You can bet your sweet S90 that it will be a surprise along the way.
If you stay in one place for a long time and work on a lot of local bikes, at some point in time one of them will come back around again. So, when the “Honda S90” listing on Craigslist showed up I was intrigued but not triggered to remember the bike from my past. A quick message to the owner yielded a response that went, “You worked on this bike 10 years ago.” Hmmmm???
We made an appointment to meet up about 15 minutes away and the bike was in a pretty sad state once you got close to it. Both tires were flat and off the rims, the engine appeared to be seized, very old gasoline was still in the fuel tank, the battery was dead and it was obvious that the bike was actually a JDM domestic version judging from the tail light, solid passenger pegs, low handlebars and a side stand that was factory attached to the footpeg bracket. The speedometer was in MPH, but it didn’t have a neutral light function, just a high beam indicator and a meter light. The headlight switch had only a H-L setting with no on-off function, so there were many puzzling features as the investigation continued.
We struck a “reasonable” deal price and the owner’s husband helped push the poor little dead S90 up the driveway, from behind the apartment building and into the back of my Tacoma. The bike was pretty complete, but the wheel hubs were all chalky from sitting somewhere wet for a long time and the rims has rust pits in various places. I was told that the bike had been run for a while, but judging from the virtually unused front tire with a 2011 date code, it didn’t go very far. The bike didn’t have a title anyway, nor a license plate, as it was a project bike at the time, made from various parts which became more apparent as time went on.
While unloading the bike from the truck, the shift lever got bumped and the engine engaged, turning backward, releasing whatever the stuck piston was doing previously. I resolved to remove the engine and take a close look at what had happened to it, while starting a parts order for tires, tubes, and a battery to get things started.
With the stock muffler removed and the intake manifold/carburetor taken away, the engine drops out with just three bolts. I hauled it up on my bench and proceeded to dismantle the top end fearing the worst. But as it came apart, the piston crown was mostly shiny from low hours, but signs of water intrusion were present. The cylinder had some staining, but the piston wasn’t seized, although the top compression ring was kind of stuck in the ring land. A little penetrating oil and some careful movement freed the ring and it appeared to be good enough to reuse again. A quick hone of the cylinder was done to rough up the surfaces just a little bit.
The cylinder head was more of a mess. Old valves were cupped and water had gone down in the intake port where it etched away at the metal just behind the valve seat. New valves were ordered and the seats were re cut with my ancient set of OEM valve seat cutters. Fortunately, the seats had enough material left to allow for valve seating, so the head was reassembled and installed back on the engine.
A new clutch cover was ordered from DavidSilverSpares.com as the original was broken where the clutch cable is secured. There are two different clutch covers, depending upon the serial number of the engine, which was harder to determine as there were no serial numbers on the engine apart from the cast in S90E and the space below was blank.
The wheels required some cleaning and the brake drums scraped clean with a screwdriver blade then wire wheeled and finished with a Scotchbrite pad. A set of inexpensive scooter tires, tubes and rim bands were ordered from an eBay seller, so they were spooned on after de-rusting the rims as much as possible. New brake shoes were ordered from an eBay seller for very little money, having come from China.
One fork seal appeared to be leaking so the forks were removed when the wheel was taken off for tire replacement. The right side fork had a screw-on fork seal holder, but the left side had a conventional fork slider with the seal retained with a snap-ring. When they were reinstalled, it turned out that they were different lengths! I wondered why the axle came out with some difficulty, but now it was apparent what that was the case. The difference was probably about an inch with the forks uncompressed, but pulling down on the front end finally matched them up enough to insert the axle once again. I have to say that I haven’t run across that problem before! Either the long fork is from a later version or perhaps from a CL90 bike instead. Finally, the front brake cable was excessively long. The early low handlebars take a shorter cable, of course and someone just ordered a high bar US version. The clutch cable was the correct length, however, as was the throttle cable
The ignition coil was replaced due to a cracked spark plug wire and general age-related damage. Once everything was correct, the little engine was wrestled back into the frame. The fuel system was next on the list so the Chinese copy of the original angled base carburetor was cleaned, which was a type that Honda used on the early engines connected to the head with a twisted intake manifold. The early carb setup requires a unique air filter and connector to put it all together. All the parts were connected originally, so it is puzzling how water had worked its way down into the intake port.
