I have a very mixed history with Bultaco ignition systems. My first Bultaco was one of the original 100cc Lobitos. I was able to start it with the kick starter for about two weeks. After that I stopped even trying...I just paddled it down the street until it fired. A little frustrating and more than a little embarrassing. I was expecting my new (to me) Campera to treat me the same. Sure enough it stopped starting.
I really can not say that the stock system was at fault. It may well have been that the Mikuni suffered some sort of failure, but I was determined not to have to worry about sparks again. If I have to ultrasonically clean a carburetor, or re-jet it, or adjust the idle....fine. I just don't want to be worrying about whether or not I am fixing the system which is at fault. It is reassuring to know that the ignition system is working. I was never sure with the stock system.
The solution came in the form of a German system from Powerdynamo. It is a beautifully made system which can include an AC power source for a rudimentary lighting system. I got the system that had the 2kg rotor. There is a lighter rotor for the Pursangs. It is not a particularly inexpensive solution (about 289 Euros including a puller) but it is the only option for the older 4 speed Bultacos.
The installation was very straight forward. You just remove the old points system which lies behind the flywheel and replace it with the Powerdynamo stator. As can be seen, it is a beautiful piece. Five wires lead from the stator. Three go to the coil and two go to the AC regulator. Very high quality wires and insulation where the wires exit the case. You would be hard pressed to screw up this part of the installation.
The rotor, on the other hand presents some interesting challenges. The instructions that come with this unit indicate that you will not be using the woodruff key that locates the Bultaco flywheel/rotor. when you install the Powerdynamo rotor. Interesting....so how do you time this thing?
Etched on the outer face of the rotor is a thin black line. Again, according to the instructions, when this line crosses the blue mark on the stator, ignition ensues. There are two problems. First and most obviously, there are two blue marks on the stator. Take a look at the picture below. There is a very prominent blue dot on the mounting plate, and then a somewhat fainter mark on the gold plate just to the left of the windings which generate the spark. Which one to use? I elected to use the obvious dot.
The second problem is that the rotor is somewhat difficult to attach to the crank at exactly the place you have selected. Assuming that you have used your dial gauge to locate the optimum point where you think that ignition should occur, the challenge is to place the rotor on the crank at that exact point and then tighten it down without disturbing its location. Trust me...this is not easy. The magnets on the rotor want to move the rotor around, and when you attempt to tighten it down...again, things start moving around. If you invest in this unit, be sure to buy the puller. You will have multiple opportunities to use it.
After multiple installations I was finally able to set the timing at about 2.5mmBTDC. I also got after the Mikuni carburetor with my new ultrasonic cleaner....and, just to be on the safe side, I mixed-up some new pre-mix. Success at last! It fired up literally with the first kick. I have gone out on several rides in the 10+ mile range and it runs perfectly. How cool is that?
The next challenge with this system is to create a lighting system...not so I can go out riding at night, but just in case. Even here in Mexico you are required to have a working stop light. My longer term goal is to participate in the occasional giro and I know that these events require a street legal motorcycle.
The next project for this bike is to install and tune the Wassell carburetor that I just picked up from Hitchcocks. It is pre-jetted for a 175cc BSA Bantam so I am expecting it to be pretty close. I picked-up this unit for purely esthetic reasons. The adapter that I have to use to fit the Mikuni to the Bultaco is unsightly and there is virtually no room left for an air cleaner. With the Wassell carburetor I can bolt it directly to the Bultaco manifold (2" center to center mount) and the original Bultaco air cleaner screws directly to the carburetor body. Wish me luck!