out the throttle slide needle guide. Note that, unless you are replacing it, you don't need to remove this part. If you do choose to remove it, use a small drift and set the carburettor on a block of wood. Gently tap the throttle slide needle guide out of the bottom of the carburettor body through the float area.

Once you have finished disassembling the first carburettor, repeat the process with the other three carburettors.

Cleaning the carburettors

Before you begin disassembling the carburettors, you should give them an initial clean. This will make them much easier to work with. You can use a stiff brush or an old toothbrush to clean the surface of the carburettors.

Once you have removed the carburettors from the bike, you will need to remove all the gas from them. The easiest way to do this is to remove the drain screws in the float bowls.

For a more thorough cleaning of the individual carburettors, you can use a solution of Pine-Sol and hot water. Soak the carburettors in this solution for at least two hours, but no longer than two days. After soaking, rinse the carburettors with clean water and dry them thoroughly with compressed air.

You can also use an ultrasonic cleaner or create an agitating parts mixer to clean the carburettors. This will help to remove any old gas residue and varnish. Be sure to rinse the carburettors with clean water and dry them with compressed air after cleaning.

Another important area to clean is the seat for the needle. This can be polished using an electric drill with a Q-tip and a small amount of Semichrome polish. This will help to remove any old varnish residue.

Finally, you can use a guitar string or a wire brush to clean the jets and emulsion tube. It is important to ensure that these areas are clear of any debris before reassembling the carburettors.

Reassembling the carburettors

Now that you have cleaned and prepared your carburettors, it's time to put them back together. Here is a step-by-step guide to help you through the process:

Pilot Jet Installation

First, install the pilot jet, also known as the low-speed jet. This part is not included in the rebuild kit, so ensure it is cleaned and checked before proceeding. Use a high E guitar string, jet or welding tip cleaning tool, or a stiff piece of wire to clear any blockages. Once clear, push the jet back into place and gently tap it with a plastic hammer to secure it. You can also apply a small amount of Blue Loctite to the jet seat for added security.

Throttle Slide Needle Guide Replacement

Next, we will replace the throttle slide needle guide. Compare the old and new parts to ensure they are identical. The guide has one thin-walled end and one thicker, bevelled end. When installing, place the thin-walled side first into the emulsion tube chamber, with the thin-walled side protruding into the mixing chamber. Gently tap the guide into place with a plastic hammer or a screwdriver.

Installing the Emulsion Tube and Main Jet

Now, screw in the emulsion tube using a 7mm wrench. Following this, replace the main jet and tighten it with a flat-blade screwdriver. Don't be concerned if a few threads are visible above the emulsion tube; this is normal. Remember to install the spring, washer, and O-ring in that order.

Mixture Screw Adjustment

Examine the shaft where the mixture screw resides and ensure there are no remnants of the old gasket or washer. You should be able to blow air into the hole and have it exit into the mixing chamber. If the hole is blocked, use compressed air or a cleaning solution to clear it. Once clear, assemble the mixture screw. Check that the needle tip is intact and not bent. Place the spring over the tip, followed by the flat washer and O-ring (with a dab of WD-40 or grease). With the carburettor upright, screw in the mixture screw until snug, then back it off by 1-1/4 turns.

Float and Float Needle Installation

Spray a small amount of WD-40 onto the float pivot pin and the holes in the float towers. Carefully handle the float to avoid bending the float tab or causing any damage. With the notched side of the float facing down, slide the float needle's wire hoop onto the tang on the float. Place the float between the float towers of the carburettor, upside down, and insert the pivot pin. Gently tap the pin into place if needed, ensuring both sides are flush with the edges of the towers.

Float Level Adjustment

It is important to check and adjust the float levels. Hold the carburettor upside down and gently tap the floats to settle them. Measure the distance from the top edge of the float to the surface of the carburettor body where the float bowl is attached; the correct distance should be 14.5 mm. If adjustments are needed, carefully bend the tab with needle-nose pliers.

Float Bowl Reattachment

Place the O-ring around the base of the float bowl, ensuring it is held in place by the nibs protruding into the channel. Place the gasket on the float bowl and fit it onto the base of the carburettor. Secure the float bowl with screws, snugging them first before fully tightening.

Throttle Slide Needle Replacement

If desired, you can replace the throttle slide needle. Remove the two small screws from the bottom of the throttle slide using a #1 Phillips screwdriver. Be careful not to damage the needle or scratch the slide. The needle clip should be in the #3 position, but adjust it according to your previous setup. Reattach the screws when finished.

Fuel Rail Tube O-Ring Replacement

Lubricate and install new O-rings on the fuel rail tubes. Ensure the tubes are placed correctly between the carburettors. There are two short tubes (between carbs 1 and 2, and 3 and 4) and one long tube (between carbs 2 and 3). Apply WD-40 or grease to the O-rings before inserting them into the carburettors.

Carburettor Bank Assembly

Lay out the rebuilt carburettors in the correct 1-4 position. Assemble the carburettor bank in pairs (carbs 1 and 2, then 3 and 4). Lubricate the fuel rail tube and choke shaft, and insert them into carbs 1 and 2. Insert the choke rod with the lightweight spring through these carburettors, ensuring the spring is positioned correctly. File down the choke plate screws before screwing them into the choke shaft, as they may have been staked to prevent backing out. Place the choke plates, ensuring the dimple faces outward. Snug the screws but do not tighten them yet.

Steady Arm Installation

Install the steady arm between the carburettors using two hex-head bolts and a 10mm wrench.

Throttle Shaft and Throttle Slide Assembly Installation

For carb 3, insert the throttle slide assembly into the mixing chamber, then pass the throttle shaft through the right side of the carburettor and the slide assembly. Rotate the idle stop to the top and thread the 5mm Phillips screw through the top of the slide assembly into the shaft. Place the bell crank over the throttle shaft on the left side of carb 3. Locate the dimple in the shaft for the set screw and tighten it with a 3mm Allen wrench. Secure the lock nut on the throttle shaft with a 10mm wrench.

Joining the Carburettor Pairs

Lubricate and install O-rings on the long fuel rail tube. Insert the tube into the right side of carb 2, then carefully slide the throttle shaft from carbs 3 and 4 through the slide assembly of carb 2. Be very careful when handling the plastic bushing on the right side of the assembly, as replacements may not be available. Join the carburettor pairs together, ensuring the choke rod from carbs 1 and 2 fits with the choke rod from carbs 3 and 4.

Final Adjustments and Testing

Before fully tightening the choke plate screws, bench-test the carburettors for leaks. If adjustments are needed, disassemble and correct any issues. Once satisfied, apply Blue Loctite to the choke plate screws to prevent vibrations from loosening them. Do not use Red Loctite, as it requires heat for removal, which can damage the carburettors.

Frequently asked questions