Honda CB550 Carbs: Understanding the Changes

The Honda CB550, a true workhorse of its era, cemented its place in the hearts of motorcyclists across the UK and beyond as a reliable, comfortable, and easy-to-maintain everyday machine. Introduced as an evolution of the beloved CB500 Four, the 550 offered that little bit extra – a crucial 50cc bump in displacement that, while seemingly minor, delivered a noticeable improvement in mid-range power and torque. This made it a formidable competitor in the burgeoning middleweight market, particularly against Suzuki's offerings.

While the CB550 models, encompassing both the classic four-pipe K series and the sportier four-into-one F series, maintained much of their mechanical identity throughout their production run, there were specific, pivotal changes that enthusiasts and owners must be aware of. Chief among these were the carburettor revisions, which significantly impacted performance and tuning, particularly for models destined for the US market due to evolving emissions requirements. Understanding these changes, along with other critical maintenance aspects, is key to keeping your classic CB550 running at its best.

The CB550's Evolution and Carburettor Revisions

The CB550 was born from a desire to offer more. Honda saw an opportunity to not only counter rivals like Suzuki but also to justify a slight price increase while providing consumers with a genuinely 'more' motorcycle. Cosmetically, it mirrored its larger sibling, the CB750, evolving from the distinctive four-pipe 'K' models to the sleeker four-into-one 'F' variants. Mechanically, the core specifications remained remarkably consistent, a testament to Honda's initial design prowess. However, the carburettors were an exception.

The most significant carburettor change occurred in the 1977 model year for the CB550K3, primarily in response to stricter emissions regulations in markets like the US. These models saw a transition to PD46A carburettors, which featured different jetting and design characteristics compared to earlier versions. While the K and F models were largely mechanically identical, the specifics of carburettor setup varied across years and models, influencing everything from fuel economy to power delivery.

CB500/550 Carburettor Specifications Overview

To illustrate the changes, here's a comparative look at the carburettor specifications across different CB500 and CB550 models:

As you can see, the 1977 K3 model marks a clear departure with the introduction of PD-type carbs, significant changes in main jet size, jet needle specifications, and a considerable reduction in float height. These alterations necessitated a different approach to tuning and maintenance for these later models.

Understanding Your CB550 Carburettors

The Emulsifier Tube and Fuel Atomisation

At the heart of efficient carburetion lies the emulsifier tube. This small brass component, located within the throttle bore, plays a crucial role in mixing air with fuel before it enters the airstream. It features a series of holes drilled along its sides, and these holes are key to how your engine performs at different RPMs.

When air enters the emulsifier tube through the air jet, and fuel enters via the main fuel jet, this process emulsifies the fuel. Essentially, it makes the fuel somewhat foamy, which helps it atomise better when it discharges into the venturi. The pattern of holes on the tube dictates how and when this emulsification occurs:

• Top holes: Primarily affect the top-end of the rev range.
• Middle holes: Influence the mid-RPM range.
• Bottom holes: Impact the low-RPM range.

Where there are no holes, the mixture tends to be richer. Where holes are present, air is introduced, leaning out the mixture. The more holes, and the larger they are, the more the mixture is leaned out at that specific point in the RPM range. The air bleed size also influences the emulsifier tube's overall function; a larger air bleed leans out the mixture, with its influence increasing as engine demand for air rises.

An old trick, reportedly circulated among mechanics in the 70s, involved carefully drilling additional holes of the same size between the existing ones on the emulsifier tube, effectively doubling them. The theory was that this improved emulsification, leading to better atomisation without needing to re-jet. Some anecdotal evidence suggests this could significantly improve fuel economy, potentially by correcting a rich condition at part throttle. However, this modification is not factory-approved and should be undertaken with caution, as results can vary.

Pulse Carbs vs. Flow Carbs

It's important to understand that the CB550's carburettors operate as "pulse carbs" rather than "flow carbs" typically found on cars. This distinction is crucial for understanding their behaviour, especially at higher RPMs. In our carbs, it's not the float bowl level that significantly drops (though it does by a few millimetres above 6500 RPM), but rather the fuel level *inside* the tube surrounding the main jet holder (the emulsifier tube).

Each engine intake stroke draws a small amount of fuel out of this emulsion chamber. As RPM rises, this level starts to drop, exposing more holes in the emulsifier tube. This bubbling action further atomises the remaining fuel. However, there's an impasse at wide-open throttle (WOT). These carburettors tend to run out of mixing ability above 7/8 throttle. At this point, the fuel level has reached the bottom of the emulsifier chamber, and fuel is rising directly from the bowl to the needle jet. This can lead to a lean condition because insufficient fuel makes the trip efficiently. While slightly raising the float level can offer a minor improvement for racing applications, it risks fuel leakage during normal operation.

