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
05/23
