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Premium Member
1984 GTV6, 1973 Berlina, 1987 Milano
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Discussion Starter #1
I haven't had a Plex for years, but as soon as I got the new Berlina running it all came back.

It's got no power at all until WHAM, the engine takes off! Give it a little more advance and now it idles at 2000 RPM.

Do I just need to find the sweet spot, or is there a trick to getting decent throttle response and a normal idle?

Thanks!
Ian
 

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71 Berlina 74 GTV 17 Giulia Q4
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No different than any other, rev to 3500rpm and set the pointer at the "M" mark. Correct timing at upper R's is much more important than at idle.
 

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It's got no power at all until WHAM, the engine takes off! Give it a little more advance and now it idles at 2000 RPM.
Are you confident the advance weights are moving out and returning freely? From the sound of your symptoms, it seems as if they may not be.
 

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1984 GTV6, 1973 Berlina, 1987 Milano
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Discussion Starter #4
I will double check the weights, they seemed fine when I had it off earlier, but I will double check.

Gigem - I agree the max needs to be set, but I know this is an issue with some Plex's, so I might have to give up a little top end to get it to idle below 2000 RPM.
 

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71 Berlina 74 GTV 17 Giulia Q4
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If that's the case then something isn't right with the weights like Alfajay mentioned or you have a vacuum leak or out of whack throttle linkage and or balance. All the above apply to both carbs or injected.
 

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1984 GTV6, 1973 Berlina, 1987 Milano
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Discussion Starter #6
The weights seem to be moving well, but I added a little oil to help them.

I also checked the timing, and it was right at the M mark around 3000 RPM, so it's not over advanced.

But it would start and then climb and idle at 3000 RPM. I retarded the timing a bit and it now idles at 1000 RPM. Will take it out for a spin after the girl's soccer practice.

It does scream from 2000 - 5000 RPM! Man is it fun!
 

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Maybe a new set of springs for the advance mechanism. Some old hot rod shops still have a distributor machine. If you give them a curve you want they can set it up.
 

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The weights seem to be moving well, but I added a little oil to help them. I also checked the timing, and it was right at the M mark around 3000 RPM, so it's not over advanced. But it would start and then climb and idle at 3000 RPM. I retarded the timing a bit and it now idles at 1000 RPM.
From your symptoms, the range of advance doesn't seem sufficient. That is, if you time it for maximum advance at 3,500 rpm at the M mark, it doesn't back off the advance sufficiently to give a <1,000 rpm idle. Conversely, if you timed it to give ~10 degrees of advance at idle, you might not get up to 35 degrees at higher rpm's.

A distributor machine, like flivesay recommends, is the best diagnostic. But even a timing light + the marks on your front pulley will tell you a lot about how this distributor is behaving.
 

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Richard Jemison
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Return to idle issue

The problem is very common. The issie is that Marelli uses the same design for the advance mechanism, although a bit smaller in the Plex version. It needs decent spring pressure to return the weights inward at low rpm. The spring design allows one of the weights to advance with no pull from the spring (long slot) and unlike the point`s version there is no drag on the rotor shaft from the points rubbing to help bring it back to no advance. Any change in springs will change the advance curve, but otherwise you will have a high idle speed, and poor lower rpm performance.

Plex is a decent system, but not the best for drivability as they age.

I increased spring pressure on the Plex unit in the race car to get it to a stable idle. However the curve is of no consequence since RPMs are never in the lower range.

A 123 distributor with selectable curves is best for a street car. One that can be programed for a aggressive curve through midrange, with a retarding of total advance at higher RPMs is the best for all out performance engines.
 

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1984 GTV6, 1973 Berlina, 1987 Milano
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Discussion Starter #10
I have a set of springs somewhere, I might try a heavier spring on theblightet spring .

I would love a 123 distributor, maybe for my birthday. :)
 

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My own experience with the Plex distributor is that the advance shaft gets sticky and fails to turn smoothy more often than other Marelli and Bosch distributors. Disassemble, clean, resassemble, much better.

