250GT Distributor Caps

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BobA
Posts: 103
Joined: Tue Nov 12, 2013 7:36 pm

Re: 250GT Distributor Caps

Post by BobA »

Unless my understanding of the mechanical advance mechanism is flawed, I the rotor actually moves as the advance is brought into play. I would seem that the factory would set the rotor to be close to one end of the cap tip and the rotor would fire along the rotor tip as the ignition advances with RPM. Does this make sense? Bob A
John Vardanian
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Joined: Wed Sep 04, 2002 12:13 am
Location: San Francisco Area

Re: 250GT Distributor Caps

Post by John Vardanian »

Thanks Dyke, so just because I am getting fatter spark with wider gap while on the bench, inside the combustion chamber I'll have misfire with too wide of a gap. This is how I read your note anyway.

Bob, I can't follow your thinking.

john
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DWR46
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Re: 250GT Distributor Caps

Post by DWR46 »

Bob: On a Ferrari V-12 distributor, the rotor does not move as the timing centrifugal weights advance, the cam moves to cause the points to break earlier, thus advancing the timing. How the weights are assembled determines where the rotor is pointed in regards to the contacts in the cap. Obviously, it has to be close to the contacts to help the spark to jump the gap to the cap.
John Vardanian
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Location: San Francisco Area

Re: 250GT Distributor Caps

Post by John Vardanian »

Hi Dyke, I have noticed equal dwell between the two points does not mean equal gaps. Have you come across it, is there an explanation?

What is a good dwell for the road cars, 24-26?

john
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DWR46
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Re: 250GT Distributor Caps

Post by DWR46 »

John: You sure ask a lot of questions!!!! Equal dwells can have different gaps due to point construction variances, lengths, angles, etc. Also there can be differences in the mounting locations in the distributor. On my Sun 501 machine, with .014" point gaps, Ferrari distributors usually read 24-28 degrees with the machine set on the "six cylinder" scale. However, machines are known to give widely varied dwell readings, so I set one set of points to .014", read the dwell and then set the other set of points to the same dwell. I do not care about the point gap on that set.
John Vardanian
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Location: San Francisco Area

Re: 250GT Distributor Caps

Post by John Vardanian »

Thanks Dyke, appreciate your patience.

john
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John Vardanian
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Re: 250GT Distributor Caps

Post by John Vardanian »

If the carbon pickups in your distributors are chewed up, NAPA Part #AL20, it's an exact fit for $1.35.

john
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DWR46
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Re: 250GT Distributor Caps

Post by DWR46 »

John: That is great information. Thanks for sharing it with the group.
Luis A
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Joined: Wed Feb 25, 2015 4:33 pm
Location: Milwaukee, WI

Re: 250GT Distributor Caps

Post by Luis A »

DWR46 wrote:Bob: On a Ferrari V-12 distributor, the rotor does not move as the timing centrifugal weights advance, the cam moves to cause the points to break earlier, thus advancing the timing. How the weights are assembled determines where the rotor is pointed in regards to the contacts in the cap. Obviously, it has to be close to the contacts to help the spark to jump the gap to the cap.
Hi guys, bringing this one up back from the dead as I am in the thick of refurbishing my dual Marelli distributors as found on the Lamborghini Espada, which is essentially the same setup you are discussing here.

Dyke, as you point out, while it is the cam that advances and fires the points earlier, isn't the practical consequence of that action that the rotor will conduct a spark when it is physically positioned earlier, by the amount of the advance, than it is when there is no advance? Hopefully that will make sense. In my case, with no centrifugal advance, my rotor is almost entirely past the cap terminal and fires off its corner. When the cam advances by 10° then the rotor will fire more centrally on the cap plug by 10°. I need to adjust both point plates (while maintaining the 60°point set phase differential) clockwise (against rotor motion in my case) to fire more centrally on the cap plug with no advance as well as when advanced by 10°, which is possible since the rotor terminal is sufficiently wide.

Is this all correct...? Thoughts?

Here's a video of the distributor on the Sun machine tonight with the firing very steady as high as 3, 600 distributor RPM:
https://vimeo.com/148023594
DWR46
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Joined: Tue Sep 22, 2009 8:23 pm

Re: 250GT Distributor Caps

Post by DWR46 »

Luis: Yes, you are correct, in that the rotor "sweeps" past the contact in the cap as the ignition advances. Years ago, I cut holes in an old cap so I could check this on distributors. You do need to position the rotor to one side of the contact when the engine is off, so it can move across the contact as the rpm goes up. Also, I saw your post on the Lambo site, and you are again correct as to the dwell. I have not seen dwell specs for those distributors. I use the same method you do, by setting the correct point gap, reading the dwell and then setting the other points to the same dwell. As to a specific dwell number, your's are very similar to those given by my Sun 501 and 504. However, I have found that most of the old machines will give different dwells for the same distributor. These distributor machines are now about 50 years old, and have had varying degrees of maintenance. Also, the dwell reading can be adjusted internally in the machine, so it all depends how your specific machine has been set up as to what numeric reading it will give you. As a result, I never worry about the actual dwell number with these distributors, I just get the gap correct and then match the dwells.
John Vardanian
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Location: San Francisco Area

Re: 250GT Distributor Caps

Post by John Vardanian »

Hi Dyke, in achieving identical dwell for the pair of points I end up with close but nonidentical gaps. Has this been your experience? Thanks.

john
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DWR46
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Re: 250GT Distributor Caps

Post by DWR46 »

John: Many times I find equal dwells do not result in equal gaps. This is due to manufacturing variances in both the point sets and the distributor itself. Dwell is what counts, as we are looking for equal amounts of coil saturation. Gaps really do not matter as long as they are wide enough to cause a good "break" to switch the circuit, and not too wide so as to result in point bounce at high rpm.
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