What we'll cover in this lesson is this:
Before we get into this I'm going to explain something that you may already understand. However, I'm not going to take anything for granted, and it's important that you comprehend this, so . . .
Also, they both will be in that position two times in any power cycle. Only once, on the compression stroke, really matters for timing purposes. However, it happens again on the exhaust stroke, and that can confuse you when you actually get around to setting the timing. Get that clear in your mind - it really matters.
"Timing" an engine means causing the points or computer sensor (located in your distributor or cone) to create a spark at exactly the right moment in the piston's travel up and down the cylinder.
SUCK, SQUEEZE, BANG, AND BLOW - the four cycles.
A four-cycle engine (and Harleys have four-cycle engines) go through four strokes to make one power stroke.
First, is the SUCK, or intake stroke. The piston is moving down the cylinder, and the intake valve (from the carburetor or fuel injector) opens. The suction caused by the piston going down pulls a mixture of fuel and air into the cylinder.
Second is the SQUEEZE, or compression stroke. As the piston got to the bottom of the cylinder, the intake valve closed, and the piston started back up the cylinder. Since the cylinder is now completely sealed, the mixture of fuel and air in the cylinder gets compressed. This is the one where timing matters, because for reasons explained farther below, the spark plug must fire before the piston gets to the top of its travel. The spark ignites the air/fuel mixture, creating an increasing pressure.
Third is the BANG, or power stroke. The delivery of power to this stroke actually started during the compression stroke, when the fuel ignited. Because the crank is spinning though, and is connected to the piston, the piston continues to be forced up the cylinder, until it reaches top dead center (called TDC for short. There is also a BDC - figure it out.), its highest point in the cylinder. At that time, it starts back down, with the increasing force of the expanding gases pushing it. Now it is spinning the crank, instead of the crank pushing it up.
Fourth, is BLOW, or the exhaust stroke. Once the burning gases have spent their force pushing the piston down, the exhaust valve opens, and piston starts back up the cylinder. As it moves up this time though, it is blowing out the burned gases through the exhaust valve and into the exhaust pipe.
WHY DOES "TIMING" MATTER
The reason it matters has to do with the fact that gasoline in a cylinder burns. (You want it to burn, not explode. If it explodes, that is called "detonation", and you hear it as a knock or ping in the engine. A gentle, intermittent ping is ok; not desirable, but ok. A real knock is bad news; it can blow up the top of your pistons and other bad things. It sounds like malicious trolls inside the motor pounding on the heads and cylinder walls with their little hammers. It may also be a gentler sound, but it nearly always sounds sort of metallic.)
You also want the burning fuel to reach its maximum pressure, which is what pushes the piston down, just after the piston passes Top Dead Center and starts back down the cylinder. Make sense?
Well, since fuel burns, and burning takes time, you've got to get it to start it burning before TDC, right? But not too soon, because if the fuel burn reaches maximum pressure before TDC, that's a bad thing, - the burn is then working against the piston travel. Timing is the measure of when you fire the spark in relation to the piston's position in its travel. The amount of piston travel that you start the burn before TDC is called "advance."
The perfect place to measure advance is piston travel because that's what the engine actually cares about: where is the piston? In other words, how far before TDC (or "BTDC") is the piston when the spark sets fire to the fuel? In a panhead or shovel it is about 7/16ths of an inch of stroke, or about 40 degrees of crankshaft rotation - the engineers discover that through developmental testing (actually, there's a little more to it than that. I'll get to it towards the end).
The testing they do will work something like this.
To be really accurate, timing must be set while the engine is running. As you can imagine though, it's sorta unhandy to try to measure 7/16ths BTDC while the engine is running - after all, you can't look in and see the piston going up and down. To get around the problem, what an engine manufacturer does is take a development engine that's not running, set the FRONT piston at TDC, and make a mark on the side of the left flywheel. (On your motor it's probably a stamped dot. You can see that mark through the little hole in the crankcase on the left, at the base of the V of the cylinders.) That development engine will also have individual degrees marked on the flywheel, with TDC as the ZERO point.
