About timing
Timing a bike refers to setting when a spark jumps the
gap between the center electrode and the ground electrode on a
spark plug and its relation to the rotation of the
crankshaft. Adjusting when this happens is very important and can significantly affect how the bike runs.
This has been achieved by using a mechanical system called
points
ignition for many years. Recently, advances in electronics have
introduced the CDI (Capacitor Discharge Ignition) which is what we will
examine here.
Lets look at the parts of a
CDI system and how they work.
- Flywheel (internal rotor, or external rotor) with magnets
- Stator plate with coils and pickup/hall sensor (as opposed to a points stator plate, which has points and a condensor)
- CDI box
- High Tension Coil
- Spark Plug
Timing and CDI's
We won't go into great depths on how the CDI produces the spark, or
how electricity works in general (we are only interested in getting the
spark to happen at the appropriate time) but I will glance over the
important parts that relate to the timing.
The basics
The flywheel is attached to the crankshaft. When the engine is
running, the flywheel spins in either a clockwise or counterclockwise
direction (depending on your engine). The stator plate is attached to
the engine and stays stationary while the engine is running. The stator
has two main ignition components; a coil, and a hall sensor/pickup. When
the flywheel spins next to the coil, the magnetic fields from the
magnets produce electrical current that flows through wires wrapped
around the coil. A wire attached to the stator carries this current to
the CDI box which has a capacitor. The current is stored in the
capacitor (a capacitor is a little like a battery) until it is ready to
be sent to the spark plug to create the spark. The hall sensor detects
changes in the magnetic field and sends a signal to the CDI box via a
wire. This signal is created by the hall effect and it happens when the
flywheel magnet changes poles over the top of the pickup (N to S, or S
to N depending on the flywheel and it’s rotation). When this signal is
received by the box, it discharges the capacitor and sends that
electrical current to the spark plug (through the HT coil which converts
the current to high voltage) and this is the point when the spark plug
fires.
Effects of timing and curves
Before we actually set the timing on a bike, lets talk about the
effects of different timing, and why some CDI boxes have a curve.
Once the timing has been set somewhere around a few mm before the piston reaches
top dead center
the bike will run. Too far in either direction and it will run very
poorly, but get it close to optimal and it will run nicely. Once it runs
well, changes in spark timing have the most effect on the temperature
of the gasses that exit out the exhaust port of a cylinder. What this
means is that if a spark happens somewhere around 2.5mm before top dead
center (BTDC), combustion will complete earlier in the cycle and have
time to dissipate its heat into the head, piston, and cylinder before
the exhaust port opens. This means that the gasses that exit out the
exhaust will be cooler. When the spark happens later, lets say somewhere
around .5mm BTDC the combustion process completes later, and the
exhaust gasses have less time in contact with the head, piston, and
cylinder walls. This means the gasses that exit out the exhaust port
will be hotter. Why do we care what temperature the exhaust gasses exit?
I’m glad you asked… the hotter the gasses, the faster the gasses will
travel down the exhaust pipe. The cooler the gasses, the slower they
will travel. So why do we care how fast they travel? I’m glad you asked…
Slower gasses will move the powerband of a pipe to LOWER rpms. Faster
gasses will shift the powerband of a pipe to HIGHER rpms. So as timing
changes, so does the powerband of a pipe. We can use this information to
our advantage. If we have variable timing we can have a variable
powerband of a pipe, right? So why not have the powerband of a pipe be
at lower rpms when the engine is actually at lower rpms and then have
the powerband of a pipe be at higher rpms when the engine is actually at
higher rpms. This is exactly what happens in cdi’s that have a curve.
They start out with more advanced timing at lower rpms (to shift the
powerband of the pipe to lower rpms), and then retard the timing at
higher rpms (to shift the powerband of the pipe to higher rpms). Some
cdi’s dont have a curve though, and you have to compromise. If you do
not have a curve on your cdi, you set the timing so the powerband of
your pipe is where you want it. Set it more advanced to lower the peak
power rpm value, and set it more retarded to have the peak power of the
pipe at a higher rpm value. There are limits to how far you can
advance/retard your timing however, as your bike may not run properly if
it is too far in either direction.
Setting the timing
Now that we know the effects of timing values lets talk about how to set the timing on a CDI.
Special tools Needed:
- Timing light
- Micrometer
- Sharpie marker
- Piston stop
• Put a line on the flywheel anywhere with a sharpie in a fashion
similar to what was done in this image (ignoring the mark on the engine
for the moment; only mark the flywheel).
• Install the flywheel but tighten it only enough so that it can't spin freely, as you'll be taking it off in a minute.
Hook up the timing light,
and either spin the engine (spark plug removed, but grounded so it
sparks) with a power drill or have a buddy pedal it, and make a mark on
the engine case (or something stationary) where the line you drew shows
up.
You now know that the spark will always fire when the line on the
flywheel matches up to the line you drew on the case. You may want to
at this point, hold the two lines together, and draw two more lines in a
more convenient place, as they may wind up being on the bottom, but
don't forget to erase the old lines so as not to get them confused.
• Loosen the flywheel nut and pull the flywheel off, but then put the
nut back on hand tight so you can still rotate the flywheel but tight
enough so that it won't wander on its own.
• Now, this is where the calipers become handy. Some people don't
realize that the little pointy bit that sicks out from the bottom of the
calipers is a 'depth gauge', as depicted in the goofy image below.
• You'll need to have your spark plug removed to do this. Be sure that
your crankshaft is in a position in which the piston is backed away from
TDC a bit, and don't forget which way your engine turns, as you could
wind up with terribly retarded timing.
