CB550 Side Covers

There are three styles of 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.

Carb O-Ring Sizes

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

Increasing your gas 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”
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.

Removing and Re-fitting CB500/550 Carbs


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.

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.

Dealing with the CB500/550 Flat Spot

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:

  1. Smooth the insides of the intake runners and match the ports.
  2. Polish the intake valves.
  3. 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.
  4. Change from the stock air filter (paper) to a K&N filter. Open up the airbox intake holes about 25%.
  5. 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.

CB500/550 Clutch Tips

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!

CB500/550 Carb Specs

Model YearCB500: 71-73CB550K 74-76CB550F 75-77CB550K3 #8211; 77CB550K3 #8211; 78CB500K3 #8211; 77
Carb setup #627B022A069AX46APD46C?
Keihin Carb Type111PDPDPD
Slow Jet#40#40#38#38#42#42
Main Jet#100#100#98#90#90#90
Jet Needle and Taper#2.52.515 #8211; 2°30', 4 grooves2.495-3°00', 2 groovesE2349F, 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 Opening1 +/- 1/81-1/2 +/- 3/8 taper 121-1/2 +/- 1/2 taper 18NANANA
Pilot Screw Opening21 1/21 1/2
Air Jet#150??#130?#120
Slow Air Jet#150?#150
Float Height (metric)22mm22mm22mm14.5mm12.5mm14.5mm
Float Height (english) 0.89in 0.89in 0.89in 0.57in0.49in 0.57in