Tuesday, May 16, 2017

Coming through in the clutch… Honda 250-305 clutch history

One of the most confusing aspects of 250-305 engine repair is sorting out the numerous versions of the clutch assembly. I will attempt to demystify it here, if possible.

Honda began the new wet-sump engine series in 1960, starting with the 250cc Dream, then adding the 305cc version soon afterwards. In 1961 the CB72 and CB77 Super Hawks emerged from the factory, followed by the new 1962 CL72 250cc Scramblers.

Honda started with a light pull, conservatively-designed 4 plate clutch used in the first generation C/CA72 250cc Dreams.  In the parts books, you will see these parts with 259 part codes, which is the code for the 250cc Dream engine series. With the somewhat higher power output of the 305cc engines, the parts were upgraded to 266 codes for the clutch pack and the primary chains. Yes, there were three different parts numbers just for the primary chains, starting with 259, then 266 and finally the 268 code part for the CB72.

The thick 5mm friction discs of the 250s, coupled with very light clutch pressure springs resulted in clutch wear and failures fairly early on in the production cycles.

From the 1960 Dream parts books we see:

The 250s had four thick friction plates and four steel plates. The center hub had no grooves for retainer springs. Early pressure plates have been seen with small metal pegs inserted around the edges with tiny coil springs attached, which helped push the clutch assembly apart during disengagement. The 305s had five sets of thinner plates. One set of clutch springs sufficed for both versions, at first. Apparently clutch wear and failures became common, so a new set of clutch parts were designed for the Dream engine applications. Notice there are two clutch outer part numbers. The first is a solid mount rear chain sprocket style and the 010 version was the improved “cush drive” model. The inner clutch hubs came in two more types using the stopper rings, fitted into grooves machined into the clutch hub. Ring groove locations varied due to the differences in the thicknesses of clutch plate pairs which were being retained by the stopper rings. 

Clutch springs were offered in 259-000, 259-010, 266-000 versions, which became progressively stronger, as necessary. Clutch centers were offered in 259-000, 259-020, 266-020 and 266-030 for 250 and 305 models, respectively. The early model clutch friction plates were molded solid fiber material. As the clutch designs progressed, the 268-020 coded friction plates were made in aluminum-backed parts with the friction material bonded to the metal backing. Friction materials varied in thicknesses depending upon the application and number of plates used. Dreams wound up with either 5mm thick, 4-plate clutch packs or the thinner 4mm 5-plate combinations. The Plate “A” (first plate on the hub) was generally 2x the thickness of the other steel plates. Steel plates come in several thicknesses, so just the right combination of parts needs to be achieved to create the best possible clutch pack build.

What is important to remember is that no matter what combination of friction and steel plates were used to complete the clutch pack, they ALL had to have the same stack height when installed on the engine. If the stack height isn’t matching to OEM specifications, then the clutch release components inside the right side kickstarter covers will not have the correct geometry to effectively release the clutch pack when the clutch lever is pulled in. The clutch lifter arm must travel through a shallow arc, starting below horizontal plane and arcing upwards above horizontal. As the clutch lifter arm moves within the clutch adjuster (fixed in the kickstarter cover), the coarse threads of the two parts create a pushing motion on the end of the clutch release pushrod, which contacts the big ball bearing nestled deep inside the clutch lifter arm.

Many people have discovered sudden loss of clutch lever action, usually when the kickstarter covers have been removed for servicing or clutch cable replacement, due to the ball bearing dropping out of the lifter arm hole. The thickness of the ball takes up considerable amount of lateral play in the clutch release mechanism, so when it is missing it is impossible to adjust the clutch free-play using the cable adjusters or the clutch adjuster on the side of the kickstarter cover.

Scrolling down these pages to the CB72-77 clutch parts list, you will see that there was a 6 plate clutch offered, a 5-plate version using all fiber friction plates and finally the last generation 5-plate clutch using the metal-backed friction plates. Obviously, in order to accommodate a 6 plate clutch pack, all the plates needed to be thinner than the 5-plate set. The last generation CB72-77 (and CL77) used a clutch center with retainer grooves placed to allow the thicker plates to be installed and retained. The last of the steel plates had anti-warp notches stamped along the inside edges to help keep the plates flat when they became heated during aggressive operation. The Plate A is quite thick and has a beveled back edge to clear the clutch outer when installed. Late version pressure plates have angled drain holes to help evacuate trapped oil inside the clutch assembly.

There were three different clutch covers first due to a change in the shift shaft lengths, then the progression from “small hole” to “large hole” outer oil filter cover sizes. Remember to check your new parts against the old ones, when doing replacement of components.