Common Carburettor Issues & Solutions

Dealing with the Notorious 'Flat Spot'

Many CB500/550 riders notice a characteristic 'flat spot' in performance, typically between 3500-4800 RPM. This isn't a fault but a consequence of Honda's design choices aimed at achieving specific characteristics:

• Good Idle & Easy Starting: Required a slightly rich idle A/F mixture (around 14.25:1).
• Quiet Operation: Achieved through long intake runners.
• Easy Maintenance & No Leaks: Carbs designed to sit horizontally.
• Smooth Power Delivery & Good MPG: Desired at highway speeds.

The trade-offs, particularly the long intake runners and angled direction change into the heads, made achieving perfectly smooth power and good MPG challenging. The richer idle needed to lean out at running speeds, and the spark advancer often reached full timing early. The result was a flattened torque curve in that critical mid-range.

Simple Solutions for the Flat Spot:

• Raise the jet needle: Move it up one notch in the carb slides.
• Smaller main jet: Install a 10-size smaller main jet (5 sizes smaller for late-year CB550s) concurrently with raising the needle.
• Spark plugs: Switch from standard D7E (NGK) or X22E (ND) to D8E (X24E) plugs. Always check plug colour afterwards to ensure it's not running too lean.
• Timing adjustment: Advance static timing by 2 degrees. You can also slightly modify the advancer springs (cut one turn off, reshape) to slow the advance curve by 5-8%.
• Rear sprocket: Adding 4 teeth to the rear sprocket raises the RPM slightly, shifting the torque curve down to a less noticeable point.
• Octane testing: Experiment with regular, mid-grade, and premium fuels to see how they affect the flat spot on your specific bike.

More Advanced Solutions:

• Intake runner smoothing: Smooth the insides of the intake runners and match the ports to the cylinder head.
• Valve modifications: Polish the intake valves and shorten their guide bosses by 2-4mm, narrowing them in the flow direction (but not thinner than 50% of original thickness).
• Air filter & airbox: Change to a K&N filter and open up the airbox intake holes by about 25%.
• Camshaft upgrade: Install a cam with about 7 degrees more duration and advance it by 3-4 degrees.

Carburettor O-Rings: The Small but Mighty Details

When rebuilding carburettors on these older Honda fours, especially those with press-in main jets, you'll often find that the metal components are perfectly fine after a thorough cleaning. What truly needs replacing are the O-rings. Unfortunately, Honda doesn't sell these individually, often forcing owners to buy complete, more expensive carb kits. However, industrial O-ring suppliers can often provide replacements at a fraction of the cost. Key sizes include:

• Main Jet: 1.2mm cross-section x 3.5 mm ID
• Float Seat: 1.5 mm cross-section x 5 mm ID
• Float Bowl Drain Screw: AS568-009 (7/32″ ID x 11/32″ OD)

Ensuring these are replaced with fresh, correctly sized O-rings is crucial for preventing leaks and maintaining proper fuel mixture.

Essential Maintenance Procedures

Removing and Re-fitting CB500/550 Carbs

Removing and re-fitting the carb bank on a CB500/550 can be a fiddly job, but with the right approach, it's manageable:

Removal:

1. Completely remove the band clamps for the rear rubber couplers on the carbs.
2. Loosen (but do not remove) the band clamps for the front rubber couplers only on the carb end.
3. Remove the air filter box frame mounting bolts. This allows the filter and airbox to move rearward by about 1/2 inch.
4. Disconnect throttle cables, bottom hoses, and the fuel line.
5. Disconnect the engine breather hose and move it out of the way.
6. Work the rear rubber couplers off the carbs. Fold the bottom of them inwards to allow the rear of the carbs to move down.
7. Twist the carb bank down at the rear and pull them back out of the front rubber couplers simultaneously. The rear couplers, being thin-walled, will crush and yield.
8. Once the carbs are out of the front couplers, work the entire bank sideways out the left side of the bike, mashing the rear couplers out of the way as needed. The small gain from moving the airbox rearward is just enough.

Fitting:

Installation is largely the reverse. Using a wedge or tying the air plenum and filter box rearward can ease the process. Leave those rear band clamps completely off the bike initially. Push the carb bank in from the left side and work them across, almost ignoring the rear rubber couplers until the carbs are aligned with the front couplers. Then, wiggle them home into the front rubber couplers. A dull, pointed 'L' shaped scriber can help guide the rear rubber couplers onto the rear carb throat nipples, similar to mounting tyres onto rims. Once in place, push the air/filter box forward and reinstall all band clamps on both the front and rear of the carbs. Reinstall filter box frame mount bolts, hoses, and throttle cables. Be prepared for bandaged knuckles – it's a tight squeeze!

