The recent acquisition of a sad-looking CL72 lead me down the path of the usual revival techniques, including a new battery, fuel system clean out (tank, carbs and petcock), plus a fresh set of tires and cables.
The carbs were pretty gummy and I tossed the bits into my ultrasonic cleaner for several rounds. There was some residual corrosion and grime in the nooks and crannies, but the bike had been running on them previously, I assumed.
CL72 carb specs are #115 main jets, #40 idle jets and a 26.5mm float level. Pretty straightforward, huh? I mounted the carbs on the bike w/o air filters just to get it running and see if the 150/60 psi compression readings would change with a bit of run time. After checking the ignition timing and valve clearances, I hooked up the carbs to a remote fuel bottle and cranked it up. It wanted full choke at first, but settled into a steady idle rhythm with no unusual noises. After about 10 minutes of running, the compression on the low 60 psi side came back up to match the 150 psi on the high side. Things were seemingly looking promising for a quick turnaround for this 62 year-old machine.
I had sealed the fuel tank and rebuilt the petcock, so it was all installed with the seat for a short test ride. Nursing it down the driveway and around the block revealed what seemed to be a big lean flat spot just off idle and an attempt to catch on at higher revs. I limped it home and started troubleshooting.
The aftermarket points and condenser set installed showed running point gaps at .14” and .018” with the backing plate centered on the adjustment slots. Reducing the point gap increases the coil dwell time, but the geometry of the aftermarket points was just off enough not to allow any meaningful adjustments. This was yet another example of the failings of non-OEM point sets on vintage Hondas.
/I had one of the last Pro-Trigger e-ignitions on hand, so decided to install it on this bike to ease the timing woes. Starting the engine up again had no effect on this sudden massive flat spot in the throttle settings. So, onto the carb settings again. I hadn’t removed or checked the main jets in the holders and discovered that the jets were #125! This was puzzling as they were 10 numbers larger than stock. I had accessed a new set of OEM needles which were set at the middle adjustment notch initially. I dug out a set of #115s and installed them, but the flat spot just got worse. It wouldn’t keep running off a nearly full choke condition.
I removed the carbs once again, rechecking all the passages and deciding to change back to the #125s and raised the needles all the way up. Once installed the engine began to show signs of normal life, but not a lot of clean throttle response. I turned back to the ignition system for checks.
In installing the E-ignition system, I removed the spark plug wires from the coil and discovered that the plug wire was not fully engaged into the central spike. The cavity was rusty and corroded so I cleaned out the plug wire hole, but then discovered that someone had connected the coil polarity backwards! I had some spare used coils, so started to check them against the installed ones. I discovered that the secondary resistance readings were 14k ohms on one coil but about 8k ohms on the other five so chose a set of those. I transferred the wire connections to the new set of coils and cut back the ends of the wires to ensure that there was clean wire strands to engage with the central spike. I discovered that the old wire end was not fully pressed into the coil housing, so that was remedied.
It seemed like one cylinder wasn’t firing as well as the other. Checking for spark on a plug grounded to the head, had one side visibly reduced in spark at the gap. I checked the NGK plug caps for resistance problems but they both were reading about 4 ohms. I screwed another fresh cap on the wire and it seemed to get a better spark.
The tank and seat were on and off the bike a half dozen times for test rides that continued to improve, but not achieve a normal function. One problem is that the original fuel tank was replaced with a 1967 tank which does not have a crossover tube function. When fuel runs low on the petcock side, there is still some fuel in the other side of the tank that is inaccessible. One test ride left me feeling that it was running on one side and when I nursed it back home the right side bowl was nearly empty. This brings up another phenomena of Honda twins with duel feed lines coming from the petcock. There are times when the petcock only feeds one carburetor, despite the fact that both lines are the same length and fuel source is sufficient. You can check it sometimes by pulling a fuel line off the petcock and nothing comes out until you plug the petcock fitting with your finger for a moment, then suddenly the fuel begins to flow once again.
There is some kind of fluid dynamics issue going on there, that is not easily understood. If one carb gets filled before the other, then when the line if full, the fuel should transfer to the other port. Perhaps an air lock occurs in the empty line that needs to be bled out to encourage fuel flow once again.
At any rate, the last test ride yielded better running results with the #125 main jets and needles all the way up, however a plug check showed the left side running too rich off the needle setting whereas the right side plug came up clean. I dropped the needle on the left side down to the middle notch and restarted the engine to ensure that it was improving on the left side.
