Question:Question:
What is this magneto? How should it work, and why doesn't it work in my bike?
Answer:
This article is submitted by Craig A. Vechorik. Bench Mark Works
Does your mag igniton BMW start on the first kick? If properly set up, it should! Although many of the old timers are aware how to set up a Magneto, there are some commonly overlooked items and procedures to remember.
First of all, disconnect the plug wire from the coil, and using an Ohm meter, check the resistance of the wire and the plug cap. It should be ZERO resistance Before all of the hair splitters out there jump my case, let me make my standard Disclaimer of the '90s and say that THIS WORKS FOR ME, and has for years. Yea, I know that BMW started using resistor plug caps in '67 in response to regualtions governing RF interference. I still don't recommend that you use them. A lot of aftermarket companys will sell you /5 or /6 plug caps, but they have a 1000 ohm resistor built in them. If you can't find a source of non resistor caps, head for a chain saw dealer. Remember, that the faster a Mag spins, the hotter the spark. At the speed that you can kick the engine over, the spark is relatively weak, and using resistor caps and plugs is to invite hard starting.
Another problem in starting can be the condensor. ALWAYS carry a spare one with you when you ride a /2, because if it ever fails (and they do) it will quit on you, and will not restart. You don't have to buy one from BMW, either. The condensors I use are for a V-8 Ford, '73 351 ci, and they have their own bracket already on the condensor, and the wire lead has a spade lug on it. This consensor will produce a strong blue spark, and last for many years.
The initial setup of the Magneto is important if it is to produce a strong spark. Check to see if the scribed line on the rotating magnet lines up EXACTLY with the bottom of the "V" notch on the brass plate that the points mount on, when the "S" mark of the flywheel is lined up with the stationary mark on the crankcase. If it is off slightly, loosen the two 6mm nuts that secure the body of the mag to the timing gear cover, and rotate the body to the left or right until a perfect alignment is obtained. I have seen folks use a timing light to check the timing while the engine is running, and to obtain the correct timing, they will move the body of the Magneto. this is NOT the way to set the timing. If you remove the centrifugal advance, you will notice that the points are mounted on a seperate breaker plate, which is held down by two screws. That is the plate which needs to be moved when setting the timing.
Another thing which will cause coil failure, (which is common) is the ventilation system for the electrics. Ever wonder what that little breather air cleaner thing is doing under the 1/2 moon cover on top of the engine? Air is drawn in through the notches on the front engine cover, over the generator and coil, through the little breather, into the Carbs. Check that little breather, clean it, lightly oil it, and put it back. Keep in mind that the timing of an air cooled engine is critical to the operating temperature of the engine, One can easily time a /2 engine staticly. By aligning the "S" mark on the flywheel to the stationary mark on the window of the crankcase, and then using a test light or an ohm meter on the points. (the wires from the coil and condensor must be disconnected) If you use a test light, you must use a battery also, to light the test light. You use the test light AFTER you have set the points to their proper gap. You rotate the breaker plate clock wise (but all I've ever seen is a digital watch P^) )to advance the timing or counter clock wise to retard the timing. The idea is that you want to postion the breaker plate so as you rotate the engine, as the "S" mark S-l-o-w-l-y passes the stationary mark, the points open, turning off the test light, or showing an open circut on the ohm meter.
In the end, except for the initial set up and starting of the engine, it doesn't matter too much where the "S" mark appears in relation to the stationary mark, when the engine is running at idle. What does matter is where the "F" mark or full advance mark is with the engine running at riding speeds. The only way to check this is with a timing (strobe) light. This brings us to another common problem. When you use the timing light, do you see two images in the window? At idle or at Full advance? If you do, shut the engine down, and rotate the engine until the points are at their widest. Check the gap with a feeler gauge, and figure out which blade of the gauge fits the gap the best. Then rotate the engine 360 degrees, and check it again. In the ideal situation, with a good advance, assuming that the shaft of the rotating magnet, that the advance mounts on, is not bent, or that you had inadvertently had a speck of dirt on the taper between the camshaft and the rotating magnet when you mounted it on the cam, the point gap should be the same. You do know that both cylinders fire at the same time, but only one of them is on the compression stroke, while the other fires during the exhaust stroke. When you see two images in the window, one side of the advance is worn more than the other, and the points are opening to a different gap from one side to the other of the advance. Slash 5 & 6's will do this also. If they still have points in them. You can cure a /5 or /6 with an electronic inductive ignition system. Too bad that there is not an inductive system for a mag. The wider the images appear in the window, the more worn out the advance is. You can see this condition in the header pipes of the engine. If one is "bluer" and the blue extends further back on the pipe, one cylinder is timed slightly different, and is doing most of the work. On a machine in this conditon, it is hard to get it to run right, it seems as if the carbs are slightly out of sync, and you get vibration at all speeds. Messing with the sync of the carbs won't help it either. the only thing you can do is buy a new advance.