Valve Tappet Adjustment

Regular valve tappet adjustment is vital for engine health and performance. Here's the procedure:

1. Remove the fuel tank.
2. Unscrew the tappet hole caps.
3. Ensure Piston No. 1 (numbered left to right from rider's position) is at Top Dead Centre (TDC) of its compression stroke.
4. Remove the point cover and align the 'T' (1.4) mark on the spark advancer with the timing mark.
5. Check and adjust valve tappet clearances indicated by 'O' in the chart below using a feeler gauge. Loosen the lock nut, turn the adjuster screw, and then tighten the lock nut while holding the screw to prevent disturbance.
6. Inlet Clearance: 0.05 mm (0.002 in.)
7. Exhaust Clearance: 0.08 mm (0.003 in.)
8. Next, rotate the crankshaft one full revolution and realign the 'T' (1.4) mark on the spark advancer to the timing mark. In this position, Piston No. 4 is at TDC of its compression stroke.
9. Check and adjust the valve tappet clearances indicated by 'X' in the chart below, using the same clearances as above.

Clutch Maintenance Tips

The CB500/550 clutches are known for wear, largely due to the design of the slanted cork block faces on the friction plates, which were intended for smoother engagement. However, heavy throttle use could lead to plate heating and warping. Here's what to consider:

• Grease Lifters: The clutch lifters wear more due to frequent shifting. Ensure they are adequately greased.
• Friction Plates: Barnett friction plates, while superior, were thicker and resulted in one less plate pair, leading to shorter life. More critically, their cork bits could wear oil pumps. Today, seek out plates with square-cut cork faces and replace steel plates, as they are likely warped after 10,000 miles.
• Oil Choice: Avoid Valvoline or Havoline oils, as they can over-lubricate the plates, causing slip. Instead, use Castrol (often considered best) or Torco oils, as Castrol and Honda collaborated in the 1970s to find the optimal blend.
• Drag Racing Trick: For drag racing, installing Honda slanted-cork plates and steel plates backwards can provide a spline-clutch-like grab for aggressive starts.

Side Covers

The CB500/550 models feature three styles of side covers. The left cover mountings are consistent across all 500-550 models and years, allowing for interchangeability. However, the right covers differ at the top forward mount position, where the mount tab is rotated 90 degrees. Specifically, the 390 and 404 covers (F style and late K model) are interchangeable with each other but not with the earlier K model 323 frames/covers.

Frequently Asked Questions (FAQs)

Q: When did the CB550 carburettor change?

A: The most significant carburettor change for the CB550 occurred in the 1977 model year, specifically for the CB550K3 in markets like the US, where they transitioned to PD46A carburettors to meet emissions requirements.

Q: Are the CB550K and CB550F carburettors the same?

A: Mechanically, the CB550K and CB550F models were largely identical throughout their model years, except for the carburettor changes introduced in 1977/1978. The specific carb setup and jetting varied between K and F models in earlier years too, as shown in the specifications table.

Q: What is the 'flat spot' on a CB550 and how do I fix it?

A: The 'flat spot' is a noticeable dip in power or smoothness between approximately 3500-4800 RPM. It's caused by the fuel mixture transitioning from rich idle to normal running, combined with the spark advancer reaching full timing early. Solutions include raising the jet needle, installing a slightly smaller main jet, changing spark plug type, adjusting timing, or even modifying the rear sprocket.

Q: What O-rings do I need for a CB550 carb rebuild?

A: Key O-ring sizes include: Main Jet (1.2mm cross-section x 3.5 mm ID), Float Seat (1.5 mm cross-section x 5 mm ID), and Float Bowl Drain Screw (AS568-009, 7/32″ ID x 11/32″ OD). It's often more cost-effective to source these from an industrial O-ring supplier rather than buying full carb kits.

Q: What type of oil should I use for my CB550's clutch?

A: For optimal clutch performance and longevity, it's recommended to use Castrol or Torco oils. Avoid Valvoline or Havoline, as they can cause clutch slip due to over-lubrication of the plates.

Conclusion

The Honda CB550 remains a testament to Honda's engineering prowess, offering a delightful blend of performance, reliability, and ease of ownership. While its carburettors saw specific changes, particularly in 1977, understanding these nuances is crucial for both enthusiastic owners and professional mechanics. By meticulously addressing carburettor setup, tackling common issues like the 'flat spot', and adhering to diligent maintenance practices for components like the clutch and valves, your CB550 will continue to provide countless miles of enjoyable riding, embodying the spirit of classic British motorcycling culture.

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