Monday, July 26, 2021

Honda CL72 with tricks up its sleeve…

I actually had a quick look at this bike over a year and a half ago, when I was dropping off an auction bike for the owner. He was riding it and then it started making a noise and then BANG. I checked for compression with my finger and it didn’t seem like it had any. The exhaust valve was tight, so I loosened it and tried to start it up… BANG!

The bike owner and I live 120 miles apart, so the bike has been sitting since then waiting for one of us to connect and get/deliver the bike. After it was sent to someone nearby, the diagnosis was incomplete, so the bike finally was brought down to me for a deep dive. The bike came here as a whole machine, so we both worked together to tear off the accessory bits and lift the motor out of the chassis and onto the workbench for a look.

Once the hardware was removed and the head lifted, the horrors of the situation developed rapidly.

                                                   CL72 pistons come in many flavors...

The whole cylinder sleeve flange was broken off and lying atop the cylinder block, floating loose. The piston was all torn up with broken sections between the top ring and crown. There was shrapnel floating around inside the cylinder on top of the piston, but little damage to the combustion chamber. At first it was thought that the engine had burned a piston, but in consideration of the appearances of all of the parts, it seems like the cylinder sleeve just failed at the top and dropped down far enough to catch the top rings in the gap, which lead to catastrophic failures of the piston.

With the cylinder block removed, the cylinder walls were actually okay, apart from a groove in the damaged right side sleeve. At first, I thought that the wrist pin clip was left out or had jumped out of the pin bore, but closer inspection showed the clip still in place. Apparently, a piece of the piston debris got caught between the piston and wall causing damage. Interestingly enough the bottom of the sleeve had odd grinding marks that couldn’t be matched to anything in the region. That one is still a mystery.

Well, the pistons were the high compression versions, even though the engine was a 1965 which had lower compression pistons. The cylinders had been bored to .75 for these pistons and they didn’t show any signs of seizures or other overheating, so apparently, the cylinder wall just had a failure in the casting. I have had one other engine do that to me after a rebuild, but the cylinders had been bored to 3.00mm oversize and it failed in about 5 minutes of run time. This engine had about 1,000 miles run in on it so it was an unexpected and unforeseen failure.

Digging deeper into the cylinder head, I noticed that the spark advance shaft had more than the usual amount of free play, advancing the shaft easily with my fingertips. When the head was torn down to check for valve damage, the sprocket advance weights were missing a return spring on one side! Not sure where the spring might be, but removing the oil pump and screen didn’t bring it to the surface.

I removed the oil filter from the clutch cover housing, as it was the late-style Big Hole version, which allows some careful removal and reinstallation without pulling the whole clutch cover off. I was surprised to find a thick layer of debris inside the filter itself, but also a lack of oil present inside. Usually when the filter shaft is removed, the oil inside the housing drools out, but that wasn’t the case here. The rest of the inside of the engine appeared to be getting well lubricated, but I noticed that the engine cases and the clutch cover had been sealed with black RTV, which can be a death knell for small Honda engines when the sealant plugs off vital engine oiling channels.

So, the cylinder head came apart to check valve conditions and all had cupped seats and faces, plus one exhaust valve on the troubled right side had a nick in the edge. One other valve had the tip face getting chewed up so all four valves were replaced after I cut new seats in the head. I replaced the missing advancer weight spring and then put the puzzle pieces all back together again.

A search for a set of cylinders or a 250cc sleeve only turned up a possible new set of cylinders from David Silver Spares, described as rusty. Photos will reveal how bad it really is. Once the cylinder question is answered, then we can chase down some new pistons. I have a half dozen 250 pistons and have found that they come in three different compression heights.. 10.0:1, 9.5:1 and 8.5:1. The early ones are marked CB72 inside the skirt while the later ones have CB72 on the outside next to the pin bore.

One must choose the piston setup carefully, as Honda lowered the compression in order to reduce the number of piston seizures that occurred due to overheating. Lugging the little 250cc engine on a 300 lb motorcycle with low torque values causes the pistons to work very hard in a space of only about .001” to .002” clearances. With the known issues of erratic spark timing and today’s questionable fuel qualities that didn’t exist when the bikes were built, these engines can and will seize their little cast pistons to the cylinder walls. Usually, the seizures are momentary, but when they occur the damage has been done, leading to lowered compression readings, smoke, and oil consumption which causes oil-fouled spark plugs or sometimes even more detonation because of oil burning.

Today’s unleaded, alcohol-infused gasoline causes the engines to run lean, which adds to the heat equation, so some careful re-jetting must be considered and tested before disaster strikes. As always, the spark timing must never exceed the II hash marks on the rotor when the engine is revved beyond about 2500 rpms. Excessive spark timing leads to piston seizures even when everything else is spot-on.