So, just when you think you have it all sorted out, the unexpected happens… The compression tested out about 130 psi, which is about 20 short of normal, owing to a less-than-ideal valve seat repair attempt. Still, it should have at least started up, but when the battery was checked, it was down to 3 volts from six. Given that the only thing that can drain the battery down is the rectifier, it was apparent that it would need to be changed. And to change the rectifier, you have to remove the engine.
I recharged the battery back to 6 volts and installed it without hooking up the rectifier, just to see if the engine would start up. I had compression, spark, and fuel in the carburetor, but it wouldn’t even give any signs of life. The spark plug appeared to be dry despite all the kicking, so the carb was removed again for a check of the idle jet and passages. Everything seemed to be in order, but it didn’t affect the starting routine at all. Kicking the engine over with the spark plug out I noticed that the spark was erratic at the plug. The spark plug cap tested normal, so all that was left was the possibility that the condenser was failing. Apparently, the part that was failing was my brain because when I assembled the coil on the top of the engine case, somehow I failed to reinstall the condenser! Of course, in order to check the coil and condenser, you have to remove the engine again.
To cure the compression issue, I removed the head again, stripped it of all the valve train stuff and took it to my local machinist, who cut nice fresh valve seats to match the new valves for $25. When the engine was reassembled the compression jumped up to 150psi, which is in the range of factory specs. Sometimes, the DIY efforts are better left for the professionals.
In the meantime, I ordered a cheap rectifier from an eBay seller who modifies those little solid state cube bridge rectifiers like Radio Shack used to sell for a couple of bucks. The unit was already made up with long proper colored wires, but when I went to install it the mounting bolt was just a round 6mm bolt and the hole in the chassis is half-round! So, at that point, the options are to either drill out the frame hole or grind a flat on the bolt. I put the bolt on the grinder wheel to make it flat on one side, but when I tried to install it, the bolt proved to be too short! Back to the shop to find a longer bolt or screw, which I found in a 6mm Phillips screw from my drawer of fasteners. I had to regrind a flat on the screw again and finally got the little unit to fit back into the frame hole.
The problem with these little S90 engines is that they are just kind of egg-shaped and it is difficult to find a nice flat space to use to raise it back into the frame without it wobbling all over the place in the process. The engine harness plug is tucked up way inside the frame, so you have to reach up and plug it in carefully while balancing the engine on a stand or jack. Finally, with a condenser installed and the new rectifier bolted up, the three mount bolts were inserted and the rest of the fasteners were installed.
Having charged the battery once more, it was installed and hooked up to the new rectifier and the bike was ready for another starting attempt. Not surprisingly, once a proper condenser is installed, the bike lit off on the second kick, as it should.
A final challenge was the fuel tank and petcock. The tank had been sealed with Kreem about 10 years ago and that product doesn’t really hold up in the long term. Plus it looks like they just poured it over the top of existing rust so the insides had been peeling away from being exposed to old gasoline left behind the last time it ran. The petcock brass tube was gone, eaten away by corrosion. I cleaned out the passages and installed what I thought was a new 4-hole petcock gasket, but apparently, it was used in the past and had hardened enough to allow gasoline to slip past the surfaces of the seal.
The correct petcock for the early model S90s is difficult to find now, as it has a crossover tube to connect the bottoms of both sides of the fuel tank together. There are numerous Chinese copies of these petcocks as they fit a number of models. There are variations with an outlet that points down, points sideways, with and without the crossover tube that also points in different directions. What seems to be the problem is that most of the fuel line fittings for these copies are for a 4mm hose instead of the 5.5 mm hose that is needed to connect the carburetor to the petcock. These petcocks are dirt cheap, so I ordered one similar to the one on the bike, which didn’t have a crossover tube fitting, just to get something fitted up to allow the fuel tank to be installed so the bike could be test-driven. Unfortunately, the first try with an eBay seller yielded the wrong version despite the fact that I had ordered the correct one to match what was on the bike now.