They start the engine up and set it to idle speed; say, 900 to 1,000 rpm. Then they advance the timing a little, until the idle is smooth. That means the fuel burn is taking place just right to let the engine idle nicely; not firing too soon, and not too late. They take a timing light and look through the little hole to see what degree marker is showing. On most motors at idle it'll probably only be around five to seven degrees of advance. They make notes, but that figure is not one you'll typically see published since you don't control it directly.
Easy so far, huh? Sounds like they're done.
Not so, Tonto. There's a complicating factor to deal with.
Advance has to increase as engine speed increases. The reason for that is that air/fuel mix burns at a fairly constant rate (there's also a factor called "combustion lag" that I won't go into here. It would just complicate the explanation more than it needs to be.), but as engine speed increases, the time for that burn to take place gets shorter. Understand? When the engine is idling at 900 rpm it takes a certain split second for the piston to get from one point in its stroke to TDC, but at 1,800 rpm it takes half that amount of time, so the fuel has only half as much time to burn.
What that means in practical terms is that when the engine is idling the timing is set at only about 5 degrees BTDC, but when it is running at higher speeds the timing has to go out to as much as 30 or 50 degrees BTDC, depending on what engine we're talking about. The rate at which the timing increases is called the "advance curve." A typical points set for a shovelhead goes from idle to fully advanced by about 1,800 to 2,000 rpm.
So, what they do now is run the engine at around 2,500 rpm, adjusting the timing by reference to power output until it is just right. That is, they set the rpm at a constant speed, then diddle with the timing until the engine is showing best power at that rpm. Then, as they did before, they look through the little window and see what degree mark is showing. That becomes the number that is published as to how many degrees "full advance" the engine takes. All flywheels for that model motor thereafter will be marked with the TDC mark, and the advance mark (probably a vertical slash in your case) will be stamped in the proper number of degrees before the TDC mark. There will also be marks for the rear cylinder, but you don't care about those right now.
Other factors enter into the need for advance, including how efficient the combustion chamber is; dual plugs in the cylinder; rich or lean mixture; ambient temperature, and load the engine is under. Points have but a limited ability to manage an "advance curve." Computerized ignitions do a little better, but they can only offer a variety of curves (the Crane HI4 offers 9, as I recall) that are preprogrammed based on a restricted number of parameters built into 'em, and you select the curve manually when you install it. A genuine engine management computer like Harley uses nowadays though, is much better. It actually measures what's going on with the engine, from temperature to airflow and several other items, then sets the timing to match those factors, choosing from a "map" of possibilities that the engineers made during actual research and development testing.
When you time an engine, the fundamental process involves having it running so that the advance mechanism is in action (either a points advance system, or computer - both react to rpm) , then looking through the timing port (that little hole on the left side) and making sure that the advance mark on the flywheel is showing in that port when the spark fires. If it's not, you adjust the advance mechanism so that the mark DOES show when the spark fires.
And how to do that will be the subject of another lesson someday.
The computer on a Twin Cam is a whole different deal - not at all like this, so although the theory of timing remains the same, the way you control when the spark fires is not at all similar. In fact, there's not much you can do to work with the system.
About Single-Fire vs Dual-Fire, and Waste Sparks
Up through the Evo engine, Harley only used one coil for both cylinders, and you cannot obtain differential timing with a single coil. What that means is that a spark fires in both cylinders at the same time. One of the pistons is on its exhaust stroke though, so that spark is "wasted." It might ignite a tiny bit of residual gas in the cylinder though, so the setup can result in a bit of a rough idle with some combinations of cams, pipes, et.
This "dual fire" system cannot be timed absolutely precisely for both cylinders; one or the other, but it's a compromise for both, but it's close enough for practical purposes.
A "single fire ignition" uses two coils and two points (or the computer equivalent,) so you can time the spark precisely to each cylinder, and not have a waste spark. You can spend big bucks on a single fire ignition, but you won't see a noticeable difference in performance except maybe on a dyno. You might notice a bit easier starting, and a slightly smoother idle, but what they want for a single fire is mighty expensive for such little return.