Back together again…

The customer tore down a spare “rebuilt” engine from his stock and it had .50 fresh pistons installed with a little bit of water damage along one side of a cylinder but it was old rust pockets and out of the way of critical piston ring travel. After cleaning up the old gasket material, re-honing the cylinders and checking gasket surfaces for being flat, the reassembly resumed in earnest. I did replace the center guide roller with one of the reproduction items from CMSNL. Once the top end was reassembled, we lifted it back off the workbench and rolled it on a dolly back to the naked chassis. It has been very helpful to have an extra set of hands available to hoist the engines back into the frames, as loading the Scrambler engines are a bit more challenging because they load into the right side instead of a vertical lift like that of the CB and CA models.

I put a floor jack under the oil pump to help position the engine as the mounting bolts are installed. From that point, it is just nuts and bolts going back together again. After about an hour, the bike looked like its original self once more. We installed a new points plate and did a quick static timing check before starting the engine. Initially, it was acting odd, only running on nearly full choke. The old petcock and carbs had some leftover old fuel residues in the bottoms of the bowls, which may have caused some fuel feed issues. I rechecked the float levels and main jets (120s in this case) and restarted the engine. It was still coughing a bit which was caused by some late timing on the right side points. Once the timing was set to the F marks, it came fully back to life and sounded healthy. 

A brief run around the block and through the gears revealed a nice shifting transmission, even running qualities, and a front brake that needed a lot of adjusting. Beyond that, it was “mission accomplished” after some 10 hours of labor, including help from the owner. I’m still not sure what caused the liner to crack and break like that, but it may have just been a flaw in the sleeve casting. Anyway, all’s well that ends well, so another CL72 is back into circulation again.

Bill Silver aka MrHonda


Tuesday, July 20, 2021

Another vintage Honda revival.. CL175K0

When your name is out on the internet and on social media pages, you can get messages from anyone and anywhere these days. Out of nowhere, I received a message from a man in the N. San Diego County area who had owned a 1967 CL170K0 for many years, riding it for awhile, then parking it for extended periods. It was basically a “make run” request, but these can get complicated and expensive at times, especially when searching for parts on a 54 year old machine.

I was already in the area for a chiropractor appointment, so volunteered to pick it up at his residence and bring it home for a rejuvenation session. Of course, the battery was virtually dry of fluids, the tires had no measurable air pressure, but the engine still turned over. Compression checks revealed 150-170psi readings. A valve adjustment was performed, but they were only off by a thousandth of an inch or so. Hopefully, some running in time might clear off any leftover residues from a valve seat or face.

Pulling the seat and tank off revealed a very burnt looking ground wire into the harness. The owner told me that he had run the bike off a jumper when the battery was dead and the charging system took out all of the light bulbs. Headlight bulbs for these bikes are getting harder to find and very expensive in many cases. However, had complete NOS units in stock for less than $85.

The whole fuel system was drained, cleaned and carbs rebuilt with new kits. One of the carburetors had a needle hold-down butterfly clip missing, so that must have been a problem for some time. I discovered that the carbs had been kitted before because the needle had a D number stamped in where the K number should be. Those parts come from the Keyster kits and are often not accurately machined.

The rear tire was a 3.50x18” trials tire, that made the bike sit on all four corners due to the circumference differences vs. the stock 3.00x18 road tire. I happened to have a new 3.00x18 rear tire in stock so it was swapped in and finally the rear tire had some space beneath it when sitting on the centerstand. The brake adjustment nut was pretty well threaded onto the brake rod, so a new set of brake shoes were installed after the brake cams were cleaned and lubricated.

The point cam on the spark advancer seemed gummy in operation, so the unit was removed and cleaned/lubed for re-installation. A fresh lead-acid battery was installed to save a few dollars on the repair bill and is plenty good enough for a bike without an electric starter system onboard.

Once the tank was drained, petcock rebuilt and new fuel lines run to the rebuilt carburetors, it was ready for a wake-up routine. I had drained the oil and serviced the oil filter spinner on the end of the crankshaft. It was refilled with 1.5 liters of 10w-30 motorcycle oil and a new set of D8HA spark plugs installed. The spark plug threads on the left side were a little bit worn, so I used a new tool that I heard about on forums which threads inside the hole and then you expand it to secure to the threads and then unscrew it to clean up the plug hole threads without having to run a tap down inside and risk leaving shavings behind.

With fresh fuel, oil, battery and spark plugs installed the bike lit off on the 2nd kick. A quick tour around the neighborhood didn’t reveal any issues. The front brake was initially grabby probably due to some rust build up inside the brake drum. Working the brake and putting on a few miles seemed to improve the situation.

The owner came back down from N. County, some 55 miles away, arriving in his VW sedan with a helmet and gear to ride it back home! I advised going that far on the freeway, but after a few surface street miles he hopped on the highway I-5 and rode some 30 miles in the right lane. I suspect the top speed for the bike is not much more than about 70-75 because the way they are geared and the lower than normal compression readings. In an hour or so, he reported back that the journey was successful and that the bike had held its own despite many miles of high rpm driving conditions.

It’s a testament of the bike’s design that it was able to sustain that kind of treatment for some 45 minutes. In the end, the bike is back on its wheels again and fully functioning once more after a 10 year snooze.

