MAGNETO BLUES
Solving hard-start problems
by Craig Vechorik
Does your magneto ignition BMW start on the first kick? If properly set up, it should! The initial set up of the magneto is important if it is to produce a strong spark. The strength of the spark produced is influenced by the exact positioning of the rotating magnet in relation to the body of the magneto at the moment the points break open. Notice that there is a scribed line on the face of the rotating magnet. Rotate the engine by hand, until the "S" mark, on the flywheel, lines up with the stationary mark on the crankcase. Then, check to see that the scribed line on the rotating magnet lines up EXACTLY with the bottom of the "V" notch on the brass plate on the front of the magneto body.
If it is off slightly, loosen the 6 mm nuts on both sides of the magneto body and rotate the body left or right until perfect alignment is obtained. Tighten down the nuts, and leave the magneto in that position. I have seen folks use a timing light to check the timing while the engine is running, and obtain the correct timing by moving the magneto body as previously described. 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 separate breaker plate, which is held down by two screws. That is the plate which needs to be moved when adjusting the timing. Rotating the plate clockwise will advance the timing while counter clockwise retards the timing. Keep in mind that the timing of an air cooled engine is critical to the operating temperature of that engine.
One can easily time a pre-1970 BMW engine statically. By aligning the "S" mark on the fly wheel to the static mark in the window of the crankcase, and using a test light or an ohm meter on the points (with the condenser and wire from the magneto coil disconnected) to set the breaker plate in the proper position, after the point gap has been set. You rotate the breaker plate clock-wise to advance the timing, or counter-clock-wise to retard the timing. The idea is to move the breaker plate until the points just begin to break open, as indicated by your test light turning off, as you s-l-o-w-l-y rotate the engine up to, and past the mark. (You have to use a battery with the light and disconnect the coil and condenser from the points) or use an ohm meter to show when you get an open connection. All this will do however, is get the engine timed very close at idle. The important, and much overlooked component in the ignition system is the centrifugal advance.
In the end (except for when you are trying to start the engine) it doesn't matter too much where the "S" mark appears in relation to the stationary mark at idle. What is important, is where the "F" mark is in relation to the stationary mark at full advance. The only way to check this is to use a timing (strobe) light with the engine running at operating speeds and full advance. Which brings us to another problem that is easily overlooked. When you use a timing light on the machine, have you ever noticed two images of the "S" or "F" marks in the window? If you do, you need to test for differiential timing.
Here is how to test for differential timing:
1. remove both spark plugs
2. get an Ohm meter and a regular writing pencil
3. detach the wire from the coil to the points.
4. attach one lead of the ohm meter to the nut and bolt of the points
5. attach the other lead to ground, anywhere on the engine.
6. The ohm meter will tell you precisely when the points open. When the points open is when the engine fires. That should be at the line above the letter S on the flywheel.
7. put an allen wrench in the crankshaft nose bolt, and turn the engine in the direction it runs VERY SLOWLY. When the ohm meter indicates the points are breaking open, put a pencil mark on the flywheel next to the stationary mark cut in the viewing hole of the crankcase. You may see that the ohm meter indicates that the points are breaking at some point other than the line above the "S" mark. For this test, no matter where on the fly wheel the ohm meter indicates the points have cracked open, put a pencil mark at that spot.
8. continue to turn the engine, one more complete revolution. Watch the meter for when it indicates the points have broken open. Put a pencil mark on the flywheel exactly where this occurs the second time.
9. Ideally, the points should have opened TWICE in a row, at exactly the same place on the fly wheel. (Ideally, it should have been the line above the letter "S".) However, you probably ended up with TWO pencil marks. The distance between the first pencil mark and the second is how much differential timing you have. BMW allowed 3 degrees differential timing. Linearly on the flywheel, that is about 3/16 of an inch. So if you see 1/4 inch or 1/2 inch difference in the pencil marks you have 8 or 10 or maybe even 15 degrees difference. This means one cylinder is running ahead of, or behind the other by that many degrees of timing. That makes one side run hot, and do all the work, while the other sort of follows along.
The cause of the problem:
1. the shaft that the advance mounts on is wobbling when it turns.
2. the advance is worn out, the lobes are no longer identical for the points to break open at exactly the same time in relation to the flywheel. Remember there are two lobes, one for each cylinder on the advance.
(This holds true for /5's and /6's also, but can be easily corrected by buying a electronic, magnetic, inductive ignition system, like a Dyna III, in which a rotating magnet is mounted to the old, worn out advance.)