Andrew
 

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I believe that the standard 'plex curve gives 13 degrees of distributor advance which is equal to 26 degrees of crankshaft advance. With max advance set at 36 degrees, static should be 10 degrees. It starts advancing at 800 rpm and is all in at 5000 rpm. So you need to be at 5000 rpm when you set the max (that would be an indicated 5500 rpm on my tach, yours may be more accurate). You should set the idle at 800 rpm or less or you will be trying to stabilize the idle on the slope of the curve and that may be difficult to achieve.
 

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1984 GTV6, 1973 Berlina, 1987 Milano
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Discussion Starter #13
I've got it to idle, but it has a flat spot just off idle.

Does anybody know if it's possible to swap just the advance mechanism between a Plex and standard Marelli?

I have a standard Marelli with an MSD 5 box on the other Berlina, but I'd rather not swap the whole shebang over. I'm curious if swapping just the advance setup would improve drivability.

I already tried a slightly heavier "light" spring, with a small improvement.
 

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71 Berlina 74 GTV 17 Giulia Q4
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Have you considered a pertronix for your S103 marelli?
 

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1984 GTV6, 1973 Berlina, 1987 Milano
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Discussion Starter #15
The MSD is basically doing the same thing as a Pertronix plus adding multiple sparks . It just uses the points as a switch.

My white Berlina runs great, but isn't as quick as the new one with the Plex.
 

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Richard Jemison
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Marelli

Does anybody know if it's possible to swap just the advance mechanism between a Plex and standard Marelli?
The plex is smaller internally (and externally) Parts are different. Springs could be used which are stiffer due to larger weights on a wider pivot point as well...
 

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The spring design allows one of the weights to advance with no pull from the spring (long slot) and unlike the point`s version there is no drag on the rotor shaft from the points rubbing to help bring it back to no advance.
Wow, I never thought of that. So distributors with points will return to zero advance more readily due to the drag from the points. When we convert those things to hall effect, magnetic, or optical we lose that factor. Another argument in favor of ditching the weights & springs entirely and going to electronic advance.

I increased spring pressure on the Plex unit in the race car to get it to a stable idle. However the curve is of no consequence since RPMs are never in the lower range.
Another good point. Sure, you can install heavier springs to ensure that those weights get pulled back to zero advance, but that will shift your advance curve out along the rpm axis (e.g., it'll take more rpm's to achieve the same amount of advance). OK for a competition car, but not so good on the street.
 

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1984 GTV6, 1973 Berlina, 1987 Milano
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Discussion Starter #18
Wow, I never thought of that. So distributors with points will return to zero advance more readily due to the drag from the points. When we convert those things to hall effect, magnetic, or optical we lose that factor. Another argument in favor of ditching the weights & springs entirely and going to electronic advance.



Another good point. Sure, you can install heavier springs to ensure that those weights get pulled back to zero advance, but that will shift your advance curve out along the rpm axis (e.g., it'll take more rpm's to achieve the same amount of advance). OK for a competition car, but not so good on the street.
Yes and no. In general I agree, but in this case that's exactly what I want. I need it to advance more slowly.

Maybe I should toss a set of points in to slow it down! :)
 

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..... in this case that's exactly what I want. I need it to advance more slowly.
Not that I disagree, but you've got me curious: why do you want it to advance more slowly?

Maybe I should toss a set of points in to slow it down!
I know you're kidding, but maybe you should! Over the weekend, a friend was extolling the virtues of a CD ignition system made by Winterburn (see: http://www.thesamba.com/vw/classifieds/detail.php?id=1557647). The Winterburn unit retains points as a trigger, as the CD units of the 1970's did. My friend likes these systems because you don't need a unique bracket for every oddball make/model of distributor as you do with the optical and hall effect sensors.
 

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Marelliplex curves

I have forgotten how to post charts from Excel and Word so I have attached a graph showing the standard and Shankle modified curves. The problem with the Shankle curve is that the initial rise is steep and it starts at 600 rpm which makes it difficult to achieve a stable idle. The advantage of it is that it gets all the advance in by 4000 rpm and it gives more advance than the standard curve from 1500 to 5000 rpm.
 

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