The SL350 Honda series of MotoSport models had a short-lived and interesting life span. Honda spawned the SL series models in 1969, starting with the one-year-only SL90, adding SL70, SL125, and SL175 and finishing with the 350 twins. Honda’s CL350s were hardly off-road worthy, being based upon the heavy CB350 street models, by adding some handlebars, cosmetics and high mounted muffler system. The initial SL350 was a meager attempt to make the bike more off-road friendly, at least a little bit, but they continued to make the bike with an electric starter which added a good ten pounds to the bike when you consider the starter motor/starter clutch parts, the starter solenoid, wiring, and a heavier battery.
The SL350K0 continued to use the basic CB/CL engine with large CV carburetors, but the power delivery might have been altered a bit by the low-slung black mufflers, mounted on both sides of the bike.
Honda got a lot more serious with the SL350K1, when they totally revamped the bike, adding alloy fenders, deleting the electric starter system, then installing low-end power-producing modifications such as a small-port cylinder head, revised camshaft, and a set of 24mm slide-type carburetors. These revisions helped to build mid-range power at the expense of cutting off the 10k rpm power peak that the earlier engines were producing. Not content with the K1 mods, Honda added 35mm fork tubes to the front end, replacing the 33mm units that were commonly used during this period.
The SL350K2 in this case, came from a BAT (bringatrailer.com) auction, which made its way all the way from FLA to SoCal. The local owner bought it somewhat on a whim, at what seemed to be a decent price for what appeared to be a restored machine. Certainly the paint and bodywork were all up to a good standard, but there were some noted “oil leaks” and running problems that were addressed by adding a set of Chinese-made carburetors and manifold adapters, plus some large pod-type filters.
Once it arrived, the bike was taken out for a test run and it promptly died about 10 miles out from the garage. After popping the gas cap open to check fuel level and perhaps lessen any effects of a restricted fuel cap vent, the bike fired back up and was driven back home safely. In an odd quirk of fate, the owner had the bike out in his front yard when another friend/customer whose CL77 I had revived after a 10-year sleep happened to drive by the house and spotted the bike. He stopped to talk to the owner and the discussion arose about the bike’s running issues. My name was given as a possible resource and I got the call the next day.
The bike and owner live about 35 miles away, near my sister’s house and my chiropractor's office, which I visit once a month. Being in the area, I swung by to have a look/listen to the bike. The bike was started up after a few kicks and initially sounded quite noisy in the top end, plus there were oil traces at various points on the back of the engine cases and around both sides of the cylinder head. After a discussion of the possibilities, I loaded it up and brought it home for some TLC and hopefully resolution to the various issues.
The first actions were to remove the beautifully painted (and signed) fuel tank and lean the bike over to the right side against the wall to keep from losing oil from the dyno cover when it was removed. Rocking the crankshaft back and forth with a 14mm wrench, it was obvious that the camchain needed to be adjusted. The locking bolt was loosened and the little snapping sound of the plunger jumping forward was heard. Rechecking the crankshaft motion again and now the camshaft was following the crankshaft motion precisely. Next step was to check the valve clearances, all of which were somewhat loose beyond specifications. With the dyno cover off, the point cover was removed and the points checked for proper timing, but only after the plate and advancer were removed to reveal a leaking camshaft oil seal. I happened to have one in stock, so it was replaced and everything reassembled and adjusted.
The fuel tank was replaced and the bike fired up with the little black carburetor chokes engaged. The engine was much quieter now and a quick test ride revealed decent power, but the bike stalled out at a stop sign, then restarted again with a few kicks. For some reason, the kickstarter mechanism seems very difficult to engage the engine at an angle that promotes easy starting. Plus the right side muffler is close to the kickstarter arm path, when pushed down with your leg.
Knowing that the valves were set correctly and the ignition timing was set to specs, the gas cap was checked by prying out the cap innards and drilling it apart. There are several discs of metal that comprise the cap vent system, but despite years of corrosion, the vent holes appeared to be open sufficiently to allow proper tank venting. When the vent system is blocked, the fuel flow is restricted or stops due to a vacuum being produced inside the tank. I cleaned the parts and used a pop rivet to reassemble the various bits, including replacing the outer cap seal.
The bike was driven again, with mostly the same results. Ran fine under power, but sitting still at idle, would tend to just die out after a minute. The bike came with a stock set of 24mm carbs, which had been “rebuilt” but was suffering from a left-side persistent leak. When the carbs were inspected, the float valve needle was found to be upside down, causing a fuel leak problem. After turning the needle around and resetting the float levels, the Chinese set was removed and the OEM carbs were installed. I had to make some new gaskets for the manifold from cork material, but the carbs held fuel just fine and the bike started up somewhat easier. But, again, the test ride was fine, but the bike stalled in the driveway.
