Bullet: Mysteries & Myths -
Don't just love but get to know your bullet
- by B. R. Gurunandan
...also called Spark Timing,
Advance Setting, etc.
The cam-gears drive a couple of idlers
which drive a shaft with a cam,
that operates the contact-breaker...or "points".
The points interrupt the flow of current
in the primary of the "ignition-coil"
inducing a high-voltage pulse in the secondary,
and that, at the plug, becomes the spark.
The idea is to get this spark to ignite the fuel-air mixture
at the right time so that the resultant combustion imparts
the maximum possible energy to the piston at any engine speed.
We have already seen the principles of ignition
in a petrol engine, but maybe in bits & pieces.
So here it comes, again.
The spark parameters we need to consider are -
Timing and Energy
The timing of the spark, like that of most things in life,
is of utmost importance.
Recall that the combustion-front formed at the spark-plug
is moving down while the piston is moving up; and ideally,
they meet at TDC and the piston is thrust down to BDC,
by the combustion-front in the Power-Stroke.
* What happens if the spark takes place too early ?
The combustion-front hits the piston yet to pass the TDC.
This "head-on" collision pushes the crankshaft
in a direction opposite to that intended,
which is nothing but the dreaded "back-kick" of the Bullet.
In less extreme cases, the engine will run,
but with a pinging sound from the collisions,
and less power due to losses in the said collisions.
Prolonged running in this condition can damage the piston,
extensively, and so is to be avoided, scruplously.
* What happens if the spark takes place too late ?
If the spark is too late, the combustion-front hits the piston
when well past the TDC, already far and accelerating rapidly away.
The transfer of energy to the piston is less,
and the mixture may be still burning when the exhaust-valve opens
discharging it into the silencer.
Maybe overheating & oxidising the exhaust-bend to blue colour.
Prolonged running in this condition overheats the entire engine,
ultimately leading to seizure in extreme cases.
About the Spark
The spark must have sufficient power and duration
to ignite enough of the petrol-air mixture-molecules
so as to ensure a self-sustaining combustion-front
in the existing conditions.
If either power or duration of the spark is inadequate,
too few of the mixture molecules are ignited,
and the combustion may self-extinguish,
resulting in the engine stopping
with a chuff and a whiff of partly burnt mixture.
No doubt you have observed this sometimes,
when starting the engine with a partially charged battery.
Or the engine may run with partly burning charge
and excessively toxic emissions and lower power.
We shall see in the Ignition chapter
that the power of the spark depends,
on the current in the primary-windings of the ignition coil,
and that in turn depends on the points-condition and dwell-angle.
The what ?!!! Dwell Angle...sorry, guys !
Advanced jargonese. For now, just take it as the time
for which the points remain closed before opening again.
Which here relates to the gap between the points
when they are fully open.
The Dwell, at any speed must be MORE than the time (microseconds)
needed for the current in the primary of the ignition coil
to rise to it's saturation value.
Or else the spark energy / power will suffer.
We will go into the details in a later chapter;
at this stage, suffice it to accept that
the Dwell is a problem felt in high-revving multis,
and NOT in a sedate thumper like the Bullet.
Of course, the points must be free of
bluing, residue, corrosion, erosion, or misalignment.
Or else the achieved saturation value of current
will be LESS than that designed, which
will lead to a weaker spark.
The gap between the electrodes of the spark-plug
is one of the factors determining the break-down voltage.
Too large a gap stresses the insulation, reduces coil-life,
and may lead to mis-fires under non-favourable conditions.
A smaller gap will ensure a spark under more adverse conditions,
BUT not guarantee ignition of the mixture,
due to the smaller power and exposure.
Obviously, a compromise is called for.
Use a Dial-Gauge to find 00.80mm before TDC
on compression stroke, set the "points-opening" there.
Set the max point-gap at 0.35mm,
set spark-plug gap at 0.45mm
using appropriate Feeler-Gauges.
Yada, yada, yada, yada, yada, yada.
Read Fallacies. Plural. And don't even bother to count !
The Dial-Gauge in place of the spark-plug will be at an angle
to the piston movement,
The piston-top is covered with a non-uniform layer of carbon deposit,
It is also a curved surface.
I won't go into the probability and the trigonometry of it, but
you should if you are about to buy a Dial-Gauge for this purpose.
The precision of the gauge will be totally wasted
by the nature of the displacement it is going to measure.
Then recall the generous slop & backlash in valve/ignition
Do you know how to compensate for that ?
Or else, won't the setting drift when the engine runs ?
Then the spark-plug. You were going to measure the gap
with a costly, high-accuracy, high-life feeler gauge.
But what about the electrode-condition ?
Ask your pre-teens brother in school.
The edge-radii (sharpness) and roughness at the electrodes
of a spark-gap strongly influences the breakdown voltage.
And so does the pressure of the gas around it.
And the nature or composition of the gas.
Read the strength of the fuel-air mixture,
and the type of fuel...yes, petrol differs from gasoline !!!
Has it all no influence on the speed of the flame-front ?
(Is it a universal-constant ?! I mean, g or pi or e !!!)
How precisely, if at all, are you going to measure ALL those ?
Aaaaaarrgh ! Isn't it a wonder that the engine runs at all !!!
But you are a real, die-hard, if amateur, Bullet-eer.
And you ask:
what if we fix the Dial-Gauge vertically
and/or clean the contact-area on the piston,
what if we avoid lash by uni-directional
rotation of the engine to adjusting-position,
what if we glass-paper the elect...
Yessir, scientistsir ! You can.
Like ANY problem on earth, a real Bullet-eer ALWAYS can.
But, I only ask, WHY ? It is totally UN-NECESSARY !!!