With the engine back in the chassis and the chain hooked up, I confirmed that the transmission is a rotary gearbox design, which is great for around-town traffic, but not if you were going to ride it in some kind of competition. The gear pattern is N-1-2-3-4-N going down, down, down, down. You can, of course back shift it to gear down, but not paying attention to the transmission in 4th gear will get you into neutral at a high speed and if you push down again, you get 1st gear when you are going 50mph. So, you just have to override your normal shift pattern in your mind to compensate for this feature.
Finally, once the bike was running and safe to ride around the block, the first thing I notice is that the clutch is slipping! I thought it felt a little off when I was kick-starting it so much, kind of not catching the engine fully and the suspicions are confirmed! Another order to DSS for some clutch plates and a few days of waiting to install them. Ordinarily, these little clutch packs are pretty bullet-proof, but not knowing the full history of the engine can lead to more surprises when it comes down to getting the bike to go down the road.
Removing the clutch revealed that one of the friction plates deteriorated so it looked like I had three steel plates and one friction plate. All the friction material was gone from the plate and bits had lodged around within the clutch hub. New parts were installed after cleaning out the old debris and suddenly I had a bike that kicked over smartly and didn’t slip when power was put to the motor. That one should last for years to come.
Historically, the OHC 87cc engine had a 20-year run of production. It replaced the pushrod C/CT200 engine in 1964 and wound up expanded in the CT110 models for the early 1980s. Going over the parts lists for the S90s is a real head-scratcher. There are three sets of engine cases, two clutch covers, different oil pump and filter screens, two series of transmissions, different cylinder heads, intake manifolds, air filter systems and carburetors. By the time the CL90 versions came out in 1967, most of the major redesigns were completed, but the early years of the Super 90 are full of modifications and parts that are not interchangeable with later versions.
Apart from the Chinese copy carburetor, which is unique for the early models, the other modification to the engine was a “performance” camshaft from DRATV.com which is a $40 investment. I have used this cam in conjunction with a big bore piston kit on a couple of CT90s and it really woke them up. The cams are a drop-in modification, having extended cam timing but no changes in the base circle or valve lift.
Electrically, the light bulbs were all blown out from someone running the engine without a good battery in place. Typically, the US bikes have a headlight switch has an ON-OFF, LOW, HIGH function all built into a single unit. The domestic bikes only have a HIGH-LOW switch on the left side and no separate ON-OFF switch function. That is controlled by the ignition switch which has OFF-ON-Lights settings instead. The unique domestic tail light has its own mounting bracket, lens unit and base not commonly found in the US. The original turn signals were MIA. The low beam of the replaceable bulb was burned out so DRATV came to the rescue once again.
In order to get the bike registered in CA it has to have an engine number, which is missing on this machine. California has its own CA-assigned stickers that are applied to either the frames or engines when cases like this occur. That will require a separate trip to the CHP office after the first go-round with the local DMV office.
Slowly, but surely, the bike is coming back to life and full function once again. Even tiny little bikes like this can consume a lot of time and effort to revive. S90s require attention to detail and a close study of the various superseded parts that were made as production increased through 1969.
Honda finally replaced the venerable, horizontal OHC 90 series machines with new CB/CL100s with upright cylinders, followed by a 125cc version, which also included the SL100-125, the one-year CT125 and eventually XL100-125 singles, all fitted with a 5-speed transmission. The basic design morphed into later XL185-200 and XR200 versions. The 90s did do extended service in the US90/ATC90-ATC125 three-wheeler models until 1985, so the linage of the original design carried on for more than 20 years.
Bill Silver aka MrHonda
2-2023
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