A quick way to spot this problem on a pre 70 BMW is to look at the "bluing" of the header pipes. If one side is bluer, and the blue runs down the pipe a lot further, that is the cylinder that is doing most of the work, and running hotter. The two lobes of the advance (that open the points) seem to wear unevenly, (true on /5's & /6's also) causing one side to have a greater height than the other. This gives you the double image in the window under a timing light. The old mechanics rule: a change in dwell (point gap) affects timing, but a change in timing that does not effect dwell is illustrated. The only easy cure for this problem is a new advance.
Other problems:
Disconnect the plug wires 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 jump my case, let me make my standard disclaimer and say that this works for me, and has for years! I realize that BMW put resistor plug caps, in the closing years, from 1967 until the end, but this was to comply with regulations related to radio interference. I don't recommend using them. Lots of companies will sell you a /5 or /6 plug cap. They will work, but they have a built in resistance of 1000 ohms, to suppress noise in electrical equipment, (such as radios) but I haven't seen too many pre 70 BMW's with stereos on them! Remember that the faster a magneto spins, the hotter the spark. At the speed you can kick the engine over, the spark is relatively weak, and to introduce resistance with /6 plug caps and resistor plugs is to invite starting problems.
Another problem can be the condenser. ALWAYS carry a spare with you, for if the condenser fails, the magneto will not produce a spark. The condenser doesn't have to be a genuine BMW part. The condenser used on a V-8 Ford, (such as a '73 351ci) comes with it's own clamp, and has a lead with a spade terminal already on it. The Ford condenser will produce a strong blue spark, and last many years.
Another thing to check to prevent coil failure is the ventilation system, which keeps the air flowing underneath the front engine cover and over the electrical components. Underneath the half moon cover (on the top of the engine) is a small, circular breather. Be sure that this breather is clean, lightly oiled, and that air will pass through it. Air is brought under the front engine cover through the "notches" on the sides. It then passes over the generator and magneto coil, through the round breather filter, to the area beneath the air cleaner, and on to the carbs.
Testimonial by Max Tietjen
Setting the timing on a R68 - R69
Because of the retard lever feature, and the fact that the breaker plate is not held down in a fixed position, this is what you must do to time an R68 - R69.
Remember that the handle bar lever control is a retard feature, and it was put on the bike due to the fact that in that early post war era, the R68 was the first high compression BMW, and gasoline in many countries to which the R68 & R69 was exported was very poor. The idea was, that if the engine pinged during acceleration, one was to retard the spark until it stopped pinging. That was all it was for. With good grades of gas, the feature is totally unnecessary, and should not be used. The danger of making it operational is that passerby's who love to touch and monkey with levers will move it. You see, you will find that while the timing (in theory) should return exactly to where it was originally set, it rarely does. It will get close, but not dead on.
Let us assume that you want the retard feature fully functional. The following procedure is how to set the timing:
First, you must set the body of the mag, so the scribed line on the rotating magnet is dead in the middle of the "V" notch on the brass face plate, when the "S" mark on the flywheel is dead in the window next to the stationary "notch" cut into the front of the viewing window. Tighten the two 6 mm nuts, and never move the body again. While moving the body clockwise or counter clockwise will change the timing, it will weaken the spark at kick start speeds. Also, keep in mind that the point gap has a given range of .014 to .016 of an inch. Widening the point gap, advances timing, narrowing the point gap, retards timing.
Next, with the advance installed on the shaft of the rotating magnet, set your point gap. Notice that on the face of the R68 - R69 magneto body there is a slotted limiting plate which can be moved to adjust just how far the spring can shove the breaker plate toward the advance.
So, start out by loosing the screw that holds the slotted limiting plate, and allow the big coil spring, on the cable, to shove the breaker plate down as far as it will. Then, using either an ohm meter (you must disconnect the coil from the points) or our magneto timing device (which you do not have to disconnect the coil in order to use) check to discover exactly where the points are opening in relation to the flywheel. If the meter indicates that the timing is too far advanced, rotate the engine until the mark is dead in the window, and move the retard lever on the handlebar, s-l-o-w-l-y until the meter shows the points cracked open. Then, move the slotted limit plate up, until the bottom of the slot is resting against the fixed peg on the breaker plate, and tighten the screw on the limit plate. Then push the lever on the bar to the full advance position, and check the timing again. If it is not dead on, try the above procedure again. Also, if it turns out that when you are pulling the lever, in the above procedure, and you run out of travel, before it comes into time, then you must go back, and CLOSE the point gap slightly. Then try the procedure again.
Now, if it turns out that when you check the timing the first time, with the meter, while the slotted limit plate is still at it's most advanced position, and the timing is still too far retarded, you must WIDEN the point gap slightly and then try the procedure again.
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