There was never much bad to say about the Honda 500 Four, so why enlarge its bores by a mere 2.5 millimeters to make a 550? Several reasons: first, and probably the least important, is the presence of Suzuki’s 550 - Honda didn’t want to lose any sales over a piffling 50ccs; second, the consumer woudld be more likely to absorb a price increase to $1,600 without squawking if their getting more motorcycle for the money; and, third, the extra displacement - tiny as it is (a guppy could barely turn around in a 50cc fishbowl) - actually added a noticeable amount of mid-range power and torque.

Cosmetically, the CB550 was a smaller version of the CB750 from the 4 pipe K model to the four-into-one F model. Technical Specifications for the CB550 remained virtually unchanged over the model lines. Like it’s larger sibling, the 550 proved a reliable, comfortable, easy-to-maintain, every-day motorcycle. Perfect for the time.

Model Guide


Serial Numbers

Model Model Year Frame Engine
CB550 K0 1974 CB550-1000001- CB550E-1000001-
CB550 K1 1975 CB550-1200005- CB550E-1029182-
CB550 K2 1976 CB550-1230001- CB550E-1067334-
CB550 K3 1977 CB550-2000007- CB550E-2000001-
CB550 K4 1978 CB550-2100001- CB550E-2100001-

1974 CB550K0

Boss Maroon Metallic Flake Sunrise Orange Freedom Green Metallic

The CB550K was introduced in 1974 as an update to the CB500 in the US market. The models were cosmetically similar; mechanically, aside from the extra 50cc displacement, significant improvements in the transmission. The 1974 CB550K was offered in three colors: Flake Sunrise Orange, Boss Maroon Metallic, and Freedom Green Metallic. The tank featured a two-tone paint scheme with the base color accented by a black panel and gold striping. The side covers were painted the base color.

1975 CB550K1

Candy Jade Green Flake Sunrise Orange

The CB550 K1 received dark green instrument faces and the tank featured a gold and black accent stripe. Two color options were available, Candy Jade Green and Flake Sunrise Orange.

1976 CB550K2

Candy Garnet Brown

The 1976 model was offered in a single color, Candy Garnet Brown. The tank paint scheme was carried over from the 1975 model. The only other significant change was to the instrument faces, which were now light green.

1977 CB550K3

Candy Garnet Brown Excel Black

The 1977 CB550 K3 paint options were Candy Garnet Brown and Excel Black. The tank was now painted in a single solid color and featured a wider accent stripe in red and gold. The gas filler was now recessed under a locking cover panel. The side covers featured a new emblem, “550 Four K”. In the US, the carburettors were changed to meet emmissions requirements. The 4-into-4 exhaust was restyled.

1978 CB550K4

Candy Alpha Red Excel Black

The 1978 model was offered in two colors, Candy Alpha Red and Excel Black. Changes from 1977 include a gold pinstripe on the side covers and a dual contour seat with a stepped section for the passenger.


Serial Numbers

Model Model Year Frame Engine
CB550 F0 1975 CB550F-1000002- CB550E-110004-
CB550 F1 1976 CB550F-2000003- CB550E-1109887-
CB550 F2 1977 CB550F-210001- CB550E-1135380-
1976 Shiny Orange 1977 Candy Sword Blue 1977 Candy Presto Red

1975 CB550F0

The CB550F featured a 4 into 1 exhaust similar to the CB750F. The tank was painted in a solid color, Candy Sapphire Blue or Flake Sunrise Orange without pin striping. The side covers featured a ‘550 Four’ decal. Instrument faces were dark green with white numbers.

1976 CB550F1

The colors options for this model year were Flake Sapphire Blue and Shiny Orange. The seat covering was dark brown and instrument faces changed to light green with white numbers.

1977 CB550F2

This model was offered in Candy Sword Blue or Candy Presto Red. A wide gold stripe was added to the tank and the side covers were painted black. The seat covering was black and the fork boots were eliminated.


The CB550K and CB550F models were mechanically identical throughout the model years. There were some carburettor changes in the 1977/1978 years which are detailed in the tech library.

