These points plates all have elongated holes
in them allowing you to adjust the plate. Sometimes if you need more adjustment, you can take a small rat tailed file and file more length to the elongated hole.
Point gap is not supposed to be used for the adjustment of timing, but sometimes we can use it for that. Wear, on the different mechanical ignition parts, may require us to use it to adjust timing. Other times the manufacturer designs it that way, even though that is not a good thing to do ! The gap of the points is supposed to control how long the Ignition Coil(s) is charged by the battery. The time the points are closed is called the Dwell Angle. This angle is given in the shop manual. This can get complicated because some systems charge the coils when the points are open and others when they are closed. We won't worry about that and anyway most of the points we will be dealing with charge when the points are closed and fire the spark plug when the points open. Just be aware that you can vary timing a little with the point gap, but you want to stay within the maximum and minimum specs so the coil will charge right.
Some twin cylinder bikes, like the old 350cc Honda twins, have both points screwed to the points plate. On these bikes you must use the point gap to adjust at least on of the cylinders. Usually, you can set the gap on one point set, turn the point plate to set the timing and then use the point gap to adjust the timing on the second set of points. Now check the gap on the second set and if it is within spec you are done. If not, you will need to juggle settings on the other set of points and points plate to get everything within spec. To help things out, some manufacturers use an extra plate
that holds one set of points and bolts to the main points plate. It is moveable, so you can keep the gap right and still adjust the timing for that point after you have adjusted the other set with the main points plate. Most four cylinder bikes that use points have this extra plate. Some twins use them too.
Points tend to blacken with corrosion when left sitting for a long period of time. You can clean them with a points file. I don't use the metal ones that look like small files. I never could get them to work right, the metal used in points is just too hard to file good. It's also not a real good idea to file points if they are badly pitted because it cuts down on their contact area.
If you do file badly pitted points you must file ALL the pits away. I use a flexible
type that has grinding compound imbedded in it, or a piece of 100 grit sand paper folded back to back. This just cleans up the point surface. If the points are badly pitted, replace them. Now blow everything clean with compressed air and clean with some contact cleaner. Finally, clean the points with a 1/4 inch strip of paper
cut from a 3 by 5 inch card and dipped in acetone. Also, put a bit of points grease on the points cam, points heel and the points lubricating felt
. If the felt is old and dry, lube it with grease and put one drop of oil on it. If the felt is old and dry, and you only put grease on it, the dry center will pull the grease into the center and away from where you want it... on the points heel. The oil kinda "charges" the dry felt center, preventing this.
The Yamaha XS1100 is the only bike engine I know of that has a vacuum spark advance.
Why have vacuum advance ? Under part throttle, air intake into the intake manifold is restricted, so a vacuum develops in the manifold. Because of this vacuum less air/fuel mixture is drawn into the cylinder. This, in effect, lowers the engines compression and slows the burn rate of the fuel mix. To get the full power out of the engine under these conditions requires more Ignition advance. The Vacuum advance gives you this advance when there is vacuum in the intake manifold. Most car engines have a vacuum advance, but it just didn't catch on for motorcycles.
If you reverse the low tension ignition coil connections it will reverse the polarity. It takes 40 % or so more voltage to fire the spark plugs on a ignition system with positive polarity. On most bikes you would have to really work at it to do this. However, some four cylinder bikes like the early Honda fours are designed this way with double ended ignition coils. One lead is positive and one lead is negative. This means two of the plugs will require a lot more voltage to fire than the other two. Not much you can do about it, but if two plugs start fouling out on these bikes, this might be the reason. To test polarity put a pencil lead between the high tension lead and the spark plug. If there is a flare from the pencil to the spark plug, the polarity is correct. If the flare is between the lead wire and the pencil the polarity is wrong.
I've found that a set of points seems to go out of time in about 2000 miles. If you are willing to live with lousy performance, they can go upwards of 8000 miles.
With the ignition on, you should have battery voltage going to the points. With the ignition on, if you take a screw driver and ground out the the moveable point, when you remove the screw diver from the ground, you should get a spark at the spark plug. With the ignition on and the points closed, open the points with a small screw driver. This, also should produce a spark at the spark plug. The spark should easily jump an air gap of 1/4" or more, to ground, outside the engine.
Most motorcycle coils are weak when compared to car coils. You can use car coils BUT
you have to do a lot of modification to the points cam so you don't over charge the coils. Seems to me the old "CYCLE" magazine had quite a write up on how to do this but I guess now that ranks right up there with performance tweaks for a 486 computer ! I like points ignition systems. If they start to fail it always seems you can fiddle with them and limp home. If something needed replacing, you didn't have to get a loan from the bank to buy the part. Too bad they are fading away like old soldiers.