Full of confidence I installed the refurbed Marelli Plex . . . only to find that I didn't have a spark. To shorten a long story I checked everything using the published specs (they are correct, btw) and found that everything was within spec. I changed to another of the 3 good amplifier modules I had. Still no spark.
The problem turned out to be driver error. The Marelli Plex instructions rather ambiguously state that you should connect the ". . . lead wire to the +B side of the coil and . . . the electronic rev counter wire, if any, to the other terminal. If there is no rev counter, leave the terminal bare
." So I did, taping off the black wire from the ign amplifier until I had everything else connected. Still no spark. Time for consultation with a higher authority. After a great Mexican food/killer Margarita lunch my friends Bob and Howard and I returned to Bob's shop. About the same time Bob found the taped off connector, Howard looked at another Marelli Plex in Bob's parts stash. Sure enough the black wire was connected to the "-" side of the coil. We connected it to the "-" side of the coil, applied some power, spun the distributor and were rewarded with a nice, fat spark acorss about a .5 inch air gap. It wasn't quite an MSD arc but it was pretty impressive for an inductive ign.
So, despite what the printed Marelli Plex instructions seem to say: hook up the black wire from the ign amplifier to the coil
The remainder of the install is easy although there are some things that are useful to know about.
1. Air gap. The air gap between the reluctor and magnetic trigger in the distributor isn't particularly critical. If you are setting up a refurbished distributor set the air gap using a business car as a feeler guage. A loose fit is just about right. You can bench test the air gap by applying power to the coil and spinning the distributor. If you get a spark you're OK.
2. GM amplifier module. These are probably the most commonly avaliable ign. parts anywhere (at least in the States). You can buy them anywhre or simply visit a Pick N Pull and harvest a few for almost no money. When installing the amplifier make sure you use good quality heat-sink compound
which you can buy at electronics or computer parts stores (Artic Silver is very good but there are lots of other choices). The grease supplied with aftermarket amplifiers isn't good enough so use the good stuff.
3. The Marelli Plex requires 2 separate grounds. The through bolts holding the amplifier heat-sink to the body is one but you should also run another ground from the heat-sink to the engine. As you can see in the photo, I ran one to a carb manifold stud.
4. Since the ign. supplies a substantially more powerful spark it's safe to open up the plug gaps a bit. I set my trusty N4C Champions to .035.
5. While I was at it, I replaced my plug wires. The Cavis wires were OK so I saved them, but I had a nice set of wires from Kingsborne Ignition so I decided to use those instead. I don't know if you know about Kingsborn but they're a small company in San Diego with excellent products. Their wires are ready-made for Alfas and have tightly fitting, numbered boots. Something I especially liked: the coil wire is a bit longer than normal allowing me to run it underneath the air cleaner.
Kingsborne Wire Werks Inc - Spark plug wires, Automotive, RV, Motorcycle, Marine, Racing, Industry
6. The 'Plex is apparently set up for 2.0 engines and so only advances to about 32-34 degrees. Mine has a lightweight advance spring so it goe to full advance at about 3k rpm and, frankly, after driving it awhile I can't tell the difference in the way the motor performs. Time will tell, however, and, besides, I can always have the distributor recurved or move on to a programmable ign.
So, how does my new/old ignition work? Just the way I wanted it to. Start ups are quicker, the idle is overall smoother, and throttle response seems better. Except for the easier starting these changes aren't all that dramatic but they are descernable. A big advantage, however, is the fact that the 'Plex is breakerless. Since there are no moving parts connecting with one another as there is with a points distributor, the 'Plex timing should remain constant until you decide to change it. The last time I used this ign. my Alfetta went something like 20k miles without a miss. However we should be mindful of more recent users who experienced failures. I intend to carry my original coil and Bosch distributor around just in case.
But perhaps the biggest attraction of the Marelli Plex is its ease of connection to an MSD multifiring, capacitive discharge ignition---one of the more powerful in the marketplace. Using a simple adaptor cable to bypass the GM amplifier, the 'Plex is plug compatable with an MSD6A ignition which is an order of magnititude more powerful than the GM amplifier driven ignition. Even better, you can also easily connect one of the programmable MSD's which allows you to design your own advance curve to fit the particular needs of your car.
Watch this space.