(LONG …read to end.)
I acquired this gorgeous, low mileage, ’76 CB550F Super Sport survivor in the Fall of 2020. The carbs had been previously touched by at least 2 experts, but it ran terribly. Soon enough, another expert (me) would go through the carbs and the bike would still run poorly. Solving this mystery became an obsessive pandemic project. How could a bike that looked so good run so poorly? Read on.
I bought the bike from a friend who is a VERY knowledgeable Honda collector and restorer. He acquired the bike around 2014 from another knowledgeable collector/restorer. Neither fellow could ever get the bike to run “like a Honda.” It would not idle well, had low power and fouled plugs within a few minutes of run time. In 2015, the previous owner sent the carbs out to a professional carb restorer with an excellent, national reputation. The carbs came back and looked fabulous, but there was no operational improvement. Little else was tried until I bought the bike in 2020. Sadly, this bike has been a Garage Queen for much of its life. Hence the very low miles …less than 800 on the clock. The carbs were ultrasonically cleaned once again just prior to delivery to me. I was told the bike still had an unreliable idle.
Once I got the bike delivered, I cleaned the plugs and went for an evaluation ride. The bike started INSTANTLY in 40 degree weather requiring almost no choke …an important clue. I motored away impressed with how silky smooth and absolutely “new” the bike felt. Within 2 minutes of easy riding, it started feeling sluggish and “ragged” then dropped a cylinder. I turned around and headed home dropping another cylinder as I pulled into the driveway. I tried a bit of easy revving before I shut off the engine. It would run OK above 2000 rpms on all 4 cylinders, but below 2000 rpms it would only run on 2 cylinders. Bingo! There was an easy ignition fix in my future!
Think again. Pulling all 4 plugs I discovered that #1 and 4 were heavily fouled with dry soot. #2 and #3 were also fouled, but not quite as bad. My initial conclusion from the first ride: Significant ignition problems compounded by carb problems. Just my luck. In my experience, motorcycle tuning problems are almost always “combination” problems.
Curiously, the OEM headers had some “discoloration.” This could be just poor quality chrome (likely) or a prior “lean” situation. Hard to say.
A few minutes of testing with my multimeter confirmed that I had one “out of spec“ coil and some very tired spark plug wires and caps. No surprise since all this hardware was original equipment and 44+ years old! This gave me my first priority: resolving the ignition issues.
Before I delved into the ignition, I followed the “Sacred Sequence of Diagnosis” - Compression, Ignition, Carburetion. Always in that order. No departures or shortcuts!
First, I did a cam chain tension adjust, valve lash check/adjust and then a cold compression test: All 4 cylinders were at 170 psi …confirming the like new health of the engine.
Next, I re-did the entire ignition with all new parts: coils, wires, caps, plugs and a DYNA/S ignition. I installed a relay to supply full battery voltage to the coils at all times. This eliminated trivial voltage drops in the wiring harness. I also replaced the OEM rectifier and OEM voltage regulator with an upgraded electronic “combo” unit by Rick’s. The MotoBatt battery is new and healthy. The charging system was verified 100% functional. My goal was a reliable rider …not a museum display.
With perfect compression, perfect ignition, new plugs and air screws at 2 turns out - I did another road test. The bike ran great for 5 minutes, then began fouling plugs. Before it began fouling plugs, it had a smooth idle with no ignition “drop out” at lower rpms. At speed, it ran smooth and made decent power but seemed sluggish and “fat." This satisfied me that the ignition was now fine and that I could address the carb problems.
I dropped one of the carb bowls to take a look. The bowl was very clean. The overflow tube was clear with no leaks. Float adjustment/operation looked fine. I determined that I had #038 idle jets (correct) and #100 main jets (incorrect, but hardly enough to matter much). I decided to remove, inspect, clean and adjust the carbs to satisfy my curiosity and resolve what I expected to be some obvious, minor problems.
The carb cleaning / inspection was unremarkable and totally non-revealing. Everything was pristine and looked fine. The brass bits were mostly OEM Honda parts. The OEM needles showed negligible wear and the needle clips were in position #2 (correct). The emulsion tubes (needle jets) looked OK and had no markings. I carefully cleaned the #038 idle jets and verified the orifice size. I decided to retain the #100 main jets. That seemed “close enough.” The main jet o-rings seemed new, and were the OEM-spec size. I did notice that they were not very snug in the jet towers. This did not concern me as the jet retainer would ensure they remained in correct position. More on that later. Read on.