Engine Air cooled 8-valve SOHC transverse four
Bore X Stroke 58.5 x 50.6mm
Capacity 544cc
Comp. Ratio 9.0:1
Claimed power 50bhp @ 8,500rpm
Torque 30.4ft-lb @ 7,500rpm
Carburetors 4 x 22mm Keihin
Gearbox 5-speed
Tires, front 3.25 x 19in
Rear 3.75 x 18in
Brakes, front Lining Area: 288.8 sq. cm X 2
Brakes, rear Lining Area: 169.6sq. cm X 2
Suspension, front Telescopic forks, 35mm
Caster Angle 64 degrees
Trail Length 105mm (4.1in)
Suspension, rear Twin shocks with adjustable pre-load
Wheelbase 1,405mm (55.3in)
Seat Height 805mm (31.7in)
Weight 192kg (423lb) wet (F model slightly less)
Fuel capacity 14 liters (3.7 gallons)
Top speed (approx.) 102mph (164kph)


The CB550 FAQ is a compilation of useful information from the SOHC/4 Forums.

CB550 Side Covers

There are three styles of covers.

CB550 Right Side Covers CB550 Left Side Covers CB550 Left Side Covers

The left cover mountings are the same for all the 500-550s all years. Swap around as you chose. The right covers differ in mounting at the top forward mount position where the mount tab is rotated 90 degrees. The 390 and 404 covers swap among frames F style and late K model, but not with the earlier K model 323 frames/covers.


When rebuilding carbs on the old Honda fours with press-in main jets, you will often find that all the metal parts are fine after a good cleaning, and all that you really need to buy are new O-rings. Honda doesn’t sell the individual O-rings separately…. instead you have to buy a complete carb kit at $10 to $15 per carb. I found an industrial O-ring supplier who was willing to do small-volume counter sales, and was able to get new O-rings at about 50 cents per carb. Sizes are:

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)

Shifter lever seal = inside diameter – 14mm, outside diameter – 24mm, depth – 5mm And the one behind the sprocket = I.D – 33mm, O.D. – 57mm, depth – 7mm

Valve Tappet Adjustment

Figure 1 Figure 2 Figure 3 Figure 4

The Procedure:

  1. Remove the tank.

  2. Unscrew the tappet hole caps.

  3. When the No. 1 piston (pistons are numbered from left to right from the rider’s position (Fig. 1)) is at top-dead-center of the compression stroke.

  4. Remove the point cover and align the “T” (1.4) mark on the spark advancer to the timing mark (Fig. 2)

  5. Then check and adjust valve tappet clearances indicated by “0″ in the chart below.

  6. Measure the clearances using a feeler gauge, adjust by loosening the lock nut and turning the adjuster screw, and tighten the lock nut. (Fig. 3)

    Valve tappet clearances
    INLET 0.05 mm (0. 002 in.)
    EXHAUST 0.08 mm (0. 003 in.)
  7. Next, rotate the crankshaft one revolution and realign the “T” (1.4) mark on the spark advancer to the timing mark (Fig 2)(in this position, the No. 4 piston is at top-dead-center of the compression stroke). Then check and adjust the valve tappet clearances indicated by ” X ” in the chart below. See item #5 above for proper valve tappet clearances.

  • Hold the adjusting screw so that it is not turned when tightening the lock nut.
  • Make sure the clearance is not disturbed when the lock nut is tightened.
Cylinder Inlet Valve Exhaust Valve
No. 1 O O
No. 2 X O
No. 3 O X
No. 4 X X

Keihin Carb ID Numbers and Jetting

Keihin Carb Guide

Increasing your fuel mileage

Submitted by Mystic (and others)

Not trying to sell anything, just thought I would regurgitate an old trick that went around the dealership I worked at in the 70′s. Since we’re about to have another gas crisis it just seems appropriate again. This is all from memory and was confirmed by myself. Don’t know if it works with other bikes but it should.

I don’t have parts in front of me now but it went something like this… Remove main jet,… Look into throttle bore and see a small brass thing sticking up,.. push down on it with a screw driver and it should fall out. Crawl around on knees till you find it and you will be holding the emulsifier tube. It has a series of holes drilled on opposite sides. Drill more of the same size holes between the existing ones, essentially doubling the holes. Do one at a time and put it back in the same way it came out. Compare to other carb to get the orientation.

