OK, I didn't understand anything about ignition systems until I could visualize what was going on. So, Here's a quick lesson in basic ignition systems. Ignition 101.
For any internal combustion engine to run you gotta have 3 things.
1 An ignitable fuel/air mixture
2 That fuel/air mixture under compression
3 A way to ignite the fuel air mixture while it's under compression
In a gas engine, the ignition is provided by an electrical spark. It take lots of voltage to cause a spark to jump across the spark plug gap while the fuel/air mixture is under compression and that spark has got to happen at just the right time.
Now you only have 12 Volts in your battery. You need maybe 15 to 30 THOUSAND volts to make the spark jump across the plug. In an AC circuit (like in your house) it's easy . . . use a transformer. You take an iron core and wrap a circuit with the voltage you have available around the core. Make, lets say, 100 winds. Now take a totally separate wire and wrap it around the core 1000 times. When the voltage fluctuates (see AC 101 for details) it induces a fluctuating magnetic field in the core . . . the fluctuating magnetic field in the core in turn induces an electrical current in the second winding. You have 10 times the windings, you get 10 times the voltage . . . but only 1 tenth the amps. That's what a transformer is. Two circuits wound around a common core. (See Electro-magnetism 101 for details). So all we need is a transformer.
Now we have another problem. The voltage in our battery is 12 volts and it doesn't fluctuate. OK, it "does" fluctuate somewhat, but not enough and not near fast enough for us to use an AC type transformer. So, somewhere way back in the 1800s, someone figured out that if we wrap our high voltage circuit (lots of windings) around an iron core and wrap our low voltage circuit (less windings) around the high voltage circuit . . . when we put DC voltage through the low voltage circuit it turns the core into an electro magnet. Now if we all of a sudden turn that low voltage circuit off, the magnetic field collapses on the core . . . and on the way it has to pass through the other set of windings. Pass a conductor through a magnetic field and you generate electricity. Pass a magnetic field through a conductor . . . same thing. So, we can in a round-about way, create a transformer in a DC circuit. That's what your coil is. All we have to do is put 12 volts through it long enough for it to build up a good magnetic field. The stronger the field, the better. Now we have to turn it off quickly. We need a switch. And a way to flip it on and off quickly. Your points are the switch.
But we have another problem . . . In a DC circuit, you have the electrons flowing in one direction and they have momentum. They want to keep flowing across the point gap as it begins to break. Not only does it make the magnetic field collapse slower (lowering the high voltage output) it causes an "arc" across the point gap. That moves metal with it. Ever looked at a set of "burnt" points? One side will have a little chunk missing out of it . . . and it will be stuck on the other side. It also causes oxidation . . . all this increases resistance. Less electricity flows. The magnetic field in the core of the coil goes down. The high voltage output goes down . . . pretty soon your spark plug won't fire. No spark=No Go. So, some person with a lot more functioning brain cells than I have figured out that you could slow this arcing across the points as they open if you add a capacitor in the circuit near the points. That’s what your "condenser" is. A capacitor. Think of it as an electrical sponge. It soaks up those electrons that want to keep moving and then squeezes them back out when the circuit starts flowing again when the points close.
So, there you have the basic circuit. 12 volts from the battery flows through the low voltage windings on the coil turning the iron core into a magnet. The points open and with the help of condenser, almost instantly shuts off the low voltage circuit. The magnetic field around the core collapses, passing through the second set of windings. Since that second set of windings has many, many more loops around the core, the voltage induced in that circuit can reach several thousand volts. One side of that circuit is grounded; the other is feed into a plug wire and down the center of the plug and jumps across the plug gap to get back to ground.
Now our last problem. We gott'a make all this happen at exactly the right time.
The points have to stay closed long enough to let the magnetic field build up. If you open the point gap too wide, they will open sooner and close later so you have less time for the magnetic field to build up. You may run at idle when the engine is running slower, but when you speed the engine up to 3 or 4 thousand RPMs, the coil just doesn't have time to do its job and the ignition starts cutting out. That's why you set your point gap. Set it so that it's opened .016" when it's opened its widest. It'll run with it wider and it'll run with it closer . . . maybe not run well, but it'll run. It'll run well at .016" . . . set it there and forget it. But only for a couple of months. That rubbing block will wear down over time. Every couple of months change your oil, adjust your valves, adjust your brakes . . . and while you're at it, set your points and adjust your timing. Every month is even better. I've been known to go three . . . OK, I'll admit it . . . a few times maybe 4 or 5 months . . . but I've also been able to look down inside my engine through holes in the block so don't use me as an example. What's the old saying? Take my advice. I'm not using it! Anyway, you have to keep adjusting the gap as the block wears down. Sooner or later you have to replace them.
