DAY ONE: I met with the seller at 3:30, loaded the bike up and headed out returning home by 4PM. The wonky transmission symptoms had really peaked my interest and I couldn’t resist tearing right into the clutch cover as soon as it was off the truck. … Spoiler alert! I had the clutch apart and back together and out for a test ride by 6PM. It really DOES have 4 gears after all… no engine case splitting required!
What was discovered, in detail, is that the clutch was a stock 5-plate clutch as seen on any CB/CL77 from 1965-onwards and it showed signs of being “stuck” for awhile in its life. The steel plates had a light coating of rust/friction material and the friction plates were glazed and shiny. The clutch springs had a bit of yellow paint on them and were not the dreaded -275-810 racing clutch springs. The springs were replaced with a new set that seemed to be the same tension and length.
The splined end of the shift shaft was twisted slightly and bent off to one side, causing the shift lever to rub against the clutch cover boss around the shift shaft seal area. This binding friction prevented the shift arm, located inside the engine behind the clutch assembly, from selecting the next gear, unless the shift lever was manually moved with some force.
With the clutch and primary chain out of the way, the shift drum end could be accessed to allow manual shifting of the gears and observing the shift arm following the drum detents. There were no obvious defects in the shift mechanism, so all that was needed was to replace the shift shaft assembly with a good used item from my spares.
In removing the clutch cover, the outer oil filter cover must be removed first. The cover came out with the shaft firmly attached. Closer inspection revealed that the previous mechanic had failed to notice the position of the shaft locating pin when the cover was installed. The pin needs to be in the 11 to 12 o’clock position so it can nestle into the recess machined in the cover shaft boss. What happened was that the pin was driven into the cover shaft boss, wedging it into the soft aluminum. Fortunately, the cover wasn’t cracked and once the shaft was removed the pin position was corrected and the cover installed easily.
There were signs of some kind of non-hardening gasket cement used atop the decaying clutch cover gasket. The old gasket was scraped off and the cover surface cleaned of the old cement before the cover was installed.
The seller mentioned that the choke mechanism was not functioning correctly. Raising the lever would close the choke, but pulling the lever down didn’t draw the choke links back into the open position. Inspection showed that the soldered arm joint had failed, allowing the lever arm to float around loosely instead of directly affecting the choke links.
The engine fires up on NO CHOKE and the idle speed screws did nothing to affect the idle speed in the way of lowering it to normal 1200 rpms. This raises a big RED FLAG to me, which generally is the result of the slides being installed backwards. The engine stumbled at part throttle off-idle and the idle speed screw’s inabilities to affect the engine all lead to the same conclusion… carburetor slides are reversed. That’s an investigation for tomorrow’s work to reveal whether I am correct or not.
DAY TWO – A look under the hood - 8:15 start time.
After removing the seat, the flasher relay assembly was noted, mounted just behind the battery. The rear fuel tank mounting bolt had a big stylized “S” on the head, which I believe belonged to a Suzuki at one time. With the rear of the fuel tank loosened up, there was enough room to access the carburetor tops to assess the condition of the slides, needle positions and to determine if they were reversed or not. Despite my “seat of the pants” evaluation of the carburetion slide issues, the day before, they came out of the carburetor bodies in the correct order, after all! However, the right side slide was dragging out and resisted being reinstalled without force. So the next order of business was to remove the right side carburetor for repairs. Once removed, there were some scratches and bore wear consistent with a warped carburetor body, however there was more to it than that. After de-burring the carb body, the slide still resisted an easy insertion. Closer inspection showed some burrs on the edges of the slide, right where someone had probably used a screwdriver to help raise a stuck slide after years of sitting with old gas in the carburetor. A little use of a smooth file on the edges of the slide allowed it to drop right into the carburetor body with little effort. New o-rings were needed, which brought up another issue with the Honda parts listings concerning the o-ring sizes for both the 250 and 305s…
16173-253-004 is shown as the original part number for the carburetor flange side of the CL72 (or CB72). The 250cc carburetor insulator o-ring is 91302-PF0-003, incorrectly shown as 28.5MM on some sites, but other listings show them correctly as 91302-PF0-003 with 26.9X2.4mm dimensions. The 253 code part supersedes to the PF0 item now. I used two of the PF0 parts on both the insulator and carburetor flange, which fit perfectly, as they should on a 22mm bore carburetor. Many Honda sites are showing the PF0 o-rings fitting the CB77s too, which have 26mmm throat sizes and 25-26mm insulators. The correct o-rings for the 305-sized carburetor and insulator parts are sized 29x2.4mm under the 16173-260-004 part number, which allows for the diameter of the o-ring groove size on the larger bodies. Obviously, you can’t use a 26.9mm o-ring on a 26mm bore carburetor and they sure don’t stretch far enough to fit into a 29mm groove. Someone, somewhere, needs to address this issue and make corrections. Honda, are you listening?
That, of course, was not the only finding that was affecting the high-idle condition. A static check of the ignition timing revealed that the ignition timing had been set at the 45-degree full advance mark, NOT at the F mark, which is about 5 degrees before TDC. A look at the points set showed them to be the Daiichi- brand copies for Dreams, which have repeatedly been proven NOT to fit, in place of the standard Denso parts. Typically, upon installation to the point plate, the whole plate must be retarded as far as the slots will allow, then the gap lowered to minimum levels in order for the ignition timing to line up with the F mark timing stamp. One of the point screws was stripped, to add more frustration to the equation. A good screw was pirated from a used CB77 point plate and it had enough clean threads to hold the contact arm securely.
With the timing set, the carburetor slide free to raise and drop, the bike fired up quickly and settled into an even cadence at an acceptable idle speed. I also noted that the fuel tank was nearly empty, which contributed to the cutting-out experienced during the previous day’s transmission repair test drive. Additionally, the air filters were modified by removing the old paper element and gluing some filter foam to the remaining filter body frames. The frames were apparently soldered together at some point, but are now coming apart at the seams, so a new set of filters is on the parts list now, as are the connector tubes to replace the cracked originals.
Addressing the high-effort clutch pull problem required removal of the kickstarter cover (after removing two of the footpeg bolts on the right side). The clutch lifter arm angle was above horizontal and the wear inside the clutch adjuster gave about 3/8” of sideplay/slack between the two parts. When the clutch adjuster threads are worn that far, much of the clutch lever motion is used up just taking up the looseness in between the parts, before there is an actual rotary motion of the lifter arm to act upon the end of the clutch pushrod tip. Installing a new clutch adjuster reduced the slack to about a millimeter, restoring the rotational motion to act upon the pushrod. With the cover reinstalled, the clutch lever range of motion increased and the lever effort was cut by more than half. Another success for the day, but there is a lot more work to be done!
By 10AM, it was time to break off wrenching on the CL72 to allow for a 100-mile round trip to Oceanside, CA, so a 1966 CB77 could be acquired and returned to Rancho del Honda, but that’s another story!
Bill “MrHonda” Silver