Mutant 1969 SL90 Honda…

My friend Don brought me another one-off bike to bring back to life last week. When he sent a photo of the engine, I knew that I was in trouble. The bike was clearly a one-year-only 1969 Honda SL90 MotoSport machine but had oversized tires, a wide alloy rear rim and the swing arm had been extended a couple of inches with some awful slugs and scabby welds. The extension work was probably to clear the over-sized rear tire.

It was the engine photos that caused me concern. The stock SL90 engine has a 4-speed transmission and manual clutch, pretty much borrowed from the S90/CL90 models, but in a new heavy-duty tube frame. The engine shot showed the SL90 engine serial numbers, but the left side cover had a Hi-Low transmission grafted onto the otherwise stock-looking engine! Very puzzling…

I did check the compression first and the readings were in the 125 psi range, which should have been closer to 150 normally. The engine is bolted down with 9 bolts. Two long ones at the rear, which are normally used, then three more on the top case where the frame side plates bolt in, plus four that came up through the solid frame pan and into the bottom of the engine case.

I nudged it out of the chassis and rolled it over to the workbench. It was a nice change from the 100+ lb 250-305 engines to be able to lift the little 90 engine up and into a snug place for screw extraction. Obviously, the whole engine had been apart and a CT90 Trail 90 transmission was installed which includes a long output shaft. All the screws were very tight and it took a lot of whacking with the 3lb hammer on the impact driver to loosen them all up.

Next, the top end was removed and more surprises were discovered. The piston was 51mm vs about 49 for a stock engine. The valves looked stock and there were no signs of engine porting. Even the stock carburetor was still in place. With the cylinder removed there were signs of water down in the cylinders for a while, etching the cylinder walls, especially near the top. As I removed the piston from the rod, I noticed a bump in the connecting rod that was not normal. My best guess is that it was treated to a Powroll stroker kit and the process to keep the top end height correct was to heat up the connecting rod in the middle section and put it in a press, shortening the rod a few millimeters.

The camshaft was an odd piece, as well. One end looked as if it had been hard-chromed, the lobes looked bigger than stock and the flange for the camsprocket was a full circle instead of just a couple of ends big enough to bolt the sprocket to normally.

So, the little engine turned out to be a big-bore, stroker, cammed-up, 8-speed, manual-clutch dirt machine, but still sporting the original carburetor and exhaust system. It’s not anything that you would normally find out in the wild these days.

Meanwhile….

Don hauled the chassis home and dismantled it for a silver powdercoat finish. He tracked down a replacement stock swing arm and rear wheel assembly. The plan was to make it as stock as possible, which included a $800 paint job for all the bodywork. The hybrid motor will be the only big departure from stock, overall.

I shipped the cylinder to DrATV in Nebraska, who has pistons in an oversize to clean up the cylinder and give it new life. Along with that, I ordered new gaskets, seals, screw kits, and a new dipstick. The carb will get a good ultrasonic bath, but it wasn’t in terrible condition, surprisingly.

The engine was covered in clay dirt, which was resistant to paint thinner, so it required an hour of scraping and scrubbing with wire brushes and screwdrivers to get all the grit out of the myriad of little casting corners.

The cylinder head was similarly gummed up, but the valves may be reusable. The previous mechanic put the exhaust valve stem seal holder on the valve guide first, then jammed the inner valve spring seat over the top of it. Both parts were damaged in the removal process, so more bits were added to the parts list. I seem to find the oddest situations of previous repair attempts on just about everything I have seen lately.

I did pull the clutch cover off and everything inside looked nice and well-oiled. The little wedge-shaped oil screen was clean, as well. The cover was sealed with Gasgacinch sealer without a gasket! The cylinder camchain tunnel seal was loaded with red RTV and the formed seal was damaged somehow during installation. At least the left side of the engine cover area was sealed with a proper gasket. Instead of an SL90 gasket kit, the CT90 Trail 90 kit will be the one of choice to reassemble this hybrid engine.

Next up will be fork seals/boots, drive chain, new cables and switches.

Digging deeper:

After a lot of cleaning of the outside dirt and grime, the assembly process began, but not without more problems. I received a box of parts from DrATV including the rebored cylinder. I had set aside the old piston and began to assemble the new one only to discover that I had overlooked the fact that the piston skirt on the old piston was about an 1” shorter because of the combination of the stroker crank and shrunken connecting rod. So, I disassembled the top end again and whittled down the piston using a combination of hacksaw and a little belt sander I have in the corner of the shop.

The head went together with the old valves and new valve stem seal and holder. The new piston crown was somewhat different than the old one, but the combusion chamber had been scooped out somewhat to lower the compression generated by the increased bore and stroke.

The carburetor was the early version with a horizontal float valve setup which doesn’t seem to be available in a carb kit that I could find. It cleaned up pretty well in the ultrasonic cleaner, so I left the original parts inside. I am concerned that it had been running a stock #85 main jet despite all the other mods including the lumpy Harmon Collins camshaft. We’ll see how it turns out in a few weeks.

