Monday, July 18, 2022

Burning carbs on the CB550 Honda

Of all the vintage Hondas I have worked on, the carburetor removals on the CB500-550s are about the worst to accomplish in a reasonable amount of time. Of course, on the heels of the CB360T trauma, the owner serves up a 1975 CB550 with 14k miles on it, which is barely running, and wants it sorted out for sale this summer. The bike was faded, dirty, and missing the centerstand. The rear tire was oversized and there was no license plate mounted on the tail light bracket.

The first problem was that the front brake was dragging. I had cleaned out the caliper when I visited the owner’s house, about 25 miles away, a few weeks back. That seemed to have improved the situation, but after getting the bike pushed into my truck for the trip home, getting it back off the truck was a challenge. I loosened the caliper mounting bolts enough to get the brake freed up so I could get it safely off the truck and up into the service rack. Knowing that the caliper was already cleaned up, the issue probably resided in the master cylinder. The little bleed hole appeared to be restricted or mostly plugged up, which causes the brake pressure to remain constant, instead of relaxing when the lever is released.

There was a big scuff mark on the top of the master cylinder cap from a small crash, so it was cost-effective to just order a replacement from and be done with it. Once it was installed and bled the brake function was restored to normal. One down and more to go…

The next big problem was that the ignition key was not a match to the seat lock! The switch had been replaced with an OEM Honda unit with a 21H key number vs. the T2979 lock for the fork and seat. I spent 15 minutes reaching underneath the seat to remove the seat hinge pins, which allows the seat to be loosened up enough to get to the two nuts that hold the seat latch pin loose. The seat can be removed but then you have to take the latch off the frame with the two Phillips screws. One of the screws came out with the impact driver and the other one was in so tight that I had to drill the head off to get it loose.

Removing the carburetors was next and that is never a fun experience judging from my past dealings with 500-550s. The clutch cable dives down between the carburetor bodies on the 550s, as well as a vent tube that also makes its way between carb 3 and 4. Early 500 Fours had a different clutch release system and the cable doesn’t interfere with the carburetors.

The battery was removed and charged, followed by the removal of the rest of the battery box, the air filter housing, then the extraction of the air box behind the carburetors. Honda wrapped the frame around the airbox and there is virtually no room for the carbs to come off of the manifolds to pull them off the bike. After a lot of wrestling and pulling, I was able to remove the carbs from between the head and the airbox. Usually, you can figure out how to disassemble these things, but on the installation, I discovered that removing the clutch cable mount at the back of the engine gives much-needed room to maneuver the air box in and out of the bike. Still having to disassemble half of the bike to get the carbs on and off is not a fun way to spend your time.

Once off, the carbs were in relatively good condition inside, but I replaced the internals with kit parts and replaced the intake manifolds with a fresh set. The biggest problem with the carburetors was that the welsh plugs located at the outsides of the 1 and 4 carburetors were missing! These are not separately available parts from Honda and online searches for a 1/2” (13mm) plug came up dry for the most part. Apparently, there were plugs like that for some lawn equipment carburetors that might work, but most listings didn’t show sizes. I had a full set of used carburetors that wound up being parts donors just for those two little plugs.

I discovered new electronic ignition systems on eBay, obviously from China, as they were $69 vs. the $150+ Dyna kits. I ordered up a kit, which came quickly, but without any instructions which could be a problem for novices. Fishing the old point plate leads out from the little clamps under the engine and then up next to the air box takes a bit of time and patience but it is mostly plugged and play. I put the rotor trigger on backward which lead to a no-start condition until I switched it back 180 degrees.

I had already adjusted the valves and camchain, replaced the spark plugs, and wound up putting on new replacement coils, as the old wires were hardened and had been shortened before to keep good connections.

So, initially, it fired up sounding decent, but the idle was erratic. I was feeding it with a remote fuel bottle so I could make adjustments to the carburetors for synchronization. The engine compression readings were 180, 150, 145, and 170 psi, so you can’t expect really even readings with variable compression issues within the engine.

I had cleaned out the petcock screen and disassembled the petcock to check for any issues, which were not a problem. Adding fresh fuel to the tank, I reassembled the tank and seat, then took it out for a short run around the block. I set out for a longer run, which went okay until I was a mile away then it died. I had only put about 1.5 gallons of gas in it, so switched to Reserve and it started back up again. On the return, it ran well up through the revs, but at the top of the hill, it started to falter again. I kept the revs up enough to get it back to the garage, but it sounded like it was only running on 2 cylinders, plus it started peeing oil off the left side of the engine beneath the countershaft cover.

