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The Unfriendly Benly … a mystery noise stumps MrHonda for awhile

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This is the second red CA95 that has come my way in the past month. The first one needed a head gasket replaced and the tail light socket repaired due to a short circuit when the brakes were applied. Not a big deal, maybe 4-5 hours of wrenching and a few parts, then off it went back to the big OC.

Benly #2 belonged to a friend in San Clemente whose Honda Dream put me through some challenging house-calls way back in 2009. I hadn’t heard from the owner much since then, but fired a message off to me asking for help on his 1964 CA95 Benly. Complaints were centered on reduced performance, gear shifting issues and an odd “chain noise” that had a rhythmic cadence to it. This time he brought the bike down to me and we fired it up to get some idea of what was happening. Sure enough, there was an odd clattering sound that sounded like it was tied to engine rpms, but maybe at ¼ speed or more.

It seemed to be coming from the left side of the engine, where the camchain and tensioners are all located. I did a “layover” with the bike, leaning it up against the shop fence at a 45 degree angle to help retain most of the engine oil while we tore into the dyno side of the bike. The footpegs had to come off, then the muffler, then the outer covers, the rotor and starter clutch hub and housing. This exposes a lot of the camchain and idler/tensioner hardware, but little seemed to be out of place or excessively worn to the extent that it would generate an odd noise such as this.

The bike had a top end done back around the time we first met and the owner did most of it himself. He had a machine shop bore the cylinders out to .75mm and installed new pistons and rings. There was some discussion about excessive end gap at the time, but I didn’t recall the details or give it much thought. After inspecting the parts and adjusting the camchain visually before closing it up again, there were no stand-out issues noticed and of course the noise was again present when the bike was fired up again. Having hung around for nearly 4 hours, the owner decided to leave the bike and return to SC. I continued to remove the clutch side of the engine looking for a “smoking gun” in a loose part or something that was worn and knocking around in the clutch area.

The only noise-producing potential item seen was a somewhat wobbly clutch outer, which is gear driven off of the crankshaft. Looking at online microfiche drawings and part numbers, apparently there were thrust washers on the back and front side of the clutch outer on early models, but the second thrust washer was discontinued on the later editions. There was a thrust washer on the input shaft, up against the clutch outer hub hole, but only the snap ring for the clutch inner hub was used to keep the parts together. There were no signs of excessive metal transfer/wear on the shaft and clutch outer hub hole, but there was definitely some rocking motion and gear backlash between the parts, when moved around statically. It is hard to say what is actually happening when the engine is turning and the helical gear thrust is transferred from the crankshaft to the clutch outer gear teeth.

I put it back together and was greeted by the same noise, once again. It was difficult to pinpoint the source of the noise as it seemed to be present in numerous locations. I have heard crankshaft rod rattle sounds that were similar to these, but there was no way to check that without tearing the motor apart… So, that’s what I did next.

Day Two of Benly marathon engine repairs…

I really prefer to work on 250-305s and hadn’t really dug deeply into a Benly engine for at least 20 years. They are not terribly complicated, but because they are somewhat of a 7/8 scale Dream, all the bits are smaller and closer together, so it becomes a jigsaw puzzle that must be de-structed and constructed in a particular sequence, that is rather tedious and time-consuming. Anyway, out the engine came from the frame and onto the bench for disassembly and inspection. SOMETHING had to be obvious to make this kind of internal noise… wouldn’t it?

After tearing down both sides of the engine (again), the engine was flipped over and the lower case half removed. I could see that it had been apart before and that someone used Gasket-Cinch instead of Honda bond sealant on the case halves, last time. After inspecting the transmission gears and bearings/bushings, I turned my attention to the shift forks, one of which was showing signs of excessive friction/burning on the fork faces. When the output shaft assembly was installed in the upper case and pressure put on the plain bushing side, the shaft would pretty much seize up due to some contact between the fork and gear slot in the transmission’s second gear. I spent more than a half hour removing, grinding and replacing the fork on the shift drum and then checking the fit onto the transmission. I recall having some similar issues with CB92 engines way back in the 1980s, where the shift forks were not ground properly and making similar interference fits with the transmission gear. Finally enough material was ground away to allow the transmission shaft to continue to rotate easily with the bushing pressed firmly into place.

I surmised that this mechanical interference was creating drag on the gear and shaft, causing excessive friction which slowed the engine down and due to the cyclic nature of the parts, causing some back-chatter between the adjacent transmission gear dogs… Sounded like a good theory, anyway. I continued to tear down the engine as some noticeable oil burning was happening in the exhaust system, leading me to suspect piston/ring issues. Sure enough, the end gap on the rings was about 1mm (.040”) instead of clearances down in the single-digits, which are called for by Honda. I happened to have acquired a batch of old vintage pistons and rings, which luckily included a set of .75 CA95 pistons and about 5 sets of OEM rings so I had replacement parts on hand, for a change. Opening a new box of OEM rings; I found them fitting up inside the slightly worn bores with a .4mm fit (.016”) instead. This is on the outer end of Honda’s specifications for new parts, but cutting the end gap by more than 50% couldn’t help but improve the oil control situation. The original pistons were a/m copies, but seemed decent enough, so were reused with the new rings installed in the ring lands. When the cylinders were lifted off the pistons a potentially disastrous sight appeared in the form of a partially installed and mis-shapen piston pin clip that was half-way out of the piston groove. If that had popped loose, major damage would have ensued.

With the bulk of the engine torn down to basic subassemblies, the more I looked the less I found. There was only about .009-010” of sideplay on the connecting rods and nothing seemed to have excessive wear or friction in any moving parts, including all the ball bearings. So, it was reassembled and reinstalled back into the chassis for testing.

Well, the good news was that the piston rings cured the oil smoke issues immediately. But, the bad news is that the mystery noises continued and then I discovered that the kickstarter ratchet mechanism wasn’t installed correctly, so the kickstarter arm only turned ¼ turn and then stopped dead. (Insert $#^!@^!@ bad words here). That’s enough for the day… just leave it alone and see what tomorrow feels like…

Day Three of Benly marathon engine repairs…

Starting fresh about 9AM, before it got really hot outside, I undid all the hardware, wiring connections and mounting bolts and dropped the motor out once more. On the bench, everything came apart once again, but more easily this time, except for the tangled up kickstarter shaft and transmission gears. Once the k/s shaft was removed, I looked more closely at the ratchet mechanism and realized that part of it hooks into the upper case, in a little boss cavity made just for that part to fit into properly. Lesson relearned after 20 years of being away from these motors.

Leaving only the pistons and cylinders on the top case, I rolled the crankshaft so that the pistons were at TDC first, then BDC. In those two positions, that transition when the big end of the rods are changing direction is where some loud clunking sounds were occurring. Looking ever so carefully at the big ends of the rods, while holding the crankshaft weights steady, I could see some radial play between the rod end and the crankpin, which must be the source of the clacking sounds. Going back on the FB forum for Vintage Motorcycles, there were numerous comments about guys remembering their days wrenching on 150-160 twins, changing out crankshafts left and right when the bikes were still new. Removing the splash shield beneath the crankshaft in the lower case, revealed some slices of aluminum that resembled rod bearing roller cages and sprinkles of magnetic material mixed together.

All of this new evidence can only (hopefully) lead to the conclusion that the crankshaft is destroying itself and needs to be replaced. Fortunately, the owner gave a green light to continue the project, so the next step is to find a good useable crankshaft assembly for a 1962-later CA95 Honda Benly.

To be continued…

Bill “MrHonda” Silver