Chassis build

Don brought back the frame and suspension bits to start the reassembly. I had to extract the old swing arm bushings and spacer from the extended one to the powdercoated stock one. Using a slide-hammer setup I was able to coax the old bushings out and reinstall them into the good swing arm. There were new aftermarket shocks but the eye ends were the same on both ends and the bottom one fits the swing arm mount fine, but the top mount is an 8mm bolt, so requires a bushing. I had one lying around, but will need to get another one before I am finished.

The steering stem was the next part to install, but the races were somewhat pitted and the package of balls that was delivered had 18 in it and the installation calls for 42! I tried Home Depot and called ACE Hardware, but no luck. I told Don that I needed more 3/16” balls and he came up with a package of them by the next morning! I had used a small Dremel grinding wheel to work down the pits in the races in hopes of them being good enough to use and they were. I had already replaced the fork seals on the fork set, so they were slipped into the stem, after some excess powdercoat was removed. With the fork bridge in place the forks rotated side to side very smoothly.

The rectifier bolts to the side of the battery box which had also been powdercoated. I had to grind off some of the coating to get a nice solid electrical ground for the box to the frame and the rectifier to the batttery box.

More cleaning time was spent on the front brake backing plate and shoes. The worst problem was that someone had torqued down the axle when the speedometer drive gear wasn’t registered into the two little slots on the hub. The tabs were flattened out and it took some time to coax it out of the drive cavity and then put it in a vise to bend the tabs back to ninety degrees so they fit the hub once again. It’s amazing how much time is spent on doing little steps to move forward in the assembly process. I turned the frame upside down in order to feed the front wheel and brake assembly into the ends of the forks, then flipped it back over again.

I tracked down new cables including one of the rare front brake cables, which have a brake switch burid in the middle of the cable. This was a 1969-only feature on most of the street bikes in that year and the correct cables are very hard to find and can cost $100+ from some sellers.

The side stand was assembled and the spring installed with my brake spring tool. Unfortunately, the SL90 only has a side stand, so the frame was sitting wobbly on the work table until the rear hub and sprocket holder were cleaned and installed with the new wheel and tire. With new shocks and oversized tires, I hope that the bike will sit on the sidestand correctly. I have to put a rear wheel stand under the swing arm and feed the rear wheel into the back end of the chassis. Once both wheels are in place, the engine can be installed once again.

The engine is certainly bolted firmly into the chassis. Normally, a S/CL90 engine bolts in two places at the back of the engine case. On the SL90, the rear bolts still are installed once the frame side plates are assembled, but then 4 more long 8mm bolts come up from the bottom of the frame’s skidplate and into the engine case. So, the engine is secured with 8 bolts. It’s probably a good thing, as the big bore, stroker engine isn’t balanced for those mods, so the bike might be more than a little buzzy.

The wiring harness was all crispy with heat/age damaged push connectors, plus, like everything else, was coated in the fine dust seen on the whole bike. I ordered some silver electrical tape to tape up the forward section of the harness where it enters the back of the headlight case. Reviewing the condition of the connectors and wiring leads, I wound up buying a better quality used harness from eBay.

The original handlebar was refitted with a new left side headlight control/dimmer switch. The right side throttle housing has one two-wire electrical lead that connects to the kill switch. Again the wire sheath was crispy and flaked off with little effort. I snaked a few pieces of heat shrink tubing together to clean up the appearance of the bike. This is not designed to be a 100 point show bike, so some corners can be cut in order to get the bike completed under a reasonable budget.

The rear fender is rubber-mounted and the rear mount wraps around the back of the frame tube. A curved mount plate and insulating rubber are unique pieces and NLA out in the world, for the most part. I ran the part number and it came up in an old list for OhioCycle’s inventory online. I sent a message to Nick and he posted it on eBay where he is doing all of his sales now. Not cheap, but it is new and will be just what we needed to mount the back of the fender. The forward section of the rear fender mounts with a little set of rubbers, spacers, bolts and washers for two spots. In the end, the tab for rear fender rubbers came to nearly $100. You have to do what you have to do when repairing a 56-year old, one-of- a-kind Honda MotoSport 90.

The speedometer packing had all flaked off, but fortunately it is a low cost item that has been reproduced and fits a number of smaller Honda models. As with all the other steps, you have to take time out to research the part number of some small bit and then find one for sale, ordering it up and adding it to the growing list. In projects like this, several hours are spent in cleaning parts, evaluating the project, researching the correct replacement parts, finding them and ordering them, hopefully on a decent time frame.