I hoped to find some blocked or compromised air circuits, but all were clear. I checked the floats and float valves (fine), made some minor float height tweaks and replaced the float bowl gaskets. I swapped in a different new air filter (aftermarket, but OEM type). Since the jet needles had been disturbed for inspection, I knew I would need to do a carb synch. after reinstalling the carbs. Based on my inspection, the bike should run perfectly.
On the day I reinstalled the carbs it was raining, so I warmed up the bike inside and did the carb synch. The bike idled very well and the synch went well. The idle air screws showed good responsiveness indicating that that idle fuel and air passages were in fact clear. I set them at 1 1/2 turns out (spec). I thought I was done and that the next test ride would confirm my assessment. I was wrong.
On the next sunny day, I went for a ride. The bike ran well for 10 minutes but seemed progressively “ragged” and ”fat” as it warmed up. Then, it started to foul plugs while cruising.
Back home, I confirmed that all 4 plugs were badly soot fouled! I cleaned the plugs, opened the air screws to 2 1/2 turns out, removed the air filter altogether and went for another test ride. The bike ran much better …confirming that my problem was over-richness. But, it still fouled plugs after 20 minutes. Weird!
Summarizing My Symptoms:1. Starts great with practically no choke …even in very cold weather.
2. Runs great when cold …progressively worse as it warms up.
3. Becomes sluggish and “fat” due to over-richness when warm.
4. Reliable, smooth idle.
5. Eventually soot-fouls spark plugs.
6. Runs best with no air filter, but still not great.
So I went to work doing research. I made a list of every possible cause of over-richness in the mid-range and above. Then, I removed the carbs to start over and systematically eliminate possibilities one-by-one. Since these carbs are so hard to remove and install, I did not follow my own usual advice of making only one change at a time. Instead, I made a sweeping set of changes based on research and experience.
Before I began, I re-verified perfect compression and re-verified perfect ignition/charging function.
I will spare you extraneous details and theory, but here are all the changes I ended making on these carbs:1. Installed a different set of #038 idle jets with the top 4 cross-drilled holes enlarged to 1.0 mm.
2. Installed another set of air mixture screws with the correct taper. The ones I removed had incorrect taper …too “blunt” (12 degrees vs. 18 degrees).
3. Replaced #100 main jets with #098 main jets (Keyster)
4. Replaced main jet o-rings with custom o-rings to obtain a snug fit inside the jet towers and eliminate any possibility of fuel by-passing the main jet. Replacement aftermarket o-rings (from 2 different vendors were badly undersized.) OEM Honda o-rings were also loose. I ended up using custom-sized o-rings: 3.5mm (ID) x 1.2mm (cross-section). (See: O-ring Addendum below.)
5. Installed Keyster #D274 jet needles (slightly “leaner” than OEM). These have 2.51mm straight portions …OEM needles are 2.50mm.
6. Clip position #2 (second groove from top …spec setting).
7. Installed new Keyster needle jets (emulsion tubes) with the top 4 cross-holes enlarged to 0.85mm. The emulsion tubes I replaced could not be ID’d, but all the cross-drilled holes were extremely undersized. I think this might have been a manufacturing error or the wrong part entirely? I also believe these brass parts might have been substituted in by a previous owner attempting to get the bike to run after a long lay-up. In any case, the jet portion of the emulsion tubes showed heavy wear upon close inspection. That does not match what you would expect in low mileage carbs like these!
8. Floats set carefully to 22.5mm (spec is 22.0mm)
9. Air screws set to 1 1/2 turns out (spec.)
10. Installed free-flow UNI Air Filter.
The next road test confirmed that these changes completely resolved all carb issues! The throttle was a little ‘lazy” settling to idle, otherwise perfect. That was consistent with the need to do another synchronization since the needles had been disturbed.
I did another synch which resolved that issue and smoothed out the idle. I made several passes at air screw tweaks and ending up at 2 1/2 turns out as the best setting. That’s at about the “limit” of proper adjustment on air screws, so I could probably move to #035 idle jets to get the air screw closer to the optimal setting of 1 1/2 turns out. I’m too lazy to mess with that since it runs perfectly with the current hardware.
