There is a big difference between adjusting the timing and altering the advance profile. Adjusting the timing at the plate affects the timing throughout the entire RPM range. This is commonly known as "initial advance" or "base timing". The advance "curve" or "slope" determines how quickly the timing advances with respect to RPM, at what RPM maximum advance is reached, and sometimes how the advance behaves under heavy loads. If you have too much advance at a given point, and you retard the timing at the ignition plate, you might have too little advance at some other point. You need to have the ability to adjust both the initial timing and the advance profile independently in order to optimize the timing for a particular engine.

See if this hypothetical example helps explain it better: (Keep in mind this example is not specific to HD engines. It is also extremely simplistic to avoid dragging in lots of unecessary details which will only cloud the point I'm trying to make. Modern electronic ignitions are *much* more sophisticated than the hypothetical ignition system presented in this example, and are therefore able to deal very well with the shortcomings apparent in this example.)

Let's say we've got an engine that runs best with 30 degrees total advance at 3000 RPM. Further, let's say this engine is running high compression and pings badly at 1500 RPM under a load. We determine the timing profile using a dial-back light and discover that we're advanced 15 degrees at 1500 RPM. After a few tests, we find the pinging at 1500 RPM can be completely eliminated by backing off the timing 2 degrees at the ignition plate. The problem is, now the timing is retarded 2 degrees throughout the entire RPM range. That means maximum advance is only 28 degrees at 3000 RPM, hence the top-end performance is degraded slightly. Clearly, we need to be able to alter the timing slope to avoid this sort of dilemma.

For the sake of simplicity in this example, let's assume that the timing slope in the first situation was linear and added 1 degree of advance per 100 RPM. (Electronic ignition modules are not restricted to linear slopes, nor do all timing slopes typically begin advancing immediately from zero RPM, but let's just avoid that complexity here.) What if we set the ignition plate back where it was, and alter the advance slope to add 1 degree of advance per 115 RPM? (A less "steep" advance slope.) With older mechanical advance units we would do this using lighter weights, heavier springs, or some combination. With modern ignition modules, we simply tell the computer to do it.

Now at 1500 RPM we'll have 13 degrees advance like the engine wants. OK, at 3000 RPM we'll only have 26 degrees advance, *but* we are not yet at maximum advance, which is determined by the ignition plate setting to be 30 degrees. We still have 4 degrees to go, and by 3450 RPM we've rolled them all on. Sure, this is still a tiny compromise, but it eliminates our pinging problem, and it gives us our back our optimum top end performance a mere 450 RPM later. You cannot achieve the same result by only adjusting the ignition plate.

Keep in mind this compromise is really only an artifact of this trivial example. Trust me, the modern ignition systems readily account for this type of situation. They are able to generate non-linear advance curves that would solve the above problem by providing exactly 13 degrees at 1500 RPM and exactly 30 degrees at 3000 RPM.

There are programmable modules that allow you to adjust the timing advance at *every* RPM via a PC cabled to the module. Sure, it's nothing more scientific that a look-up table, but you sure can't do that with springs and weights. These same modules can record RPM profiles and play them back on your PC so that you actually have a built-in pseudo-dyno to help you dial in the tuning. Additionally, some modules have the ability to stage the rev-limiter for drag racing applications, and often have mu

 

Hi, I am somewhat new to this.. have a 1200 Sportster (99). Had a mechanic replace a faulty ignition. He put an Advance unit, tach adapter, ion dual fire dyna in after cleaning the carbs.
This is my first bike. Rode the thing all last year. Had a different sound (glug-ga-glug) at idle and idled at 10..rpms. Now idles as though the choke were out at 20+ (gluuuugggg- infinity). It runs hot and fast (again just as though the choke were out).
Is this just symptomatic of the new firing system or is there something that needs adjusting, as explained above?
Please advise.