A fellow asked me this off line but I figured it might be of interest to a number of us Airheads…
I have a ’78 R100/7. It’s my first BMW. It broke down a few weeks ago and I never got it started again. The problem is I have no spark. The local shop is no help unless you bring the bike in and I don’t trust them. I don’t think it would be a coil because there’s no spark on either side.
Actually, I consider this to be an easy sort of problem to solve: it’s the “It’s not running right” ones that take a long time sometimes….
OK, your ignition system has a few parts, that are in series. If one is not doing its job, the system goes down. No Spark? I assume you have power, the engine cranks over and so on. The ignition system is comprised of the following, in this descending order: (under gas tank) green wire supplying power to coil, coil, jumper wire to other coil, second coil, black wire to condensor (under front cover of engine), condensor, wire from condensor to points, points. That is it. Your problem should be there somewhere.
Here’s how I would troubleshoot it: Disconnect battery, pull front cover, remove gas tank, remove spark plugs, hook battery back up. Look at the points, sometimes one of the points plate screws will, if loose, back out and touch the spring of the points (which is hot) and ground out the system. Have you had the cover off recently? Sometimes the wire from the condensor to the points will get loose and get caught between the cover and the engine block; when you tighten it down, it cuts the points wire.
Put an allen key in the end of the crankshaft (at the alternator) and turn the engine. Do the points open and close? If they are old, the rubbing block of the points may have worn so the points no longer open. They might be so pitted and used up that no current passes (but I would expect SOME spark in that case). Or they may be loose and have slipped closed.
No help so far? Visually check the system for loose wires or other obvious defects. Get a test lamp (small 12 volt bulb and wires) and start looking for juice. Attach the lamp to the condersor posts and manually open the points. You should see the lamp go on as the points open. If so, the problem is downstream at the points. Replace them and set gap and timing. No power, with the points clearly open? The problem is upstream. Pull the points wire off the condensor push connection site and start checking all electrical parts of the ignition system. When the light goes on, then the problem is between there and the condensor. I’d first check where the green wire attaches to the coil. No power there? Then the problem is not with the ignition system but with the wiring supplying it. It just takes time, and a test lamp, there IS a solution there to be found. In a sick, perverted way, I sort of enjoy electrical problems of your sort because there is a “yes-no” answer there to be found.
Approach it as a puzzle to be solved!