The spark plugs were removed and found to be somewhat fuel-fouled, possibly due to the Keyster carb kit parts installed. Compression readings were about 145 psi, which is on the low end of Honda’s specifications. There was more than a little blow-by coming out of the breather hose, which dripped some oil solids down on the swing arm where the tube terminated. I figured that the engine, which turned out to be from a K1, and bored. .50 oversize hadn’t had enough running time to seat in the new rings, assuming that it was machined and assembled correctly.
Oil was dripping into the right side cylinder head cover/cam bearing housing from a leaking tach drive seal and might have been leaking past the o-rings that seal the valve adjuster shafts. With some careful work, you can remove the cam bearings, one at a time, and inspect the bearings, gasket, and o-rings. One of the o-rings had a bit of an odd kink to it, but nothing looked out of the ordinary. The tach seal was replaced and the right side cover was installed. I had already replaced the left side cam seal but pulled the bearing back anyway to check the o-rings, which were kind of stiff. I found some thinner o-rings that were the same OD and added them to the cover shafts. The other source of oil leaks seemed to be in the top camshaft cover for the engine and possibly the left side of the head gasket.
Checking the ignition system included running tests on the aftermarket coils that were installed. Ohm tests showed about 3 ohm primary resistance vs. 4.5 for factory coils and both showed about 10k ohm readings on the secondary side. But when the bike was running, a dynamic test light was hooked up revealing that the right side was firing steadily, but the left side was firing erratically, I had already made a small adjustment to the point fixed contact which was showing contact at the edge of the set instead of more towards the center. Cleaned and re-gapped again, timed accurately, the left side continued to spark erratically which drags down the right side cylinder function. This can alter the vacuum signals to the carburetor mixture circuits, perhaps resulting in fuel fouling. I had already lowered the slide needles to the leanest position, but the idle jets may be out of spec. Idle mixture screws were not terribly responsive at idle.
So, at the moment, the combination of erratic ignition firing and possible idle jet calibrations was the probable cause of the stalling problems. A new set of 4.5-ohm coils were ordered, but in checking the charging system the rectifier wires, which were wrapped with electrical tape, appeared to have a loose positive battery output connection that had been “repaired” with a small bent piece of wire hooked up to what remained of the original electrical connection on the rectifier. Another rectifier was ordered, but digging through my spares, I discovered a replacement unit, which was installed and seemed to work, but just to about 12.5 volts with no lights on. With lights ON the battery voltage started to dip below 12v, so there were more problems elsewhere.
So going down the checklist to consider for engine stalling problems:
Low compression/leaking valves
Incorrect ignition timing due to gap problems, dirt/corrosion on point contact faces
High resistance values on the spark plug cap
Incorrect coil primary winding values
Fouled spark plug/defective spark plug
A faulty ignition coil or wiring connections
Air leak at carburetor flange
Inoperative idle circuit/plugged idle jet
Spark advancer return springs weak or worn
Low voltage to the ignition system
All of the above were corrected, except for the compression issue.
Establishing some communications with the FL seller, it was determined that he had bought the engine from eBay, as well as the cylinder head, which was installed without checking the valves. Beyond the engine running issues, the transmission has a bit of a double neutral when shifting from 1st to 2nd. That could be some kind of issue with the shift drum or stopper, but the owner doesn’t want to spend a lot more money on solving the problems that have arisen.
In having to refinish the rear sprocket cover, I discovered that the stainless steel Allen head screw was close up against the shift pedal on the upshift. I replaced the Allen head screw with a standard Phillips screw to give more clearance.
The best scenario, with it as-is, would be that the piston rings seat in and the valves find some improved sealing, as the miles accrue and the bike begins to be more useful and less troublesome. Sometimes, it doesn’t take much of a building error to spoil a bike project like this when one component step goes south or is overlooked.
In the end, I replaced the rear sprocket with a special order 38t unit from the Sprocket Specialist company. This reduced the rpms by about 500 in top gear. There was some rattling noises on the chain guard, which turned out to be because the inner portion of the guard was not seated properly in the raised guides.
A final test ride yielded good power, easy shifting for the most part and was easier to start that in the beginning. The plug check looked clean on both sides, but it still wanted to die off after 30 seconds if left unattended. The hot compression test was 120-130psi, so still below specs, but perhaps it will improve with some miles added to the engine build.
It was quite time-consuming and everywhere I looked there were issues that needed to be corrected or at least noted for the owner’s benefit.
It was here for a month… I wonder what’s next?
Bill Silver aka MrHonda