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Discussion Starter #1
I've a '74 Spider that has me vexed. The motor is a 1750 and I think it has a Mirelli Plex ignition system. So, is this a Plex?

The distributor. It has one pickup, I thought the Plex had two?



Connects to here (wire already removed):



Specifically at this module (in the back under the coil):




OK, the problem is that the car is advancing too much at low rpm causing a lot of detonation. The timing is set at 0 deg at 600 rpm. If I safety wire the advance weights the problem goes away and the car runs fine, but weak at high rpm as there is no advance. I replaced both advance springs using one from a GM recurve kit and the other stiffer spring is a new stock Mirelli spring. No change, loads of detonation (scary, really scary). I also retarded the ignition to the point the car runs like crap but could still hear a little detonation. Yes, the amount does change as I tweak the base advance.

I figured something was wrong with the distributor itself and was going to replace it but first removed the old one to see just how worn it is. Its not. I've rebuilt a number of distributors over the years and really there isn't anything mechanical wrong with this one. The shaft is good, there is no discernible play at all. The advance mechanism is tight. The only thing wrong is cracked wire insulation coming off the pickup.

So some questions;

Is there any electronic advance control going on here? Does the black box behind the coil have any advance control?

Are new pickups available for these or is there something else that works? If for no other reason than the bad wire insulation I'd like to replace the pickup.

Anybody have any ideas as to what could be wrong causing the excessive advance?


Thanks for any help!

Bianchi1, your idea is still on the table, in fact I was going to try to find the distributor tomorrow but at this point I'm not sure my distributor is the problem.
 

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It is a Plex one point system. Does the system have a very good ground? Did you check/cleaned the dizzy's cup? is the under the sensor magnet good, broken or de-magnetized? I have one or two NOS new pick-up sensors and some modules ( the black boxes you mentioned), too, however I need to find them first...
 

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Discussion Starter #3
Hmm, the ground is a good idea, I haven't checked it. The pickup mag is good and strong and the cap is clean. I think I'll take the entire coil and mount off, clean it up and check the ground. It is after all an Alfa..always check grounds!
 

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Question; has this been an ongoing issue or has it just recently started?

OK, the problem is that the car is advancing too much at low rpm causing a lot of detonation.
Symptom of a primary advance spring gone weak (the primary spring is the one with many turns of fine wire and fits snug on the spring perches).
Can you verify actual advance readings with an advance type timing light? Reason I ask is that timing is not the only cause for detonation (lean mixture, engine temp, fuel octane too low, sparkplug heat range too high are some other causes).
 

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Discussion Starter #5
Question; has this been an ongoing issue or has it just recently started?
It actually started quite a while ago, over a year back but at the time I had no spare time to work on it so I parked the car. I've recently come into lots of free time :rolleyes: so am working on it.


Symptom of a primary advance spring gone weak (the primary spring is the one with many turns of fine wire and fits snug on the spring perches).
The primary spring I've been using from the GM recurve kit is stronger than the stock primary.

Can you verify actual advance readings with an advance type timing light? Reason I ask is that timing is not the only cause for detonation (lean mixture, engine temp, fuel octane too low, sparkplug heat range too high are some other causes).
Yes I can, once the distributor is back together.

Fuel mixture has been set using my Innovate LM-1 O2 logger and is at 12.5 to 13:1 during the detonation. Engine temps look normal on the gauge and detonation happens well before the car is warmed up. Right out of the garage actually.
Spark plugs are Champion RN9YC which are listed as original equipment equivalent. I'm open to suggestions for alternate plugs.

This leaves fuel. I always run the best premium I can find which around here is 91 octane and in the past that was good enough. The car has always run very cleanly so I don't think I have carbon buildup to the point of causing problems. A good Italian tuneup doesn't help and detonation disappears as revs rise.

I checked Centerlines online catalog and they do carry pickups and black boxes for the Mirelli Plex so my plan is to pick up those, and a new rotor and cap on Monday (Centerline is about 30 minutes from my house). I'll reassemble the distributor and see what happens. I don't think those parts will fix this but they are getting old so I'll give it a shot. After that I'll check the timing progression with my light and see where its at. I have a sinking feeling it will be normal leaving me scratching my head as I'm running out of ideas here.
 

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Discussion Starter #8
I got the distributor back in the car with a bunch of new parts installed and as expected still have the detonation problem. I used my timing light to plot the advance curve:



Its not very smooth but does hit the points within what I think should work. The only other curves I've seen plot straighter but overall are within a couple of degrees of what I have. This set up is with the heaviest spring from the GM HEI recurve kit in the primary position, and the stock Mirelli secondary spring. Detonation starts at 1800 rpm and stops (or motor music drowns it out) at about 2800 rpm.

So, I'm looking at something perhaps in the cylinder(s) causing a problem. Carbon buildup or hot spots. I'll probably start with some Sea Foam to clean the cylinders and if that doesn't work I may have to pull the head to look around.

Any other ideas I'd love to hear them!
 

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Ed:
When I first bought my 85 Spider it used to ping like crazy during acceleration using 89 octane, which I thought odd, since it was supposed to run on even lower octane, and the ignition advance was controlled by the ICU. I discovered that a PO had swapped out the Bosch distributor and disconnected the ICU and went with a Marelli points distributor. I have been using 93 octane and the problem went away, and the car has excellent pick-up using the stock advance (although I have not verified that the advance weights are in fact stock, I am assuming that they are). If 93 is nt available locally, maybe try an octane booster? Otherwise you may have to retard the ignition.
 

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timing light

Maybe the timing light is out of calibration . My 2 year old Craftsman dial back was off by 5° . Not under warranty . Bought a new digital one & everything runs without pinging ( Alfa's set at 37° max )
 

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If your car is having ignition problems, the answer might be dangling from the end of your key chain. Inside the ignition switch are plastic parts that are sensitive to extra weight pulling down on them. According to manufacturers, if you're carrying around more than four keys, or other types of weight on the same key chain with your ignition keys, you could be damaging these parts. Add to that the constant movement of the keys swinging back and forth when the car is in motion.


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