CB72-77 clutch springs went through a series of part number changes from 268-000 for the 250s, followed by 275-000/010 for the 305s, then to the very stout 275-810 racing clutch spring. For most normal stock street applications, the 275-000 spring (equivalent to the 323-003 CB500 Four spring) works fine with stock clutch pack parts. These springs give a nice balance between light lever pull and sufficient pressure to prevent clutch slippage. Use of the later 275-810 springs will give the rider a very heavy lever pull experience, but does ensure that clutch slippage is pretty much non-existent.

When there is wear inside the clutch adjuster threads, there is a progressively lessened amount of clutch pushrod travel to disengage the clutch plates. The wear increases the amount of force needed to disengage the clutch to the point where the combination of heavy springs and worn clutch release parts can cause damage to the early version kickstarter covers. Later versions of the kickstarter covers were designed to move the clutch cable joints more forward, closer to the cover retainer screw mount.  Increased strengthening ribs were incorporated into the cover design to help prevent breakage which is especially common in the Super Hawks, which have forward-kicking kickstarter levers.

Well-lubricated clutch adjusters with minimal thread wear will enhance clutch lever pull efforts in a positive way. Heavy clutch lever pull effort is often a combination of dry and worn (and misrouted) clutch cables, worn clutch lever pivot bolts/holes and excessive clutch adjuster thread wear.
CA clutch parts list (1969 book):

                                                CB kickstarter cover... ready to install.

Final conclusions:

You cannot indiscriminately mix and match Honda friction and steel clutch plates and have a normal operating clutch on a 250-305. Steel and friction plates come in various thicknesses and designs.
You MUST use the clutch plate pack that matches the inner clutch hub retainer wire grooves AND use the retainer wires, as designed. These “stopper” wires are becoming extinct, so I have used picture hanger wire or very thin safety wire to use for the same function. Installation of the plain wire requires drilling an additional hole in the hub and securing both ends of the wire inside the hub where the stopper wire ends usually wind up after installation. If you leave the stopper springs out, you will have difficulty in selecting neutral when the engine is running and you are at a stop light/sign and need to release the clutch in neutral while you wait.

The best solution is to use the 1965-ish and later CB/CL77 clutch assembly, which includes the cush-drive clutch outer and the sturdy 268-020 friction plate sets. See photo below for comparisons to your clutch assembly. Remember if the clutch pack stack height is not correct, clutch engagement/disengagement issues will arise.

Using a CB/CL77 clutch is perfectly fine in a 250-305 Dream. You might play with using some of the lighter clutch springs to lighten the clutch lever pull, as strong springs are not needed for Dream applications. The Honda 425 code springs are the superseded parts for the Dream clutch springs.
Using Barnett clutch kits will not allow use of the inner hub stopper springs, so clutch release at a stop sign/signal light will be compromised. Barnett plates tend to drag when cold and the spacer plate they use to compensate for the changes in plate thicknesses will cause problems for street use. IF you are racing your bike and don’t care about street use, then going for heavy springs and heavy duty clutch plates may be the way to go, but you will have difficulty getting the transmission into neutral without the stopper springs in place.

Watch out for late model “shallow spline” replacement parts, used on 1966-67 models. You can use the shallow spline hub, crankshaft sprocket and countershaft sprocket on earlier bikes, but not the other way around. The OD on the shallow spline parts are the same as the deep splined parts, so it is the ID which causes conflicts when trying to use early parts on late shafts.

 Honda has superseded the 259/268 steel plates to 329 code parts from the XL350.

Friday, May 5, 2017

Bringing them back alive…

It was another week of bike wrangling at Casa de Honda as the month of April winds down. Recent sales included a 2015 Sym 150 Wolf (after the BAT sale of the CR93 tribute bike, a Suzuki GN400 and the 2400 mile Yamaha XS650). Most recently, I swapped out a mint condition 2013 PCX150 scooter for a non-running 299 original mile 2008 Honda 250 Rebel.

If that wasn’t enough, I spotted a survivor 1968 CL125A Scrambler in the local Craigslist postings, which had been on and off their site for a week or so. When I contacted the seller to find out about the basics like: Does it run? Is it registered currently? Are there any big problems with it? Etc..
The initial response was “I don’t know much about motorcycles.” Digging deeper in the communications, it appears that the bike was bought because it “looked cool” and it was a first motorcycle purchase for him. As I expected, it was a good 10-footer, but when you got close-up it became obvious that the bike was basically all original: cables, tires and body paint. The tank had been re-sprayed with some kind of close to Honda Silver color, but the rest of the Candy Blue paint was untouched, apart from the scratches, dings and general wear and tear that you would find on a 49 year old baby Scrambler.