The SL90 was about to start up some 10 weeks after arrival. It had a fresh battery, 150 psi compression, new coil and condenser and rebuilt carburetor. Kick, kick, kick.. check for spark and there wasn't one. I jumped the points with a screwdriver and it was pretty much non-existent. I cleaned the points carefully and the spark improved slightly. The bike actually did start up and run for a minute or so. I shut it down and it wouldn't start again. Same minuscule spark issue. I removed the $15 eBay coil that was listed for all 6v Honda singles and the primary side read 4.5 ohms. That's great if you have a 12v system, but not for a 6v bike. I checked the old original coil and it read 1.6 ohms. When I plugged the oil coil back in, the spark at the plug was nice and crisp. 

The bike fired up on 2 kicks. This bike was powdercoated, so the frame needed some places scraped off for engine grounding. What I discovered, was that the coil, on most 90s, bolts to the engine crankcase and the condenser rides on one end, so both are grounded, even though the coil doesn't need to be grounded. On the SL90, the coil is mounted on the backbone of the frame beneath the fuel tank on a split bracket that clamps to the frame tube. It finally dawned on me that the new condenser, mounted on the end of the coil wasn't getting grounded due to the powdercoating on the frame tube. I ground of some of the coating and made a secure ground for the coil bracket, thus also for the condenser. Interestingly, the engine did run briefly without the condenser functioning due to a lack of proper ground.

A whole new disaster was discovered after the engine was fired up, when a huge oil leak developed beneath the left side dyno cover and Hi-Lo transmission unit. Once the cover was removed, the gasket was checked as well as the oil seals. All were as expected, however there is a cavity below the Hi-Lo transmission area that had a curious drilled and threaded hole. Oil was gushing out from that spot and drooling down into a partially blocked drain path. I initially used some red Lock-Tite thread locker on a small 6mm screw, then covered it with some GOOP to help seal the area. I didn’t give it all enough time to set up and when it was restarted again, the leak returned. Apparently, there is something in the transmission area just behind the hole and the screw was dislodged and the leak continued.

Round two was to cut down a shorter screw, use thread locker again and only thread it in about a 1/8th of an inch. Then I mixed up a batch of epoxy and filled the cavity, surrounding the screw. I made a little dam to close off the cavity from leaking further and let it all set up overnight. That should fix the problem, permanently. This was another example of spending extra hours discovering one-off customer mods that were not expected or functional and having to find solutions for effective repairs.

This bike has been a nightmare since it arrived, packed with silt and dirt in every square inch of the machine. I had to replace the wiring harness as all of the connection insulators were fried to a crisp. I could only find a better-used harness from eBay and piece it all back together again. After the battery was installed, the ignition switch failed to make the simple 2-wire connection internally. I removed the switch and pried the back off only to find more of that silt and dirt inside the contacts of the switch plate! I’m convinced that the bike was submerged in a river of some kind, which can only explain how much dirt got into so many places.

The switch repair was successful and finally power was being distributed out to all the lights and ignition system. Even the horn works!

Getting close to the finish line…

Well, the first test ride was around the block and down the street for an initial checkover. The next one was further down my test ride course and ended abruptly when the main jet fell out of the main jet holder. Mechanic error! After a ¾ uphill hike back to the house and my truck, I recovered the bike and reinstalled the main jet. I have been very puzzled that this engine, with all of the modifications done to the engine is running a stock exhaust and a stock carburetor with a stock #85 main jet. I decided to play it safe and put in a little bigger jet, but when I tried a #90 jet on my tapered jet reamers for comparison, then I discovered that someone had drilled/reamed out the stock #85 jet to just about #90 size. Well, that mystery was finally solved.

The next run, closer to home, was more at a fuller power setting and when I returned, oil was puddling beneath the bike! First it was oil coming from behind the right side cylinder head cover. I pulled the cover, checked the surface of the head and cover and found a little gouge in the head, near the screw hole. I added a bit of RTV to fill in the gap and put a double gasket on, which fixed that leak.

Another test ride and another leak from a different place. This time, oil was seeping at the base gasket area where the camchain comes through. The gasket set was not OEM and sometimes the replacement parts are not at full specifications. I had to unhook the exhaust, pry up the intake manifold and gently tease the cylinder head off just enough to access the moulded camchain packing piece. It look absolutely fine, so I gooped it up with some MotoSeal and reassembled it without disturbing the cam timing, which was a small miracle. I had removed the camchain tensioner spring to take the load off the tensioner so it wouldn’t push on the camchain and disturb the timing.

Given that the engine is springing oil leaks on new gaskets and seals leads me to consider that perhaps the increase in displacement is overwhelming the simple breather system and the excess pressure is forcing oil past anywhere there is a weakness. Or, the gasket material and seals are not of OEM quality.

The last gasp repair attempt was a glob of RTV spooned up into the gasket seam area. After hardening overnight, the initial test run looked promising.

I needed more time to finish this bike in time for it to appear at the upcoming Steve McQueen car and bike show. This was already Wednesday before the event, so it had to be right, right now. Don called me on Monday... the bike didn't leak a drop and it went on the transport truck, headed for S. America. 

Whew!

Bill Silver

aka MrHonda

www.vintagehonda.com