• Expand the calipers a few inches, and stick the depth gauge in the
spark plug hole. The base of the calipers should be resting flush
against the cylinder head, and try to maintain perpendicularity as if
you hold them at an angle, your reading will be off. Assuming you opened
the calipers enough, pressing the base of the calipers against the plug
hole should force them to close a bit.
• Rotate your crankshaft in the normal direction of operation while
holding the calipers in place. On a French engine like a Motobecane or
Peugeot, this is easily accomplished by simply holding on to the
external clutch bell, but on an internally clutched engine, you will
need to use the flywheel itself to rotate the engine.
You should see the reading on the calipers dropping, and as the
piston rises, the rate of change on the calipers will shrink. Watch the
numbers fall, and when they no longer change at all, you have found TDC.
You may want to repeat this process one or two times just to make sure,
as if you rotated past TDC the numbers still will not change and your
piston will begin to fall. It is a tricky process and the only way to
get it right is with patience and focus.
• Now that you have found TCD, you can very carefully remove the
calipers and observe how far the depth gauge is still sticking out. If
you are using digital calipers, hit the 'zero' button, or if you are
using analogue ones, set the dial indicator to line up with the current
needle position.
• Now that you are 'zeroed', you can slowly open the calipers until the
dial reads X millimeters where X is your desired timing (1.2mm, etc).
Once you have that set, turn in the locking knob so that way you won't
lose your work.
• Almost done. Turn your crank backwards just a little bit, and then
reinsert the calipers. Very slowly turn your crank in the normal
direction of operation until you feel the piston ever so slightly hit
the dept gauge. You now have your piston set at Xmm bTDC!
•
French-style engines: Remove the calipers. With one hand, very
carefully hold the crank shaft still by holding on to the flywheel. For
the love of Mithras don't let it move! With your other arm, rotate the
flywheel so that your two markings from before are lined up and try to
tighten the flywheel nut a bit just to make sure nothing shifts.
Other engines: The same logic applies, but you may want
to completely loosen your flywheel first, as you are unable to firmly
hold your crank in position. Once free, very gently reapply it so your
sharpie lines match up and with the calipers in their previous position,
ensure your piston is still at the desired position and then carefully
reapply the flywheel nut. You may want to double-check your work as
there is a chance that things have shifted while doing this.
• HuzzAh! Insert a plug stop or a piece of rope or use a strap wrench on
the flywheel to hold things in place (strap wrench on the flywheel is
the best option), and tighten that thing down and you're good to go!
If your CDI has a curve
I picked 1.5mm BTDC above as an arbitrary number. Lets talk about
where you should set your timing for your specific application. If you
have a CDI that does not have a curve then you will probably want to set
your timing somewhere between 1.0mm and 2.5mm BTDC. Setting it much
lower than 1.0mm BTDC will cause the bike to run poorly. But setting
your timing towards the lower end of the scale will move the powerband
of the pipe to higher rpms giving your bike higher top speeds
(theoretically. If your bike produces enough torque to pull your gearing
at high speeds then it will go faster). Setting it much higher than 2.5
and your bike will also run poorly and you run the risk of other
problems such as detonation and/or pre-ignition. However setting it
towards the higher end of this scale will yield more power at lower rpms
for better takeoff and midrange.
If your CDI has a curve on it, it makes things a bit more
complicated, but if you understand the implications of variable timing
as laid out above, then you can use this to your advantage and increase
power at all rpm ranges. Let’s take a simple curve example: The stock
timing curve of a Puch HPI ignition. The curve looks something like
this:
^
|
RPMs |
| | | *** |
| | | *** |
| | | *** |
| | | *** |
| | | *** |
| | | _______ |
| mm -> |
|
The curve from this box is (roughly) linear. As the rpm’s increase,
the timing retards. So when setting the timing for this application we
would set the timing at idle to be on the more advanced end of the scale
laid out above. 2.5mm would be a good setting at idle, because it will
give us better low end power. Then as the rpm’s increase the timing will
automatically retard itself to give us more power at higher rpms.
Sounds like the perfect curve right? It’s pretty good, but idling at
2.5mm BTDC can be rough sometimes. But if we set the timing to 1.5mm
(for better idle and less chance of detonation/pre-ignition) and as RPMs
increase, we run the risk of actually having the spark retard itself so
far that it’s firing AFTER the piston reaches TDC which can rob us of
power.
Some CDI’s have an initial advance to them before retarding back
to the original timing at idle, or even further retarding it past the
point where the timing was at idle. The curve looks something like this:
^
|
RPMs |
| | |
|
| | | *** |
| | | *** *** |
| | | *** *** |
| | | *** |
| | | _______ |
| mm -> |
|
When setting the timing on a CDI with a curve like this, you want to
set your idle timing somewhere in the middle of the scale. Somewhere
around 1.8mm BTDC. This is because if we set it at 2.5, the timing
advance might put the timing at 4mm BTDC at the top of the curve and
that can put us in extreme danger of detonation and/or preignition. When
set at 1.8 the advance will give us low end and midrange power, and
then increase our top speed by retarding the timing and shifting the
powerband to higher rpms exactly when we get to the higher rpms.
This is a vary basic tutorial on CDI timing. Now go out there, use this as a base for experimentation, and go faster.
Supplay
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jet # 110. Tanpa reamer, intake manifold standard. Box filter terpasang
supaya debu tidak tersedot waktu motor dibawa ngebut nyalip bus, atau
truk. Gasss terus pokoknya.