Both bikes needed new batteries and fuel system maintenance. Despite the stinky green fuel that exited the petcocks, the carburetors were relatively easy to clean after wrestling them out of their respective chassis. These two bikes must be somewhere in the TOP 10 of “hard to remove” carburetors category. The CL125A carb is sandwiched in between the cylinder head intake port and the rigid air cleaner tube that flairs out over the opening in the frame. The only way that I could see to remove the carburetor was to take off the two 6mm nuts, then, using Vise-Grips, work the two intake studs out of the head, so the carburetor could slide sideways out of the frame.

The idle jet was plugged up and all the rest of the jets and emulsion tubes were cleaned and checked for open passages. This was Honda’s first attempt, that I know of, to create a CV (constant velocity) carburetor. The slide has a delicate rubber diaphragm secured to the top by way of a plastic plate with 3 tiny screws attached. In the middle of the plastic plate is a spring-loaded plunger which is pressed down by the carburetor arm which resides inside the top cover. While CV carbs on 350 twins from 1968-73 used a light spring to press the slides down in the carburetor body, the tiny slide on a 125 (remember this is a 360 degree firing engine with a single carburetor) is too small to control in the same manner, so Honda put a big heavily weighted spring in the top cover to push the slide down firmly whenever you let off the throttle.

While the diaphragm was probably original and felt/sounded very brittle, there were no holes or tears in the rubber, so it was gently reassembled and seemed to hold vacuum properly. The carburetor flange is sealed with a 32mm o-ring, which has been discontinued from Honda. I had been buying some various sized o-rings from eBay sellers in China for use on Honda Dream carb flanges, so one of those was popped into the groove when the carburetor was installed.

The fuel tank was purged of the old gas and refreshed with some of 7-11’s finest 91 octane brew. With a new piece of OEM Honda fuel line, the carb was again reconnected to the fuel supply. The original battery was a small electronics store item, which was marked “Removed in 2003.” The battery wire ends were changed to the small push-on connectors, but I had ordered an old-school lead-acid battery which is big enough to support an electric starter, but these bikes never had that option. I snipped off the wire ends and found some ring terminals that would fit the 6mm battery bolts normally used with these batteries. With the fully-charged 6v battery installed, the switch was turned to ON and the bike lit up with all functions working as designed. Yeah!

With fresh gas on the tank, A few good kicks and the bike started up, sounding sweet and healthy with no smoke out the muffler (thankfully no muffler rot for some reason). The original tires were rock-hard, so a brief and gentle spin around the block was enough to confirm that the transmission and clutch seemed to be working properly.

2 weeks later…
The little CL125A was treated to a new set of tires and tubes, plus fresh carburetor parts that amounted to $60 worth of purchases. I re-learned a valuable lesson about setting up the carburetor diaphragm system when I installed the central plunger in upside down. There is a long and short end to the spring-loaded plunger and when it is installed with the long end up, the throttle goes to wide open revs as soon as the engine lights off. This is VERY disconcerting, to say the least. After a few rechecks, I finally looked at the illustrations of the carburetor and saw that the long end of the plunger goes DOWN. Properly installed, the carburetor assumes normal function once again.

The exhaust system is all stock and the drain hole at the back was still open and there seemed to be plenty of material underneath the muffler, instead of finding the usual rot that happens with time and operation. When the muffler was removed, a surprising hole was discovered on the back side of the muffler relief where it curls around the rear shock. Apparently the rear mounts were bent or not installed correctly and the shock wore a whole in the muffler body. A trip to my friend Rob North’s shop resulted in a mended muffler which then needed a hard-to-find muffler packing for the slip-in left exhaust header pipe. After tracking down a small batch of them, the muffler was finally installed and should function as designed.

NOS old school cables were ordered and installed. The clutch cable installation requires removal of the rear sprocket cover which revealed that the front sprocket was a 16t instead of the stock 15t size. Test rides showed that the bike was unable to sustain a good speed up the long hill to the house from the local PO. It would rev high in 2nd to about 40 mph, then just bog down in 3rd gear under full power. A 15t sprocket was ordered and installed when the cable was changed. The clutch adjuster was found to be extremely tight in the side cover, requiring cover removal and a good bit of time to extract the adjuster and then refit it in the hole so it turned more easily. All these little tasks wind up taking up quite a bit of time to correct during a revival process…

Next improvements are a new right side fork ear, fork seals and a new headlight rim.