For
those of us who were alive and involved with 1960-80s Honda
motorcycles before the advent of “unleaded gas” and “10%
alcohol gasoline,” we remember that Honda motorcycles were
carefully jetted for maximum power, economy and reliability, right
out of the crate.
Honda CB72 carburetor with "power jet" function.
Reviving
those 40-60 year-old motorcycles generally requires fuel system
cleaning and a careful inspection of the carburetors. Before the
introduction of CV (constant velocity) carburetors on the DOHC
CB/CL450 twins, the vast majority of Honda’s carburetors were made
by Keihin and were simple throttle-valve units, consisting of a float
valve, float, main jet, idle jet, carburetor slide and tapered slide
needle. Changing the fuel calibration of the basic designs to
accommodate various versions of each application was generally
required when the exhaust systems were changed from CB=Street to
CL=Scrambler. With mostly identical engine specifications between two
series of machines, the change from a low-mount to high-mount exhaust
causes the carburetors to act differently and so Honda’s engineers
carefully made appropriate changes to needle taper, main jet and
sometimes the idle jets to bring a smooth fuel delivery to each
application.
For
the most part, Honda offered many sizes of main jets, idle jets and
needles to re-calibrate the carburetors, as needed. In the early
1960s, Honda changed the basic shape of the float bowl from a rounded
shape to one that was more squared-off on all sides. There was no
need to change the calibrations as the body shapes changed. When
owners began to alter either the intake or exhaust system for more
“performance” the original calibrations are no longer valid.
Changes to the intake or exhaust systems often require a change in
the jetting of the carburetors due to an increase in airflow or
alterations in the intake or exhaust tract length. Changing the tract
lengths changes the resonant frequencies of the systems which can
enhance or detract performance from the original settings.
For
example, the difference between the 305 Honda Super Hawks and
Scramblers lie in both the exhaust header pipe lengths, muffler
lengths and the length of the intake to air cleaner runners. The
engines have the same pistons, cam timing, valve sizes and carburetor
bodies, but the standard main jet of a CB77 is #135 while a CL77 runs
jets down around #130. Even the pilot jet sizes change from #38 to
#42 between the two models.
The Scrambler’s long, long, header
pipes have a much different resonance frequency that bounces a
pressure wave back to the exhaust port that can affect the incoming
intake charge in the cylinder. The longer intake and exhaust tracts
of the Scrambler enhances some of the mid-range torque, while the
shorter CB intake and exhaust lengths enhance higher rpm performance
on the road or race-track. Scramblers are geared shorter than CBs, so
feel “more powerful” than Super Hawks, but the CB77s will hit
nearly 100 mph vs about 85 mph max on a Scrambler. While 305
carburetors with 26mm slides have 2.0mm cutaways, the CL72s have a
3.0mm cutaway but the CB72, with the same-sized carburetor has a
2.0mm cutaway. You have to look over each item carefully, if you are
tempted to start mixing and matching Keihin carburetor parts from
similar models.
Be
aware that prior to 1967 all of the Honda carburetor jets were JIS
thread pitch and thus not interchangeable with later ones. Main jets
in ISO dimensions have a ring marked around the outside edge of the
jet, whereas the JIS threaded jets were left plain. This can cause
great confusion when you have a model like the 1965-69 S90, CL90,
SL90 models. which bridged the 1967 changeover gap. When people start
trying to mix-match a box of loose parts from models like these,
problems will arise.
Fast-forward
to the 21st Century and most all markets have succumbed to
at least 10% alcohol-infused gasoline which is fairly unstable in
long-term storage and has less fuel energy that pure gasoline. In
order to compensate for the loss of fuel energy (and a slightly
different fuel viscosity), the rule has become that bumping the
carburetor main jets up at least one size (5 to 10%) will equalize
the air-fuel ratios that these old engines were used to seeing. After
setting up stock CB77 Super Hawks to original stock settings of
#42/135, I found the bikes to having flat spots and seemed to be
struggling at highway speeds. Swapping the #135 main jet for a #140
pretty much cured the mid-range/top end power problems and even
helped the normally cold-blooded Super Hawks to warm up quicker and
have better overall throttle control.
Similarly,
I have had to re-jet a number of CB400F Super Sport fours from #75 to
#80 main jets, in order to overcome the same lean conditions. I made
that change to a bike that I sold to a young man from NY. He flew in
to San Diego and drove the newly rebuilt machine all the way back to
NYC in 4 days, running it at 70-80 mph for most of the trip. When I
first drove the bike, after a careful check over (and it had an
electronic ignition), the bike struggled at freeway speeds, feeling
flat and struggling. A simple step up in the main jets solved the
problem and allowed a successful cross-country journey, which I think
might have suffered a melt-down due to the original lean jetting
coupled with alcohol gasoline.
