I am on several
forums dedicated to vintage Honda motorcycles and see a lot of
similar questions and complaints about various aspects of owning and
reviving these 50-60-year-old machines. I thought I would go over
some of the basics once again for newbies that are just getting into
the hobby. So, let’s start at the beginning… You bought it and
now… See * at the bottom of the story first.
1.
It won’t start…
Does it turn over and feel like there is some compression being built
up in the cylinder (s)? Test compression first, even if you don’t
have a gauge. Put your fingertip in the spark plug hole and turn the
engine over, however you can at the same time. Holding the throttle
wide open gives the best results. If you do have a gauge, it should
be reading anything from 125 to 180psi. If you are looking at
75-90psi, it won’t start and run at all. Try adjusting the valves
first to see if they are leaking compression past the valve
heads/seats. If that doesn’t help then it is time to pull the
head/engine and find out if the valves are tight/burned or the piston
is scored/broken from seizures in the past.
2.
It turns over AND
has compression… Does it have spark at the spark plug? Determine if
the bike requires a battery or if it has a magneto (mostly 50-80cc
singles). A bike with a magneto ignition system doesn’t need a
battery to make the coil spark, but if you start up a bike that has a
battery in it for the lights and horn and the battery is dead, any
light bulb that is normally lit will be blown out from the
uncontrolled charging system output.
3.
For bikes with
battery-powered ignition systems, you MUST put in a fully-charged
battery to make the engine run. If you jump-start a dead/dying
battery enough to get the engine to run, the light bulbs are also at
risk. Most Hondas of that era do not have voltage regulators,
preferring to use the fully-charged battery as a buffer to absorb the
charging system output and prevent over-voltage situations.
4.
So… now you have
compression and spark, perhaps, but does the spark come at the right
time? The ignition points are basically just variable electrical
switches. Setting the point gap at the highest spot on the point cam
is the starting point of the ignition timing exercise. Also, the
point’s contact faces must be clean and shiny to maximize the spark
energy in the coil. So, set the gap at around .012-.016” and then
ensure that the points are closing together and making clean contact
with each other. If the point faces show a > > instead of | |
contact pattern then the condensers should be replaced. Bad
condensers also cause a lot of visible arcing across the point gap
when the engine is running. BEWARE of any aftermarket copies of the
original Nippon Denso, Hitachi, Kokusan, TEK and other OEM point
sets. Many copies have incorrect dimensions for the point rubbing
blocks and setting the ignition timing is nearly impossible. Daiichi,
SEV, Century and other brands are not recommended. OEM points will
have ND or a different symbol for Hitachi and Kokusan stamped on the
point bases.
5.
Ignition timing is
set by the points just opening at the F (firing) mark. Moving the
backing plate back and forth will help you to dial in the correct
ignition timing. On some twins, you have to set the left side points
to the normal gap, move the backing plate to where they open on the LF
mark, then you will have to change the right side point gap
open/close until those points open at the F mark.
6.
Once you are sure
that the ignition system is setup properly to start the engine, then
you have to determine if the mechanical spark advancer is doing its
advance/retard function properly. Point cams can get hung up on rust
or old grease on the cam base plate shaft and that will alter how the
spark timing occurs. If the point cam is slow to return to full
retard at idle, then remove, mark and clean the point base shaft and
the point cam, so that they both move easily back and forth. Use
special point cam grease on the point cam to reduce excess rubbing
and friction during operation.
7.
So, if your
compression and spark timing are all correct, then it is onto the
fuel system for cleaning and testing for full function. For
carburetors which have been left standing with old fuels in the float
bowl for months/years, the chances of the bike starting normally are
very slim. For the engine to start up the idle jet must be clear and
the adjacent air bleed ports in the carburetor throat must be open.
The carburetor float
chamber must have the correct level of clean, fresh fuel in order to
feed the idle and power/main jet circuits. Each carburetor has a
specific float level adjustment which must be adhered to for proper
operation. Also, there are air bleed ports in the carburetor inlet
that must be clean for proper fuel mixing. Also, be sure that the
bowl vent passages in the roof of the carb body are open.
If the carburetor
needs to be cleaned, use an ultrasound machine to clean out the small
passages of the carburetor body and any removed OEM parts. In many
cases, the aftermarket carb kits are not accurately made, so try to
use as many of the original parts as possible during reassembly.