This tube mixes air with the fuel coming up around the needle and makes it sorta foamy (or emulsified) Apparently it atomizes better when it enters the airstream as a foam? The main jet and needle/needlejet do the fuel metering. somewhere there’s an air jet feeding the emulsifier area. None of these things have changed and no re-jetting is required,.. It just emulsifies better.

I did this on my 75 CB550 right before a 750 mile trip. Prior to that it got around 50-52 mpg. After the mod with no other changes it got 60mpg. Now I’m old and forgetful but I remember this because gas was 60 cents a gallon so it cost me exactly a penny a mile or $7.50 for the whole trip!!! I also remember my buddy had a CB360 and we got the exact same mileage so he was impressed!

Does anyone else remember this? The grapevine I heard it through was comprised of reputable mechanics and I heard that there was a Honda bulletin describing it but I never saw it.

Possibly it just corrected a rich condition at part throttle and will only help if your bike needs it,… so I’ll go with the standard disclaimer. “Your mileage may vary” More I was curious about this myself so I did some googling and turned up the following:

From The Secret Life Of Carburetors:

The effect of the emulsion tube will depend on the hole pattern. Here is how to read it: First, hold the emulsion tube upside down and inspect the hole pattern. Holes at the top of the emulsion tube will affect the top-end of the rev range. Holes in the middle will trim the mid-rpm range, and holes at the bottom, the low-rpm range. Where there are no holes, the mixture will be rich. Where there are holes, the mixture will be leaned out. Just how much the mixture is leaned out by the presence of holes depends on how many, and how big. The more holes present, the more the mixture is leaned out at that point. Because it is fed with air from the air bleeds, the emulsion tube’s overall function is influenced by the air bleed size. A larger air bleed leans out the mixture, but at low rpm and small throttle openings, the air bleed has little influence over the mixture. As the engine’s demand for air increases due to an increase in throttle opening and rpm, so the air bleed’s influence increases. At high rpm, just a few thousandths change in the air bleed diameter can have a significant effect on mixture. One other aspect of the emulsion tube and well is that they act not only as a means of calibration but also as a control element for fuel atomization. By emulsifying the fuel prior to it reaching the booster, the fuel is easier to shear into fine droplets at the point of discharge. Generally, the more it is emulsified with air in the emulsion tube, the easier it is to atomize at the venturi. From Rotary Engineering:

Emulsion tubes control the metered fuel and air introduced into the Carburetor. When air enters the emulsion tube through the air jet and fuel enters through the the main fuel jet this condition emulsifies the fuel delivered into the carburetor. The emulsion tube has a series of small holes from top to bottom which regulate the fuel mixture. These holes allow air and fuel to enter the main circuit and emulsify fuel. Low speed engine conditions or engines at idle do not require use of the emulsion tube or the main circuit. As engine speed increases the fuel level in the float bowl drops uncovering these holes and allowing air from the air jets to enter the main circuit resulting in a lean mixture. As the engine speed increases the fuel level in the float bowl continues to drop. This uncovers even more of the holes in the emulsion tube, which makes the air jet have a greater effect on the low to high rpm fuel delivery mixture. From Pre-emulsion bleed formulas:

From memory, as the air and fuel are flowing at low speed, the air only enters the emulsion tube through the holes high up, but as speed increases, the air travels further down the well. If the top holes are to big, to much air enters the tubes at the top at high speed and does not mix well thus giving poor fuel delivery quality, if they are to small, not enough enters at low speed, giving poor fuel delivery quality. Also, the bigger the air leak into the emulsion tube via the air correction jet and the emulsion tube holes, the leaner the mixture as the pressure drop across the main jet is reduced and there is more air introduced to the metered fuel. Interesting stuff.


More (HondaMan)

I would make a minor correction to the author from Rotary Engineering, as regards our carbs, though:

It’s not the float bowl level that drops, in our carbs, but the level inside the tube surrounding the main jet holder (aka emulsifier tube). This is the result of the limited flow from the size of the main jet itself. The float bowl level does drop in real life: at engine speeds over 6500 RPM, the level drops about 2-3mm.

