Just checking back to report success in installing the 123 Distributor, and to say thanks for all the help. It was actually fairly easy in the end; the LED to comfirm that static timing is correct is a great feature of the 123.
For other novices who may read this in the future, here are a few things I learned along the way:
1. This is best undertaken with two people. I suppose it could be done alone if you've done it before, but first time out, bring a friend. In total, we spent a couple of hours however, realistically, after you've done it once, you could probably pop it out, change the curve, and go for a drive in 30 -40 minutes.
2. The roll-forward-in-5th/plugs-out approach to find TDC works well and easily.No need for elaborate calculations (fortunately!
). My friend who was helping me taught me a good 'pirate tuner' trick from his 60's Hemi-Dart days that is worth repeating:
As the two of you roll the car forward, put your finger over the plug hole for #1. You will feel the compression building as you approach TDC. You will be close enough by feel to stopping at TDC that you can then finalize by just using a flashlight to see the timing pointer and gently moving the car back and forth. Since you're only moving it an inch or so either way, I don't think there's much danger of jumping the chain or anything by moving the engine backwards.
2. Setting the static timing with the 123 is very easy because of the LED. However there are a couple of small tricks that can save you some time.
a. Figure out roughly which direction you will want the lead wires to the coil to point on the new coil before starting.
b. Turn the rotor in the new distributor so that the LED is already showing through one of the four holes in the aluminum disc under the rotor.
c. Then pull the old distributor, noting the alignment of the adjustment/lockdown plate and the dog-teeth on the bottom when they are pointing to TDC. You will want to replicate that alignment between the plate and the dog-teeth when you put the plate on the new distributor body.
d. Even after removing the lock-down bolt on the alignment plate (not the attachment nut to the block) it was a very tight fit on both the old distributor shaft and on the new, as it should be. To remove it from the old shaft, some gentle persuasion with a flatblade screwdriver and a SFH were required. To fit it to the new shaft, remove the O-ring and then spread the gap in the alignment plate with a flat blade screwdriver, leaving the blade of the driver in the gap until you've slid it up the new distributor's shaft till it touches the bottom of the distributor body. While you are doing this, you want to recall the aligment of the dog-teeth to the slots in the block, the direction in which you want the coil leads to point, AND to make sure that the LED is visible through the one of the holes in the plate. Yes, you can almost mostly accomplish all three things, probably by sacrificing the ideal shortest-route for the coil leads... the least important point of the three.
d. Now drop the new distributor in for a test fit, and make sure that the position of the alignment plate leaves you room in the advance/retard slot to rotate the distributor either way. Again, sacrifice the direction from which the coil leads depart the distributor body because that's actually unimportant except for cosmetic purposes.
e. If you've done all of the above, the distributor will drop right in, and when you hook up the positive lead per the instructions and turn on the ignition, you will be pointed at TDC with a pretty LED gleaming up at you through the hole in the aluminum plate. Now you can tighten down the bolt on the lock plate, leaving the nut off the block/adjustment slot stud.
f. Now pull it once more and pull the set screw out to uncover the 16 choices for the advance curve, if you hadn't set it already during the open-package-and-tinker-with-new-toy phase. It's nice to make sure it's really on the curve you want before you bolt everything in.
g. Make sure that you have the plug wires on correctly before fitting the cap - even if you've done everything else correctly, the car is not going to fire if your plug wire sequence is off by one plug e.g. #1 plug wire hooked to #4 on the cap or some other variation.
Remember if it doesn't just crank right up, the simplest solution is the most likely; especially since you've had the reassurance that the distributor is installed right because of that glowing LED.
h. You will be able to retard the timing well enough to do a test run just by turning the distributor till the point where your friend at the wheel reports a decent idle speed. My car is currently idling a little faster than it was with the old distributor, I'll detail tune it later. For now, tighten down the advance/retard nut on the slot and you're ready for your test drive!
Overall Improvement: Even using the curve that is the 'match to the original' curve built into the Marelli, the engine is much crisper and revs more quickly. Although expensive, I'll say that this distributor is a part that actually makes a noticeable improvement to the way the car runs. I will install new plugs and run it for a week or so before I try any of the other curves.
Post Script: There was nothing easily noted as being wrong with my old distributor, in that there was no wobble, and no marks on the advance mechanism. Still cleaning and lubing the advance mechanism hadn't solved the problem of sootly plugs
and a intermittent stumble under acceleration at 5200- 5300 (indicated) rpm. Most of the improvement attained here could have probably been achieved with a rebuilt distributor or perhaps even just with a good cleaning, lube, and replacement of the springs on the advance on the old one for most people. . Still, it's very nice having the option of changing the advance curve, and given that my distributor was more than 40 years old, a new replacement was the smartest option for me. YMMV.