Yesterday’s
carburetors are deeply affected by today’s fuels, which are
generally loaded up with 10% alcohol to reduce emissions and stretch
out the fuel supplies. Because the fuel is somewhat diluted, there is
less energy released when the fuel vapors burn. This situation
creates a lean ratio mixture condition that generally needs to be
corrected with larger-sized jets, at least for the main jet circuits.
You must check all
the carburetor components carefully as they can be damaged by the old
fuel vapors and solids that remain inside the float bowl after months
or years of neglect Most older carburetors used small brass floats
which can be compromised by the acids in old fuels which etch into
the metal and create pinholes. These tiny holes will allow gasoline
to enter the float lobes causing them to lose buoyancy and start to
sink into the float bowl. When that occurs the float cannot control
the fuel level in the bowl, which creates an overflow condition.
When fuel flows out
of the overflow tubes at the bottom of the bowl, you have problems
with either the float valve not shutting off or the float itself.
Figure out which is at fault and repair it before moving forward with
the startup cycle. If you are sure that the float level is set
correctly and the float valve is doing its job, then look carefully
at the overflow tube in the float bowl. They have a tendency to split
along the lengths of them, causing persistent fuel leaks at the tube
exit.
8.
When setting up the
carburetor body, install the idle mixture screw and back it out about
1-1/2 turns to begin with. Turn the idle speed screw in until it
contacts the bottom of the slide and just starts to raise it upwards.
Be aware that idle screws come in two functions: Air screws and fuel
screws. When you back out an air screw, more air is added to the idle
mixture. When you back out the needle-like fuel screws, more gasoline
is added to the mixture. For carburetors that bolt onto the back of
the cylinder head, look for warped flanges and flattened out o-rings
that seal the carb to the insulator, which seals to the cylinder
head. Insulators will either be sealed with an o-ring or a flat
gasket.
Obviously, if you
are working on a twin or four-cylinder model, your work will be
multiplied and you will have to make sure that the carburetors are
synchronized properly on all cylinders. Twin-cylinder bikes can be
synchronized by watching the slides lift off of the idle speed
screws. To begin, turn the speed screws in until you see the slides
just begin to lift. Then, adjust the cables at the top of the
carburetor using the screw adjusters. For 350-450 CV carbs, you will
have to watch both of the carburetor cable arms to ensure that they
both move at the same time. REMEMBER: For twins with standard
slide-type carburetors the slides are side-specific. You must see the
slide bottom cutaways in the throat of the carburetor. They must
always face the air filters. If you reverse the slides, you will get
fouled spark plugs and a very high idle when you start the engine.
Yes, they will install backward!
Fuel: When the
1960-70s bikes were built, fuel quality was much higher in octane
ratings. Regular fuel was 90 something octane and premium fuel was
100-105 octane and all of it contained lead to protect the valve
seats. Unless you are buying your gas at a boat marina, airport or
from a race gas station, you will have to deal with the usual 10%
alcohol infusion with octane ratings of 87/89/92. Gasoline chemistry
has changed radically in the last century and the octane ratings used
in the last century are not necessarily a match for today’s fuel
ratings. My rule of thumb is to use premium fuel for vintage Hondas,
either with or without alcohol. What I have discovered is that with
alcohol-infused fuels, the engines run leaner and often need a 5-10%
increase in the main jet sizes to compensate for the alcohol
additives in today’s fuels. My recent experience is that most CB77
Super Hawks run best on a #140 main jet vs. the stock #135 main jets
specified from the factory. If your machine is tuned to full normal
specs and is still giving performance problems, try stepping the main
jet up a size or two.
*Of course,
always start the process by checking to see if the engine has enough
oil to meet the marks on the dipstick. Low oil levels can often lead
to seized and damaged engine parts, so making sure that the engine
has sufficient oil in the engine. You might want to drain and flush
the engine oil FIRST before you go forward with the above steps. If
chunks of metal or a lot of aluminum specks come out of the oil,
there may be expensive issues going on inside the engine that won’t
be helped with the above troubleshooting and tune-up steps.
If you have followed
all the above steps, you should have a running motorcycle once again.
Bill Silver 8/2021