These engines seem to suffer from flattened o-rings that create oil leaks into the little chamber of the countershaft cover. Other common oil leaks come from defective oil pressure switches and there is also a very large diameter o-ring that surrounds the oil pump where it bolts to the body. After replacing the o-rings on the pump, and test riding it again, the oil drops kept dripping when the bike was on the side-stand. That usually only means one thing… the shift shaft seal is leaking. So, off the cover comes again (the first time required drilling out one of the screws) and confirming the leak at the seal. I don’t usually stock parts for 500-550s, so it was back to eBay for a $10 seal and to wait some more. It took 4 days for a small seal to come from 100 miles away and the part number on it wasn’t a match for what was listed on the auction page or parts lists, but apparently, Honda superseded the part number and the older seal is the same dimension as the upgraded one. One more problem solved.

In the meantime, I noticed a little gasoline drip from the petcock lever, so drained out 4 gallons of gas that I had just put in and removed the tank to check the petcock parts again. There is only one little o-ring that can cause a leak on this style petcock, so I thought that this might be the cause of the leak.

I noticed that there was a special flat washer on the edge of the workbench that seemed familiar, so when the petcock came apart again, I discovered that it had been left off of the reassembly last time. More senior moments…

BUT, as I was draining the gas tank for the petcock check I lifted the gas cap and heard a little swooshing sound as if there was a vacuum being created by the exiting fuel flow. I had just put a new aftermarket gas cap on the tank after seeing the disaster cap on the CB360 that was here previously. What I noticed was that the little v-shaped vent hole on the cap showed some kind of white material that seemed to be blocking the hole. The cap pieces are held together with a small screw, so I unscrewed the cap assembly and found a tiny piece of what looked like cotton that was sitting over the vent hole. I can’t determine if all of these caps are made the same with little cotton balls inside, perhaps to soak up any gasoline splash inside the tank, but I wasn’t leaving it in there. A good 15-minute test run revealed no fueling problems as had happened before. All the header pipes were at the same temperature using the laser temp meter, so removing the cotton ball seems to have resolved the stumbling and stalling condition.

With the petcock reworked and the fuel tank installed, I filled the tank back up again. Rode the bike again with good results overall. I aired the tires up to specs and thought the end was in sight, but then checked the date codes, which ended in 00 and 02, so new tires, tubes, and rim bands were ordered.

While checking the electrical system out, I discovered that this was one of the bikes that Honda had put the infamous beeper on the turn signal system. The signals wouldn’t flash at idle and the beeper made a horrible sound, so it was disconnected. The flasher unit, which was an original Signal Stat 142 unit, which apparently is no longer in production. These are the tiny rectangular flashers the size of two sugar cubes with a little rubber mount that clips the unit to a bracket on the electrical panel. Fortunately, the people at offers an aftermarket unit for about $12, so that went on the order list. The flasher came in promptly and the turn signals are working fine now.

I initially checked around for a centerstand and the mounting hardware, but nothing came up and I got no responses from the Facebook forums that I am subscribed to for vintage Honda bikes and parts. I fed the part number and description to the owner, who miraculously found one that was incorrectly identified on eBay as a Super Sport part (different than the K models). It was purchased and shipped to me for installation. This became another time-consuming and frustrating repair process as one of the exhaust pipes had to be removed to access the frame mounting points in order to push the pivot shaft into the frame mounts and the stand tubing.

The stand was fitted up okay, but then the challenge to try to reconnect the return spring reared its ugly head. With the exhaust pipes in the way on the right side, the access to the end of the spring and the hook on the stand was restricted. I tried all kinds of tricks to try to get the spring connected and even went to the auto parts store and bought a brake spring tool, which was cheaply made in China and didn’t really work in this kind of application even when some extended tubing was used to gain more leverage on the spring end. Finally, I found a thick 5/8” lock washer and used it to bridge the gap successfully. Not pretty or factory approved, but it works perfectly fine, but I spent more than a half hour of my time, wrestling the parts back in place. It’s so much easier to change the rear tire when you have a centerstand to prop up the bike!

The left side dyno cover badge was scrubbed off on a right side tip-over and the replacement parts are in the $100 range, plus the three little clips that are $6-10 each (3 required), so I thought I better check with the owner before putting that shiny piece on an otherwise dull-looking daily driver bike. The reply was a thumbs down on that question.

The bike did get an oil and filter change on top of everything else, so at some point, it will be sent back home with a full service and hopefully regular use at the beach this summer. With fresh rubber and a brake upgrade, it can resume being actively ridden again after probably a 15-year sleep.


  Bill Silver aka MrHonda


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