It helps a lot to begin this process at what I call "battlesight zero", what I mean by that, is when the timing pointer is on the TDC or "0" tick mark of the crank balancer, that we have checked (using a piston stop tool) that the #1 piston is indeed truly and exactly at TDC, the idea here being we want to be sure the timing pointer is telling the truth about the timing degrees we are seeing with the timing light. This is long to read but if you understand it better, you'll only have to do it once and you'll thank yourself every time you drive it.

Put another way for emphasis when the #1 piston is exactly at TDC on the compression stroke (using a piston stop) the engine timing pointer should coincide exactly with the "0" mark on the balancer. Everything ignition related including timing and valve closing events is based on this null point.

There are a few reasons why the pointer may not be accurately indicating true TDC, but this defect has caused mucho headscratching and wasted hours over the years in many, many different engine families. Be sure to verify TDC before proceeding, just as a "belt and suspenders" approach.

With that out of the way, you kind of have to get it into your head that under the overall ignition timing umbrella there are two main systems at work, the centrifugal/mechanical timing, and the vacuum advance timing. They are two completely independent and separate systems for advancing and retarding the ignition timing as engine conditions require. The mechanical advance uses weights and springs and is solely based on engine RPM. The vacuum advance is based solely on engine load. While they are separate or independent of each other they work together, ordinarily when one is advancing the timing, the other is taking some out, and vice versa. The exception is at steady speeds at light loading where it tends to be additive.

What the engineers found very early on, there is no way to achieve the very high necessary ignition timing advance numbers under lightly loaded lean fuel condition using weights and springs alone.

The two systems in turn are also measured and adjusted separately, in isolation. The mechanical advance is always set first. Disconnect and plug the vacuum advance port. I always tell people to forget it even exists, for a while anyway, when sorting out the distributor.

A dependable source of confusion is when you hear terms like "total timing". This logically sounds like it would include both the mechanical & vacuum advance degrees, but this is not the case! When you hear something like "36 degrees total timing all in by 3000" what that means is 36 degrees without vacuum advance.

"Total" timing numbers do not count vacuum advance. A surprising number of people who should know better are not aware of this. Total Timing is an old drag racer term that stuck in the wider automotive community, but performance engines never use vacuum advance. Yet another aspect of ignition timing that can be a bit bewildering, when drilling down deeper into the shop manual (nod your head "yes"...) the published numbers in the specifications section for both mechanical and vacuum advance are listed in distributor degrees, not crankshaft degrees.

The distributor was intended to be setup and adjusted on a distributor machine by Ford, and the distributor turns at half speed relative to the crankshaft. Therefore any given timing advance specs in the shop manual for a particular RPM or vacuum level will be exactly double at the crankshaft.

SO, what you want to do as a practical matter is give your particular engine what it wants for your particular location and fuel quality, elevation, compression. 34° to 36° mechanical advance timing would be a good place to start. Most every OHV engine ever made will be happy somewhere in there on pump gas. Sometimes a bit more. Extending the timing marks out to 50° BTDC using timing taps will help you when using a timing light. So will the flourescent colored numbers. The rate of timing advance, how fast, or "when" the advance is coming in at a given RPM also very important to check. A short test drive or three with the vacuum advance disconnected will be very helpful sorting this out. This can get tedious, but there is no other way short of a distributor machine.

What you want to measure is the mechanical advance at higher RPM. Most people like to set the initial timing to somewhere a bit north of 10 degrees. Different internal reluctor slots and spring stiffness combinations were offered for different engine applications. What the DIYer has to be careful of is checking the mechanical advance at 2000, 3000 RPM and beyond. One reason is because the factory distributor or internals intended or included may have been replaced after these many years and may not be suitable for vehicle weight and other factors. It's also very important to check that it is operating smoothly without sticking up and down the RPM range and returns to the same base timing. The initial timing (in one sense) doesn't really matter too much, so long as there is no starter kickback. It is more important that the advance on the far end, at high RPM and cruise is suitable.

The factory stock distributor in my Truck wasn't done advancing till north of 4000 RPM, they used VERY stiff springs and a "lazy" advance curve. Your T-Bird advance curve will be much different than a truck. This is one issue with remanufactured distributors - the rebuilders may have included the wrong advance curve mechanicals.

These adjustments have a huge effect on how the engine performs. Once you get the mechanical distributor timing curve sorted out to your liking, then the vacuum advance should be re-connected and the canister itself adjusted for the correct timing advance particularly under very lightly loaded steady cruise conditions.

This is difficult or at least confusing for the DIYer to measure. Because there is no load at all on an engine in neutral, there are consequently very high ignition timing numbers observed on the balancer with the vacuum advance connected.

Even the actual advance on paper might seem to be "too much". The engineers didn't seem to think so, typically at steady highway cruise on flat ground about 50° BTDC is right in there, maybe more! The ignition advance at idle makes little to no difference anywhere else in the "curve".

If you understand how the two systems work together you will understand why this is so. Optimum timing here actually allows the engine to run much cooler esp. in stop and go driving. As it turns out a sharp engine tune contributes to elevated levels of NOX emissions, something that the engineers had to address when federal pollution emission standards were mandated. This adds even yet another layer of confusion to a widely misunderstood topic!