Another case of "Fool's Paradise", like the torque-wrench !
Oh, Now be Serious, Nandan
Hey, I am ! Cross my heart, I am !!
Anyway the logic of what I said above is sinking in already:
......if there are so many im-measurables,
what's the point of going overboard measuring a few ?
A fat lot it's going to help ! Isn't it ?!
OK, wise-guy, then how to adjust the timing ?
Set it to start without back-firing, and peak the idle-speed !
And/or set/test the centrifugal-advance on a test-run.
Can it be so simple ?
Then why are Gauges & Strobes in use everywhere ?
OK, Let us imagine.....
What if we were tuning a multi-cylinder engine ?
Even a twin ?
Some cylinder(s) may be timed OK, other(s) badly out.
You could never make out from the running or starting or sounds.
THAT is where those Gauges and all come in.
To SYNCHRONIZE all cylinders, close to each other.
NOT to optimize the ignition timing per-se.
That can be done in multis ONLY by the computers
of VERY sophisticated Engine-Control systems.
And adjusted manually by us in thumpers ! :-)
Someone tell REM.
And the wannabes fiddling with in-appropriate toys...
That sauce for the goose is NOT the sauce for the oysters.
As you have gathered already from above,
is to adjust the timing by performance-parameters.
Which balances the various input parameters,
( WITHOUT precisely measuring any of them )
to the required, optimized output!
Listen to the horse tell where it tickles !
The single-cylinder engine speaks our language !
There are many ways to set the timing of the Bullet.
As always, you have to select the one best for your
conditions and requirements.
If you are mostly riding thumping-cruiser-style in traffic,
you need easy starting and a reliable idle.
And that is where you should optimize your timing.
If you scorch highway traffic at high rpm most of the time,
you need to optimize your timing up there.
Ideally, you should have both. With extra effort, you can.
Later in this serial we shall see how. But for most of us,
the returns may not be commensurate with the effort.
Because the engine will run sweetly enough in both ranges
in most of the cases when adjusted in either one range.
(Ooops, sorry, no photographs ! Believe me, I tried.
I'm sure most of you will manage fine without photos,
the rest just hang on a bit. I'm trying still.
Bullet-eers never give up easy )
Maintain a spare set of points and plug. That way, you
can clean them off-line, getting the bike on road in the
shortest possible time without compromising on workmanship,
even if you are somewhat pressed for time.
Always understand what you are about to do and why.
Mark the initial positions, and preserve the old plug
until you are happy with the new setting ! ;-)
But don't be afraid to experiment. There is no danger
of spoiling anything within minutes by misadjustment.
Don't be in a hurry. Don't expect to find the perfect
setting within a few minutes the first time you try it.
Start with a clean plug, it's gap set to about 0.6mm,
(which is the thickness of an unpainted hacksaw blade)
a set of clean points, also set to the same gap,
and the spark-timing set to about TDC.
To start with, you may wish to err to the retard side,
where the penalty is hard-start, whereas,
too much advance can result in a sprained ankle or worse.
Whenever you remove the points, check that the centrifugal
advance system is OK, ie, not jammed or obstructed.
Oil the pivots and grease the sliding surfaces.
To find TDC, remove the spark-plug and feel for the piston
with a stiff wire. Don't use a BRITTLE thing like a pencil
or an artist's painting brush....Go ahead, ask me why :-)
Engage 4th gear with the centre-stand on spacers, so the
rear-tyre doesn't touch the ground.
The piston is far more easily and precisely positioned
by turning the tyre than with the kick-starter.
If the air filter and carburettor were also just cleaned,
set the carb to earlier settings to start with.
Start and warm up the engine.
All engine adjustments are done hot.
Because that is how the engine runs.
Approximately double the idle-speed by turning the throttle
stop-screw, so that changes in speed can be more easily
observed and also, the engine won't stall while adjusting.
Now turn the points-plate from one to the other end of the
slot. Somewhere in between, the engine-speed will peak. At
that position, find the jump in idle-speed again with the
mixture-screw of the carburettor.
( We are talking of the standard Micarb )
Repeat these steps and be sure that the speed is peaked.
Then lower the idling speed to normal.
This is not the end-point but the real starting-point !
Only now begins the serious tuning !
Now find the combination of ignition and carburettor
settings that are most suitable for your style of riding.
Yes, you are pretty close already, but not quite there.
Ride around a lonely spot and tweak the air-screw and
points-plate till idling & take-off are perfect.
If it is high-speed touring you are going to do, then
there is a bit more to do. Mark the slow-speed setting,
and increase the advance by about 5 degrees.
Try accelerating briskly from 80kmph,
or run up an incline at such speeds.
The engine will display a tendency to knock.
Now reduce the advance in small steps untill the engine
is free from knocking at high speeds and loads.
This setting should be very close to the slow setting
if the design, construction and the maintenance
of the advance system have been good.
If it is off by >5 degrees (??) it needs investigation.
The engine must start with a GENTLE kick when warm,
it should idle slowly without stalling,
it must rev from idle without faltering just off-idle,
it must be capable of fast, economical cruising.
These are parameters you have to quantify for yourself.
The safety-window is wide enough for diversity aplenty !
Do not blindly set something and get yourself used to it.
First, experiment all around the base-settings.
You know now the danger-signs,
so there is hardly any risk of courting disaster.
You should really explore the whole range of settings
and find what you LIKE, instead of liking
what you found where you were told to look.
Yeah, I am all for a Common Civil Code, no doubt, but
the right adjustment in your Bullet is what satisfies YOU.
Proof of the pudding is in the eating, or,
"Dulhan wohi, jo piya man bhaaye" !
By B. R. Gurunandan
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your queries to Nandan.