However, this 2-3mm would have little effect inside the emulsifier section, as the holes are spread out over about a 10mm length. But, the main jet itself limits how fast the tube will “fill back up”, and it is this pumping action that defines these carbs as “pulse carbs” instead of “flow carbs”, like found on cars. Each engine intake stroke sucks a little fuel out of the emulsion chamber (above 1200 RPM, anyway), and that level starts dropping as the RPM rises, exposing more holes, “bubbling” more the remaining fuel for atomization.

The impasse comes at wide-open throttle (WOT). These carbs run out of mixing ability above 7/8 throttle: that’s the nature of the beast. This is why all bikes like these don’t seem to “have any more” in the last 3/4 turn of the handle: the fuel level has reached the bottom of the emulsifier chamber, and the fuel is rising straight from the bowl to the needle jet, and not enough makes the trip: it runs lean. Raising the float level of the bowl helps a little, at the risk of leaking around the edges when running at more normal speeds. Our “standard” change for road race applications was to raise the float bowl 2mm in the 750 (24mm) and then seal the float bowls with new gaskets about twice a year.



Remove completely the band clamps for the rear rubber couplers on the carbs. Loosen the band clamps for the front rubber couplers only on the carb end. The air filter box frame mounting bolts must be removed so the filter and air box can move rearward about 1/2 inch or so. Remove throttle cables, disconnect bottom hoses and fuel line. Disconnect engine breather hose and move it out of the way. Work the rear rubber couplers off the carbs at the rear. Fold the bottom of them in on itself so the rear of the carbs may move down. Twist the carb bank down in the rear and pull them back out of the front rubber couplers simultaneously. The rear couplers will crush some with the band clamps gone. Once the carbs are out of the front couplers work the bank sideways out the left side of the bike mashing the rear couplers out of the way as needed. The rear rubber couplers are thin wall, more compliant, and flop around way more than the front ones. With them yielding, and the 1/2 to 3/4 inch gain from moving the air and filterbox rearward, you get just enough room the get the carb bank out.

Fitting: Going back in is pretty much the reverse. Using a wedge or tying the air plenum and filter box rearward eases installation. Leave those rear band clamps completely off the bike. 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. I use a dull pointed “L” shaped scriber to guide the rear rubber couplers onto the rear carb throat nipples in the same fashion tires are mounted onto rims. Push the air/filter box forward and reinstall all the band clamps on front and rear of carbs. Reinstall filterbox fame mount bolts. Reinstall hoses. Reinstall throttle cables.

Bandage knuckles.


If you’ve ridden lots of other bikes, you’ve probably noticed a “flat spot” between about 3500-4800 RPM on the “mid-four”. This is caused by several factors, all relating to certain things Honda wanted this bike to do:

  1. It was supposed to idle well and start very easily. This requiired an idle A/F mixture of about 14.25:1, a little rich.
  2. It was supposed to be quiet. Long intake runners (distance between the carb slide and intake valve) helped this.
  3. It was supposed to be easy to maintain and not leak fuel, so that carbs were to sit horizontally.
  4. It was supposed to be even and smooth through the whole throttle range.
  5. It was supposed to cruise easily at highway speeds 55-75MPH and get good MPG.

It did all these things, and very well. But, the tradeoffs of design caused by the long intake runners (#2) and the angled direction change into the heads (#3) made #4 and #5 harder to obtain. The richer idle had to lean out at running speeds to get good MPG. To smooth all of these things out a little, the spark advance curve was made quick, quicker than the other fours of the day. The result: between 3500-4800 RPM (or so), the mixture was slipping from richer-than-normal to normal while the spark advancer had already reached full timing. It made the torque curve flatten out in that range, where most bikes are just getting stronger.

Here’s some simple things you can try to smooth over this “smoothie” feature.