Now that the bike was almost ride-able a new issue of concern arose. Initially, the clutch feel was normal and the bike dropped into gear without the big bang when the plates are stuck together. The bike had been sitting for about six years, so the stuck clutch symptom is not unexpected. However, when I was shifting at low speeds the transmission began to jump in and out of gear while riding, and then it became increasingly difficult to find neutral. Sometimes the light would come on and it was still in gear, all of which leads me to the conclusion that the dreaded engine pull is required to sort out the clutch and transmission woes. This becomes an all day event of removing, disassembling, repairing and reassembling the engine again, then the reinstall into the chassis. These are the reasons why the CL72-77s bikes are not high on my list for repairs and overhauls.
FYI, even if you know what you are doing, a transmission job on a Scrambler is around 6 hours, plus parts. I pulled the engine out, reluctantly, then had to pound away at most all of the screws, then discovered that the clutch cover gasket had been glued on with Honda Bond sealer. The second gear dogs were rounded off and barely had any overlap, which was the cause of the gear jumping. The center ridge on the low gear bushing was rounded down leaving a tiny worn-off ring lodged inside the low gear opening.
I have a number of transmissions from later-model engines, which use the back-cut gear dogs and better gear dog overlap to begin with. The low gear bushing was replaced and almost an hour spent removing the old factory sealant between the cases and then slowly scraping off the clutch cover gasket residues. The clutch plates were replaced with what looks like Barnett clutch discs, but the hub was the original version with 3 retainer wire grooves, but no retainer wires. I re-drilled the hub to secure the first plate and added the vented outer cover. In cleaning the clutch cover I discovered some grooves in the top of the cover where the old primary chain was rubbing up against it, perhaps during the Jumping of the Gears experience. Reluctantly, I installed one of my few remaining new primary chains to ensure that the engine would have a long life. I buttoned up the engine and installed it into the frame. Done for the day. Will finish up in a day or so, then move on to the other projects that are remaining in the shop.
Wrapping up the engine repairs and installation, more problems arose, partly from my lack of care on the assembly of the engine’s bottom end. When the kickstarter cover was installed and the clutch adjuster turned near the index marks, the clutch cable was limp when I pulled in the lever. I started the engine and tried to put it in gear, but it stalled immediately. I first assigned the clutch adjuster misalignment to an extra steel plate installed so the outer clutch plate would just engage with the plate stack. First I double checked the kickstarter cover to see if the steel ball had fallen out. It was still in place, so that left pulling the clutch cover to see what was going on inside.
With the cover off, several things stood out as problem areas. First, the used snap ring that holds the inner clutch hub in place had dislodged, disconnecting the clutch function altogether. I new snap ring was installed and the clutch stack reassembled. When the engine is assembled with the kickstarter cover, you can see how adding or subtracting a plate affects the position of the clutch liver and the clutch adjuster alignment marks. In this case, I removed the extra plate and the marks came closer to alignment, plus the clutch lifter was functioning more normally, arcing through a smooth path when the clutch lever was pulled. As I started to install the oil filter, I noticed that the sprocket was spinning on the crankshaft end! If you are not careful with the conical washer staying in place when tightening down the crankshaft nut, the parts will not stack up correctly and the sprocket fails to spin the oil filter. So, it was a 3 in 1 followup repair, now fully functional again. The third and forth days were devoted to engine removal, repair and re-installation. Final tuning is the last step.
It was still hard to start with the choke, so I opened it fully with the engine still cold and with part throttle, it fired up and caught revs almost normally. I let it warm up then tried the quick around the block test and noticed that the part throttle flat spot was still somewhat present, but once the revs got up around half throttle, it pulled with some authority. I tried a longer half mile downhill/uphill test run and the bike accelerated cleanly with a big handful of throttle, but was still a little lazy off idle. The carb needles are still in #3 slot and the main jets are still #125. After a little cool-down, the plugs came out looking white, giving the indication that the bike was still running somewhat lean, although the sound of the engine seemed to be running perhaps rich at part throttle. At this point, it is trial and error until the end result is a smooth engine take-up from idle towards mid-range and beyond. Ultimately, the final combination was #120 main jets and the needles all the way up. I must have had the tank and seat on and off more than a dozen times, trying to eliminate problem areas and do more diagnostic work to find the causes of poor running episodes.
I
spent the better part of 4 days, trying to noodle out the reasons for
the sudden engine mis-behaviors and had only gotten about 90% of the
issues solved. Make a change, try it out. Make another change, try it
out…. It ain’t easy being MrHonda sometimes. I’ve never run
across a 250-305 that has defied so many attempts to resolve the
running issues. When everything is “correct” as far as
compression, ignition timing, carb settings and ignition component
conditions, it should run as normal.. sometimes that is not the case.
Bill Silver
aka MrHonda
www.vintagehonda.com
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