QuestionI have a 1993 Honda XR650L. I removed the tranny and had the dealer rebuild it. When I picked it up the mechanic told me that someone put an aftermarket crankshaft in it and when he tried to install the pulse generator rotor, he discovered that this crank did not have the splines found on the honda crank which allow the pulse generator rotor to be installed only one way. After I put the bike back together, I found that it was stalling when I put it into gear, and when I got it going it backfired really loud. I am 100% confident with how I set up the timing chain, and everything other than the tranny work perfectly until after the rebuild. So my impression is that the timing is retarded and I need to rotate the pulse generator clockwise which is the direction the crank turns so it will spark earlier. I am not sure if this is right, nor do I know how much to turn the pulse generator? Do you have any ideas? Thanks so much for any suggestions you have.
AnswerHi Rick,
I think your ideas are on track.
Ideally, you should use a degree wheel and TDC dial indicator to adjust the timing of the pick-up coil. You would need to know timing specs.
Otherwise, try and make minute adjustments until you get the desired results.
Please keep me posted.
FYI... I return tech information files by email. There was no email address with your question as requested and explained in my instructions.
Respectfully,
Mark Shively
Ignition Basics
By David Kucharczyk
A stronger spark does not always equal more HP. If your stock ignition is reliably lighting the mixture, you can add as much spark energy as you want, but it won't make any more horsepower. To date no independent dynamometer test I know of has shown any increase in horsepower over stock wires that were working properly.
Spark Ignition Requirements
To ignite a stoichiometric air-fuel mixture (14:1) approximately 0.2 millijoules of spark energy are required. Very rich or lean mixtures can require as much as 3 mJ. Most coils are capable of storing 60 to 120 mJ of energy and will produce an output voltage of over 30 kilovolts. Spark duration for a coil storage ignition (where the spark energy is generated by the collapse of a built up magnetic field of the primary coil winding) runs between 1 and 3 milliseconds. Capacitive discharge systems tend to have shorter spark duration.
The energy generated by the coil must be delivered to the spark plug gap with enough energy to still ignite the mixture. Power is lost across the distributor gap, via leakage of the wires insulators and any in-line resistances such as the wires and resistors inside the spark plugs themselves.
Longer spark duration and larger spark distance both increase the probability that a given mixture will be ignited by the spark. Turbulence in the mixture itself also helps ignition, although too much airflow can blow out a spark or prevent its formation.
Anatomy of a Spark
To understand spark plug wires, it helps to understand some of the physics behind a spark. You get a spark when an insulator breaks down and allows a rapid discharge of electrons across it. A spark consists of four distinct phases. The first phase is a sudden decrease in resistance, which leads to the second phase, which is marked by current runaway (a feedback condition where more current flow causes lower resistance and thus even more current flow). The third phase is indicated by a voltage collapse as the low resistance path discharges the built up energy and finally the fourth phase which is indicated by a low voltage with high current flow through the insulating material.
Here is a voltage vs. time plot of a typical spark from an automotive ignition coil. The voltage rises rapidly until breakdown of the mixture occurs (1), which is followed by voltage collapse and stabilization at a high current and constant voltage (2). Spark duration (t) runs about 1.5 mS.
The initial breakdown is caused a high electric field density. An electric field has a direct relationship to voltage. The higher the voltage, the stronger the electric field. Electric fields have a tendency to accelerate any charged particles within them such as free electrons or ions. As the particles are accelerated, they hit nearby atoms and knock electrons from them, which then are in turn accelerated by the field. If the field is strong enough, the reaction cascades until a large enough number of the atoms in the insulator are ionized to cause breakdown and conduction.
For a given voltage, field density is inversely proportional to the radius of the surface. Therefore, arcs are most likely to start at any corners or sharp points as these areas will have their greatest field density.
In a spark plug, gas (air and vaporized gasoline) provides the insulator which is broken down. Generally, the initial breakdown voltage is higher in gases with higher molecular weights, so adding gasoline to the mixture increases the breakdown voltage. Increased pressure also increases the breakdown voltage. The relationship of pressure vs. breakdown voltage is also affected by the distance between the two electrodes and a combined pressure X spacing number is used. At pressure X spacing values over 1 (N/m^2) m the relationship is nearly linear, i.e. a 10 X increase in pressure will yield almost a 10 X increase in breakdown voltage. Air has a breakdown voltage of 1000 volts at a pressure spacing value of 10.
What all the above means with regard to igniting the mixture in a motor is; the higher the cylinder pressure and the more fuel there is, the higher the voltage needs to be to start a spark.
Once the arc is started, any airflow over the electrodes has a tendency to carry the ionized (conducting) gas away and blow the arc out. As soon as the arc breaks apart, the voltage rises again and the arc may re-form. If the airflow is high enough to carry the partially ionized gas away faster than it can re-form, the spark will blow out.
Spark Plug Wires
Spark plug wires are made of a conductive core and typically silicone rubber insulation. The core is usually a carbon impregnated fiber or wire. The conducting core is bent over at the wire end and the boot metal is crimped over the core/insulation of the wire. Many aftermarket wires come with no boots, which allows one to add the boots of choice and cut the wires to the desired length.
Most OEM wires use a carbon fiber as this produces minimal RF noise. The down side of carbon fibers is high resistance and thus reduced spark energy. Solid wires have very low resistance, but typically produce unacceptable amounts of RF interference, frequently causing interference in radios (in the installed vehicle as well as nearby radios). This interference can also produce problems with any electronic equipment used inside the car such as fuel injection, panel meters and ABS systems. Spectral output from ignition wires can be anywhere from 10 kHz to over 10GHz. Some wires use a solid metal core that is helically wound to cause the wire to look inductive at high frequencies and minimize EM radiation.
Silicones have a resistivity of 10^9 to 10^11 ohms per meter (between 1 and 100M ohms per mm). Given a core diameter of 2mm and insulation diameter of 7mm that's only 2.5mm of insulation from the core to the outside world and most wires actually leak a considerable amount of energy to their outer shell.
If the surface of the wires becomes contaminated, it can bleed a significant amount of power out of the wire to ground at any point it touches metal or a grounded surface. One way to check for this is to view the engine compartment in total darkness. Corona discharges will be seen at any larger bleed off points. Non-visible coronas are not a guarantee of current leakage however. If possible, it is best to keep the wires from touching any metal or coolant hoses (coolant is highly conductive with respect to air or silicone).
