First I would encourage anyone with a points type ignition to read the article "How to make your bike start and run better" as it has a lot of good information. Of particular interest to some would be the explanation of what function the ballast resistor serves on our machines. They don't give the whole story but the most important points are there.
It is key to understand that it must be used either as a separate entity, or built into the coil itself. 2 and 3 ohm coils should not be used without one in our application as the primary currents are too high.
One of the greatest benefits from going to better coils than the OEM units as supplied by Ducati is that we can make use of the larger spark plug gap as mentioned in the article. I personally have seen this change alone have a dramatic effect on the way older bikes will start, idle and run EVEN IN THE PRESENCE OF THE NEW COILS.
The coils themselves are only part of the solution. Take advantage of them and go for the larger gap. I currently use NGK BP7HS plugs in my 750 GT and it starts first kick every time, idles peacefully at every stop, pulls like a frightened sheep from near idle and cruises happily at 80 mph for hours on end in all weather. I can assure you this was not the case with the original set-up.
One of the statements made in the article pertains to the dwell period of the points on our bikes. They didn't give numbers so I wanted to see for myself if the cam profile should be changed as described.
With the help of a chart recorder I recorded the contact voltage in the primary circuit for both cylinders simultaneously. I also monitored battery voltage on a third channel for interest. After applying some filtering to reduce the amplitude of the ringing during the points opening phase, I got some pretty neat waveforms. In the diagram you can see all of the phases of the ignition cycle.
In the diagram both cylinders are shown (not sure if vertical/horizontal cylinder labels are correct) but I will use the upper trace for the description. At the "POINTS OPEN HERE" label we see the rise in contact voltage from zero followed by the characteristic oscillation beween the coil and the condenser.
After about 4 ms the signal settles down to battery voltage. There is a period where the contact voltage drops to about 10 volts then jumps back up again. This is just an interaction between this cylinder and the other cylinder - remember that they are fed by the same ballast resistor and changes in one can be reflected in the other - more on this later.
At about 24 ms after the contacts opened, they close again for about 8 ms. The coil is energised over this period to get ready for the next cycle. The total period of a cycle in this case was about 32 ms and from this we can accurately calculate the rpm of the engine remembering that the cam rotates at exactly half crank speed.
Here we get 1/0.032 sec which is 41.7 cylcles per second times 60 sec times 2 crank revs per cam rev gives us 3750 rpm which is about what the tach said when I did the test.
The details I was most interested in were the dwell period and the possibility of points overlap. If we look at the dwell period as being 8 ms at 3750 rpm, that equates to 4 ms at 7500 rpm which is sufficient for the automotive coils to reach max energy.
As noted in the article, we don't want this period to be too long as it wastes (precious) energy. The other concern of points overlap can be seen as not an issue here.
Look at the dwell periods for each cylinder and you can see that they are offset by at least 4 ms. That equates to about 45 degrees of cam rotation which is good. If the points were to overlap, we would want to use a separate ballast resistor for each coil.
As far as the interaction that does exist between the cylinders, this is of no consequence as it occurs when the contacts are open and after the circuits have settled. This can be seen in the diagram.
Have a look at your own setup and refer to the article. You too may benefit from the changes!
chart diagram