Bear
in mind that by the mid-1970s, the EPA was beginning to lean on
motorcycle manufacturers for their part in the increases of carbon
monoxide and hydrocarbon that the world was experiencing. Motorcycles
were carefully calibrated for emissions during start-up, idling,
off-idle performance and at full throttle, just enough to keep them
from seizing their air-cooled pistons during extended riding
conditions. I think we have all seen the idle mixture screw limiters
that were required to keep owner’s from destabilizing the factory
settings. Lean mixtures due to slide needle taper designs were
another part of the equation and soon owners were putting little thin
shims beneath the needle heads to raise them up slightly to enrichen
the fuel ratio mixtures at part-throttle.
1980s
Kawasaki motorcycles were famous for requiring 5-minute warm-ups,
just to get the bikes down the driveway due to lean idle/part
throttle calibrations required to keep the engines clean enough to
pass the EPA regulations. Kawasaki was always pushing the envelope on
performance machines, but the more power they make, generally, the
more pollution is created in the process.
In
order to enhance engine performance, companies like “Dyna-Jet”
began to develop “Stage” kits to help owners get a handle on
carburetor calibration issues with either bone-stock machines or for
ones that had a 4:1 aftermarket exhaust or replacement air cleaner
element or full system. These were sold under the guise of being a
“racing” product that was not to be installed in production
street bikes, especially in strict-California emissions zones. Dyno
tests generally showed some improvements to horsepower, throttle
response, torque readings and overall drive-ability with installation
of the kits, but obviously the previous emission calibrations were
out the window and any measured emissions were substantially higher
than those that were create at the factories.
The
days of carburetors are numbered due to the introduction of efficient
fuel-injection systems and computer-controlled ignition advance
curves. But in the meantime, we have to do our best to work with what
we were dealt with from the factories dating back sixty years. So, to
begin with, we have to clean, clean, clean the carburetor bodies and
all associated components, then start to make educated guesses about
what the engine will need in the way of corrected fuel-air ratios at
all engine speeds and conditions.
Just
assume that the use of alcohol-gas is going to make the engine run
lean with stock OEM jetting settings. Once the fuel passages are all
clean (ultra-sonic cleaners are great!), then a careful look at each
internal component is next and then consider obtaining some slightly
larger sized main jets to begin the calibration experimentation.
Multiple test rides will give you a “seat of your pants” feel of
what the bike is trying to do and what it wants in the way of fuel
needs. When you feel the flat-spot or performance plateau/misfire,
try to pull the choke up slightly to see if the problem gets better
or worse. If it improves, you need to richen up the mixture. If it
gets worse, lean it out again.
Reading
spark plugs is somewhat of a black art, as well, but an initial look
at the overall appearance of the plug tip will tell you if it is
running lean (all white tip), rich (black and sooty),
fuel/oil-fouling (wet end) or has a nice tan insulator without a lot
of build-up of carbon or oil.
The
bike should start up with sometimes full choke for just a moment
until it is running, then reduce choke to around 1/2-3/4 while the
engine warms up. Backfiring back through the carburetors/air filters
is a sign of a too-lean fuel condition. If the bike fires up with no
choke, it is generally running too rich at idle and off-throttle. The
engine should idle down normally after about 4-5 minutes of operation
time and the air-mixture screws should cause the engine to change
idle speed and quality when they are turned in and out within a
quarter-turn of normal. Always make the final idle mixture
adjustments when the engine is at full operating temperature.
If
the idle mixture screws are not responsive, then you have issues with
jet sizes, blocked air/fuel passages inside the carburetor body, air
leaks at the intake manifolds or warped carburetor bodies at the
flanges. Ensure that the suggested float levels are use without
fudging more than a millimeter or so. If you have fuel running out of
the bowl’s overflow tube, you either have a float valve not
sealing, a float level that is too high or a crack in the brass
tubing of the bowl overflow tube.
While
Honda bolted carburetors directly onto the cylinder heads in the
early years, they began to recognize the high-frequency engine
vibrations could cause fuel frothing and inaccurate metering at
various engine speeds. The new-generation 350s and 450s used
rubber-fused manifolds to secure the carburetors to the engines with
both damping qualities of the rubber, as well as some heat insulation
features which helps to stabilize metering in both hot and cold
driving conditions.
As
an example: The little 1960s Honda 160-175 twins had solid-mounted
carburetors, bolted to aluminum intake runners, which were then
insulated from the head with phonolic blocks and o-rings. But, when
Honda revised the 175 twin engine up to 200cc in the early 1970s,
they created rubber intake manifolds to improve fuel metering and
vibration dampening of the carburetor bodies.
Some
of the most confusing carburetor sets are featured on Honda CB/CL350
twins. There were several different carburetor body styles and 4-5
different calibrations suggested for the bikes, depending upon
whether they were CB or CL versions (and the early SL350s, too). In
the case of the 350s, there were different camshaft timing periods
between CB and CLs, so that, alone, is enough to cause changes in
calibration settings. These early CV carburetors featured both a
primary and secondary main jet to help with the transitions from part
throttle to full throttle conditions. Sometimes, all the changes
needed were to bump up the primary main jet to the next size in order
to smooth out the throttle response. One of the problems with 350
twins is that they vibrate quite a lot and the original mufflers for
both CB and CLs tended to either break or rot out prematurely.