I just verified my work with a long test ride. The bike runs fantastic now …like a Honda! Great power, linear throttle response, no bogs, hesitations or surges and no plug fouling!
In the end, I made a lot of small changes, but I think the actual main culprits and “fixes” were:1. Main jet o-rings too “loose” …allowing raw fuel to by-pass the main jet.
2. Main jet too large (#100 vs. #098)
3. Worn out needle jets (emulsion tubes).
4. Cross-drilled holes in the needle jets were WAY too small. Manufacturing error?
5. Cross-drilled holes in idle jets too small.
6. Wrong taper on air screws
7. OEM-type (paper) aftermarket air filter too restrictive.
Based on how well the carbs responded to jetting changes in the direction of “more air,” I also think my stock exhaust may be slightly more restrictive than Honda intended …perhaps a manufacturing variance on my pristine example.
Early on, I was so desperate that I even considered performing the “Ferracci mod” on the exhaust. This procedure was once recommended to me by Mr. Ferracci himself as I was dealing with a stubborn tuning challenge on my ’90 Ducati 750 Nuevo Sport. This mod is well-known in Ducati circles. It involves perforating the internal OEM muffler baffles to reduce restriction and improve flow. Adding exhaust flow effectively reduces over-richness. Adding 1/2” holes in two baffles is roughly equivalent to a change of about one jet size. The procedure can be done in about 2 minutes without any disassembly.
But, I’m glad I decided not to do that. In the end, it was not necessary. My pristine exhaust is irreplaceable and greatly increases the collector value of the bike. The “Ferracci mod” does increase exhaust noise and I like the super-quiet Honda exhaust I have now. So do my neighbors.
Full Disclaimer:I’m a notorious carb “expert” and I (generally) know what I’m doing. Every set of carbs and engine combination is unique. This set of mods worked well for me. They might not work well for anyone else.
Fuel Issues: Air-cooled, 4-stroke Hondas of this vintage are generally jetted a bit on the “rich” side to allow easy starting and reduce warranty claims in warm climates. I think Honda may have over-compensated on this issue.
I run non-ethanol gasoline exclusively. If I were to run ethanol-diluted “corn gas” commonly marketed in the US as E10 pump gas, it would change the final jetting somewhat. “Gasohol” is inherently leaner than real gasoline. I don’t run modern pump gas except in emergencies …too many shortcomings in that fuel! My jetting is optimized for real gasoline.
There’s one takeaway that you can count on. Always follow the “Sacred Sequence of Diagnosis” - Compression, Ignition, Carburetion. Verify perfect compression and ignition before ANY carb tinkering. Otherwise, you’re probably wasting your time and adding to your frustration.
Main Jet O-Ring AddendumFun Fact: As the Founder of Randakk's Cycle Shakk (now retired), I've personally commissioned the manufacture of literally millions of o-rings used in carburetor rebuild kits. No brag, but I know more about carburetor o-rings than most people.
Which is why I was so pissed when I bought a CB550F carb rebuild kit during this saga from a prominent vintage motorcycle aftermarket vendor (not Randakk's) that contained poorly sized main jet o-rings.
The spec for the o-ring in question is 4.1mm x 1.1mm. The jet is a "push fit" type with a spring-type retainer. So, the fit of the o-ring is critical. A "loose" fit o-ring can allow fuel to bypass the jet, totally corrupting the air/fuel ratio ...sending it WAY rich.
The cross-section of the o-ring sent to me in the kit was only 1.0 mm! That was way too "loose" to seal anything.
So, I secured more main jet o-rings from another prominent aftermarket vendor (also not Randakk's). They sent me a different size of o-rings ...also seriously undersized!
Next, I sourced correctly sized, new OEM Honda o-rings in Honda's "packing set." Alas, even those were not snug enough to satisfy me. The fit of the main jets inside the towers was still a rather "sloppy" fit.
My solution: I bought a selection of commercially sourced o-rings in various alternative sizes ...all slightly larger than spec. Each had the correct chemical composition and durometer rating (hardness). I reduced the ID on some as well, introducing more "stretch" fit into the jet's o-ring groove ...to "thin" the effective cross section allowing a larger cross section to fit when it would not otherwise.
I ended up using this size as optimal:
3.5mm x 1.2mm
Problem solved, no issues!