Now, we have a spark . . . what are we going to do with it? We've got one coil and 4 spark plugs. A DISTRIBUTOR! It does what it says. The coil feeds the spark to it through the center of the cap and it passes the spark on to the rotor button. The rotor button in turn points to the cylinder that needs the spark and the plug wire grabs it and runs it to the plug. Then it moves to the next cylinder . . . while it's turning, the lobes in the distributor open and close the points to create the spark.
Now, all we gott'a do is make all this happen at the right time. When is the right time? We need to borrow a lesson from 4 Stroke Engines 101. It takes 4 strokes of the piston to make one cycle. Intake stroke, piston moves down with intake valve opened and draws in and air/fuel mixture. Compression stroke, both valves are closed and the piston moves back up compressing the mixture. Power stroke, the mixture is ignited and expands rapidly as it burns pushing the piston back down. Exhaust stroke, the piston comes back up with the exhaust valve opened and pushes the burnt fuel/air mixture out. It takes two revolutions of the crankshaft for this to happen. But the distributor only moves around once! The other cylinders are offset by one stroke each so that you have one of those strokes happening somewhere in the engine every half revolution of the crankshaft. OK, back to Ignition 101 . . .
The ideal time to ignite the fuel air mixture varies with engine speed and load. The spark starts a fire that spreads rapidly down towards the top of the piston. Ideally, the fireball will hit the top of the piston just as it passes Top Dead Center. At that point if you put pressure on the piston, you'll transfer the power to the crankshaft. If you hit the piston with the fireball before it reaches top dead center you're trying to force it back down against the direction of the crankshaft rotation. Your little air-cooled engine can't handle much of this. Save it the agony. Pull the heads off and take a cutting torch to the top of the pistons. At least you won't damage your heads that way. If you start the fire too late, the fireball has to catch up to the piston before it can push on it. So you gott'a fire the plug soon enough so that the fireball hits the piston as it's passing top dead center.
Now we have ANOTHER problem. The fuel burns at a set speed. Your engine runs at a varying speed. At idle, you can fire the plug at 1 or 2 degrees Before Top Dead Center and the fireball will reach the piston at precisely the right time . . . but at 4000 RPMs the piston might be halfway down before the fireball catches up. So, another very smart engineering type guy came up with a way to move the points around inside the distributor by mounting them on a plate that moves according to the amount of vacuum from the carb (determined by the load on the engine). Another guy came up with a way to move the top part of the distributor shaft according to engine speed with weights and springs mounted down under the points mounting plate (see Centrifugal Force 101 for more details). THEN, somebody figured out that the best way to do it was to use vacuum AND centrifugal force to vary the timing. Those are the BEST distributors . . . Dual Advance. You might see them listed in parts catalogs as SVDA. Single Vacuum, Dual Advance. A DVDA is one that uses vacuum to Advance and Retard timing according to engine load. You'll run across one ever now and then . . . but RARELY will you find one with the retard side hooked up.
Now, your advance curve is built into your distributor. Some are better than others, but none are perfect. You only have one setting. With the engine opened up, under a load, cruising down the highway, you want the spark at the ideal time. To early you'll have a melt-down, too late you won't have as much power and you'll burn more gas. If it's got to be off somewhere, you want it to be off at idle, not going down the rode.
So how do you set the timing? Turning the distributor body moves the time the points open in relation to the position of the crankshaft. So does changing the point gap. So does the rubbing block on the points wearing down. That's why you have to set the points regularly. Set them at .016" and THEN adjust the timing. When the distributor reaches maximum advance, you want it firing at a maximum of 32 degrees before top dead center.
Now, if that's all to complicated, Ret.Bugtech says all that in a lot simpler terms. Something like . . . adjust the points to .016". Mark top dead center on your crankshaft pulley. Use a timing light with degree indicator and set the distributor so that when you reach maximum advance the top dead center mark is showing at the split in the case with the dial set to 32 degrees before top dead center. Let the timing at idle fall where it may. Lock it all down. Don't worry about any more . . . for a month or two anyway.
I gotta get some sleep. :sleepy1:
