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Ignition timing for CDI

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CDI Timing for Mopeds 101, originally posted in the forums.

Video

For people who like videos, an alternate timing method than the one explained below can be seen in this video.

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.

  1. Flywheel (internal rotor, or external rotor) with magnets
  2. Stator plate with coils and pickup/hall sensor (as opposed to a points stator plate, which has points and a condensor)
  3. CDI box
  4. High Tension Coil
  5. 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:

  1. Timing light
  2. Micrometer
  3. Sharpie marker
  4. 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.

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