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'74 2liter GTV with a Marelli PLex....

all of a sudden it won't start, coughing/sputtering backfiring etc.....fuel system looks Ok I suspect its ignitiin related.

has a Marelli Plex; however, I can't find a similar diagram or system similar to what his one seems to be: the coil negative term goes to ground through a condenser. not grounded directly.

is it possible the condenser is bad and causing the ignition system to not function?

anyone else have a Marelliplex with a condenser or had similar problems?

thanks

David
 

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Is your car Spica? If so, I wouldn't be too quick to blame the 'Plex. Keep in mind that most igntion problems are fuel-related (and visa versa).

Do you have access to a conventional points ignition? A great diagnostic would be to swap in a conventional distributor & coil (I think the 'Plex coil must be paired with electronic ignition) and see how the car runs.

If running a Kettering igntion suggests that the 'Plex really is the problem, you might look at:
- The wires connecting to the transducer in the distributor - they tend to fray & fatigue.
- The electronics module that mounts with the coil is common to 1980's GM igntion (is it called "HEI"?), and is available at all US auto parts stores. For a few bucks you can just swap out that component, though your problem could also be the coil/transducer/wiring.
 

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The B+ on the coil goes to the 12v supply from the ignition switch. The other coil connection is connected to the tachometer.Sometimes a 0.68uF capacitor is inserted in the wire to the tachometer. The ignition should work with nothing connected to this terminal except the "internal" wire that goes to the ignitor unit on the heat sink. The heat sink of the unit must be well grounded. A 2 pin connecter is for the cable that connects the coil unit to the magnetic pickup in the distributor. I have had a couple of ignitor units fail and they both went completely dead with no warning. They are available at local parts stores as they were used on US engines too.
 

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You've tried the obvious stuff, like ensuring fuel supply, plugs aren't fouled, distributor hasn't rotated?
To see if your spark is OK, remove one plug, lay in on the [electrically conductive] engine with the HT lead attached, and turn over; do you see a good spark? In this scenario if you run it long it might start with alarming noises from the open cylinder, but you're trying for just one or two rotations to see if a spark jumps the gap.
Andrew
 
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