  1. Raise the jet needle in the carb slides one notch. Install a 10-size smaller main jet at the same time (5 size smaller on last-year CB550). Switch from the standard D7E (NGK) or X22E (ND) sparkplugs to the D8E (X24E) at this same time. Check the color of your plugs to make sure it does not get too lean, which can happen if you’ve installed indivdual air filters and/or headers that actually work (most did not), or longer, megaphone-type mufflers.

  2. Advance the timing 2 degrees static, but cut off one turn from the springs on the advancer and reshape the end of the next coil so the springs will fit back onto the advancer mounts. This slows the advance curve about 5%-8%, depending on the year of your bike.

  3. Add 4 teeth to the rear sprocket. This raises the RPM a little, which shifts the lower-than-normal torque curve downward a little to a point where the torque-vs-acceleration is not so noticeable.

  4. Test out your octane ratings. Start with a tank of Regular, then try Mid-Grade, then premium. You’ll see how they affect this “flat spot”. Here’s some harder things you can do to smooth this anomaly while increasing the power a little:

  5. Smooth the insides of the intake runners and match the ports.

  6. Polish the intake valves.

  7. Shorten the intake valve guide bosses about 2-4mm. Narrow them in the flow direction, but don’t get thinner than 50% of the original thickness.

  8. Change from the stock air filter (paper) to a K&N filter. Open up the airbox intake holes about 25%.

  9. Install a cam with 7 degrees more duration and advance it 3-4 degrees. Action Fours used to make one of these in the early 1970s.


PD46A Carb Rebuild Guide

This guide was written by forum member, Brian B. It is an excellent step-by-step carb rebuild guide with photos. The PD46A carbs were used on the 1977 and 1978 CB550′s.


Carb Sync Video


First word: GREASE! The clutch lifters on these bikes wear more because there is much more shifting going on. Grease those lifters!

We tried all kinds of clutch mods. The stock clutch would wear because of the slanted cork block faces, which were designed to let the oiled plates squish out the oil slowly and ease the engagement (part of the “smooth 4 effort” of the 500). Heavy-handed throttles then caused plate heating and warpage, making the faces engage less, then they wore quickly.

Barnett jumped in with their superior friction plates, but they were thicker, so their sets had 1 less plate pair than the Honda set. Result: same grip, less life. Even worse: the Barnett cork bits wear the oil pumps, causing low oil pressure after a while.

Solution: today, the plates are available with square-cut cork faces. Find these and use them. And, replace the steel ones, too. They’re warped if you have 10,000 miles on them, believe it. Also, DON’T run Valvoline or Havoline oil. These excellent oils overlube the plates and make them slip. Instead, use Castrol (best) or Torco (next best) oils. Castrol and Honda worked together in the 1970s to get the right blend: trust ‘em.

If you’re drag-racing: get the Honda slanted-cork plates and put them in backwards. Put the steel ones in backwards, too. They’ll grab like a spline clutch and break theat rear wheel free at the green light!


Model Year CB500: 71-73 CB550K 74-76 CB550F 75-77 CB550K3 77 CB550K3 78 CB500K3 77
Carb setup # 627B 022A 069A X46A PD46C ?
Keihin Carb Type 1 1 1 PD PD PD
Slow Jet #40 #40 #38 #38 #42 #42
Main Jet #100 #100 #98 #90 #90 #90
Jet Needle and Taper #2.5 2.515 - 2°30’, 4 grooves 2.495-3°00’, 2 grooves E2349F, 3rd groove ? E2350F, 2nd groove
Needle Jet Air Bleeds ? 0.9 x 2 (1,2,3,4,5) 0.7 x 2 (1,2,3,4,5) ? ? ?
Air Screw Opening 1 +/- 1/8 1-1/2 +/- 3/8 taper 12 1-1/2 +/- 1/2 taper 18 NA NA NA
Pilot Screw Opening 2 1 1/2 1 1/2
Air Jet #150 ? ? #130 ? #120
Slow Air Jet #150 ? #150
Float Height (metric) 22mm 22mm 22mm 14.5mm 12.5mm 14.5mm
Float Height (english) 0.89in 0.89in 0.89in 0.57in 0.49in 0.57in


This is a video series by youtube member SocoMoto.

Part 1 Part 2 Part 3 Part 4 Part 5