Replacing the damaged components with OEM items (of which there were
several types) was often costly and the parts were hard to find due
to demand. Owners resorted to use of other slip-on muffler systems
which had different lengths and back pressure features, all of which
caused metering problems back at the carburetors.
Many
of us “old-timers” know that the carburetors are the LAST thing
to blame/adjust when engine performance is suffering. Setting up
carburetors to operate correctly requires a sound foundation in
several aspects of the engine’s systems. Go back to the beginning:
Compression test, valve adjustment, camchain adjustment, ignition
timing settings, spark advancer function, coil/condenser/point
conditions, spark plug caps and even the correct spark plugs are all
to be checked and confirmed before diving into the carburetors.
If
you have a low-compression engine that is using oil and fouling spark
plugs, the carburetors probably are not the root cause of your engine
performance issues. If the point contact faces are all pitted due to
age/miles or a bad condenser, you will have erratic ignition output
at the coils. If the coil leads are crispy and cracked, the sparks
might be jumping to somewhere else than the gaps of the spark plugs.
Spark plug caps should have no more than 5k ohm resistance
measurements and should be screwed onto the spark plug wire ends
which have been trimmed back to bare wire.
Aged
condensers cause points to arc randomly or excessively which
adversely affects ignition coil output. Make sure that you are using
a fully-charged, load-tested battery and that the charging system
output is keeping the battery charged during operation. Engine
vibrations can cause loose connections in the wiring harness and
individual connectors to loosen or disconnect altogether. When Honda
started putting ignition switches up in the center of the fork
bridge, the pull on the switch connector often causes the switch
connection to come apart, partially or completely. Bikes with KILL
switches need to have the switch contacts checked and the wire
connections secured so the coils get a steady feed of voltage.
All
of these aspects of motorcycle function and maintenance can come into
play causing performance issues that are first attributed to “the
carburetors.” Obviously, if the carburetors have been sitting for
months/years with old fuels in the bowls, then they need careful
cleaning and scrutiny for normal function. Carburetor work is made
more difficult with the alcohol in the fuel, which attacks most
rubber parts, especially float bowl gaskets! If you drop the bowl off
of a carburetor and the gasket is not the original glued-on version,
chances are that the gasket will swell up and will NOT fit back into
the carburetor body grooves. Cleaning with soap and water then drying
in the sun or gently with a heat gun will generally allow them to
reclaim their original shape and size. If you are doing a lot of
jetting or carb work, plan on getting several sets of bowl gaskets to
have on hand for quicker service procedures.
To review:
Test
drive evaluation…
First,
start the engine and listen to it to hear if it is going to need a
prolonged period of “warm-up” or if it will take enough throttle
to drive off after a minute or two of running. Run the bike up gently
with slowly increasing throttle openings. Listen and feel if the
engine is taking throttle cleanly or if there is a hesitation or
misfire occurring at certain throttle openings. While driving, if it
is safe and easily accessible, pull up the choke lever or knob
slightly during the time when the misfire or roughness is occurring.
If you feel a surge of power or increased smoothness in the engine
performance, then you will need to enrichen the fuel mixture at that
point.
Off-idle
hesitations can be resolved with either an increase in the size of
the idle mixture jet and/or raising the needle slightly with shims.
If the hesitation is more in the ¼ to ½ throttle settings, then you
can raise the needle a little bit more and/or increase the main jet
size slightly, one step at a time. Remember that if you increase the
main jet size, you have enriched the whole fuel ratio from around ¼
throttle all the way up to WOT. If you get a good full-throttle
response, but the mid-range is sagging and the plugs start looking
dark and sooty, start to lower the needle back down one step at a
time.
It’s
a balancing act, but one well worth pursuing in order to maximize the
engine’s performance over a wide rpm band and to conserve fuel that
might be wasted in an engine that is running excessively rich. Pull a
spark plug or two and look at the tip to see if they are getting a
nice color on the ends which indicates proper jetting.
Excessive
rich conditions can wash down the oil on the cylinder walls, leading
to ring wear or even piston seizures in extreme cases. Excess fuel
will contaminate the engine oil, as well, leading to lubrication
breakdown. There is a whole chain of events that happen when you
start to “adjust” the carburetor settings, so think it through
first, before you begin wholesale changes. On many bikes, just
removing the carburetors is a huge pain, so you don’t want to be in
the position of having to do it more than once or twice, at the most.
Take
time to understand each stage/function of a carburetor, so any
changes you make are done with caution and thoughtfulness about what
you are trying to achieve. Carburetor wizards are getting harder and
harder to find these days, now that fuel injection is becoming more
and more common. Take time to learn the craft for yourself and to
help your friends who are in need of timely and educated repairs.
Bill
“MrHonda” Silver
8/2020