Bill Silver


1974 CB750.. from New York!

I still don’t know who exactly “referred” a Honda CB750 customer to me from the beach area, but a nice young guy contacted me to see if he could get his head gasket replaced due to a huge oil leak. Both the owner and the bike came in from New York, where the elements are not kind to motorcycles left unattended for long periods of time. This one was pretty crusty, rusty and corroded and I couldn’t help but wonder why someone would want to put more money into it, but the request was to “Fix it, please”. I told him that I wasn’t about to tear the 175 lb engine out of a CB750 anymore without help, so he said he would give a hand, wanting to know more about the process and what it needed.

The bike was sent over on a flatbed tow truck and we gingerly rolled it into the driveway for a closer look. It was covered with grease, oil, rust, corrosion and looked to have had more than a few mods. Undaunted, we did a quick scrub on the bike, hosed it off, air dried it with a compressor hose and set about dismantling the bike to extricate the greasy lump from the chassis.

After 2 hours, the engine was on a dolly and up on the workbench for disassembly and inspection. Overall, the inside of the engine looked decent, but three valves had wear-through on the stem tips, so those went on the parts list. Obviously, all the valve stem seals needed replacing, but the cam and rockers all looked reusable. Parts were ordered and I hand-cleaned the top end parts as much as I could.

New intake manifolds were ordered, but the eBay seller shipped DOHC manifolds instead of SOHC versions, which were useless, as they are much larger than the early engine parts.

Correct parts were finally shipped from my friends at and reassembly continued over a period of a few days, as gaskets and seals arrived.

My customer/helper went out of town for a week, so I wrangled the engine onto a bike lift and wedged it back into the frame hydraulically and with minimum amounts of exertion. All the bolts and fittings needed to be cleaned off on the wire wheel just to make them easier to install. After another 2 hours, the engine was back in the chassis and ready for cleaned-up carburetors and electrical connections.

A sad finding during installation was the fact that JB Weld epoxy was all built up around the countershaft sprocket area of the engine cases. Apparently, it threw a chain and damaged the cases, so the shop just puttied it up and sent it back out again.

The bike had an aftermarket regulator/rectifier unit spliced into the system, plus there was another mystery module mounted up near the rest of the electrical components which I left alone. The bike came with a Li-Ion battery that weighs about 6 oz but wasn’t holding a charge in the past. I hooked it up to my automotive charger on 2 amps and AGM setting and it eventually reached the desired 14-15v charge rate. The electric starter cranked the engine over with the charged battery and it did fire up after the cleaned carbs were reinstalled and a petcock screen (which was missing) placed where it belonged.

The engine fired up and sounded fairly good. The old coils had fried wire ends where they screwed into the plug caps so they were trimmed back and new caps installed to give it the best chance of running properly. So far, so good… no leaks except for the oil filter housing where someone had cut the old bolt off and notched the hole where the bolt goes through. Luckily, I had a spare CB550 filter housing handy, so that solved that problem.

The front brake needed attention, but getting the caliper off the bike was another challenge. The two big 8mm Allen bolts that hold the two caliper halves together were seized solidly. The workaround was to remove the whole front fender, then the caliper mounting bracket and then take the caliper to the workbench, forcefully loosen the two bolts, put the caliper back on the hydraulic line so I could pump the piston out and then finally it was all disassembled. The caliper piston had a ring of rust pits all the way around so new parts were ordered, along with new pads.

Once the bike was back together, I took it out for a 15 minute test run, which went okay. I put the side covers back on the frame and went for lunch. When I came back to the bike, the power to the switch was gone! Nothing there. Checked the battery, which was down to 7.7 volts, but still lit up a test light. I put the battery back on the charger and dug into the fuse box connector which looked like it had been replaced or at least some of the wires had been redone. Pulling on the red wires in the connector yielded a poorly crimped wire end and one wire pulled right out of the spade connector. I replaced the connector lugs with new ones on fresh copper and crimped it tightly together. With the rejuvenated battery, the bike came back to life with power all around.

More problems surfaces as whoever had setup the bike with low bars used a damaged turn signal switch on the left and a headlight ON-OFF switch on the right side, but there was no way to select Hi-Low beams! Not only that, but the headlight was pulsing on-off. Putting 2 and 2 together, I realized that one of the modules in the wiring side of the bike was a headlight pulsing unit, which was for safety according to the maker.

In the process of checking over the lighting components, I noticed that the tail light was not coming on nor the brake light. I checked power into the light on the harness wires and 12v was present. Removing the taillight revealed that the light socket had fractured in half, so the ground wire was connected to the back half, which was separated from the tail and brake light hot leads. Back to eBay for a $20 used tail light assembly which cured that problem.

Eventually, I felt that the bike was safe and reliable enough to give back to the customer, who rode it back home about 15 miles on the freeway reporting that it was running great! I do want to try some slightly larger main jets in the carbs, due to the pod filters and a/m 4into2 exhaust pipes, but for now, the mostly dead CB750 has been given new life for now.

  Bill Silver 7/5/21