Author Topic: Dual Points question...  (Read 244 times)

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Offline Mike_B_SVT

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Dual Points question...
« on: March 18, 2019, 06:07:38 pm »
Hey Fellahs,

Was checking my points yesterday as part of my pre-driving season maintenance.  Put a piece of rubber in each set and measured them individually and found something... interesting.

Front-most set of points produced a higher idle RPM than the rear set.  Front set was idling at about 750, while the rear set had me idling at about 600.
To me, it seems that would indicate one set of points has a higher resistance, causing a lower energy spark, maybe?  But what would cause something like that?

I checked the point surfaces, and the rear set appears evenly "etched" while the front set is actually "off" a little bit (big crescent moon on point face).  No obvious (to me) pitting on either set though ~ they only have about 3000 miles on them, if that.

*Dwell on each was about 25 to 25.5, with a total of about 30.5 or so.

So what would cause the difference in idle, and what do I do about it (if anything)?
~ Mike B.

2003 Mustang Cobra Convertible
Sonic Blue 1 of 354

1970 Cougar Eliminator (Competition Gold / Black Decor Interior)
1 of 70 with 428SCJ, Ram Air, 4-speed.

Offline geraldt52

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Re: Dual Points question...
« Reply #1 on: March 18, 2019, 07:42:09 pm »
...So what would cause the difference in idle, and what do I do about it (if anything)?

I never knew anyone to try that, but as a guess I'm thinking that the advance comes out different running on one set of points vs the other...which could cause a difference in idle speed.

Offline Mike_B_SVT

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Re: Dual Points question...
« Reply #2 on: March 18, 2019, 07:53:18 pm »
...So what would cause the difference in idle, and what do I do about it (if anything)?

I never knew anyone to try that, but as a guess I'm thinking that the advance comes out different running on one set of points vs the other...which could cause a difference in idle speed.

Hmm...  I didn't think about that.  I figured the idle was low enough to prevent much (if any) advance from happening, but maybe not?  I guess it is possible that my little flap of rubber insulator was dragging on the insides of the distributor body, maybe causing a slight difference in advance between the two point sets?
~ Mike B.

2003 Mustang Cobra Convertible
Sonic Blue 1 of 354

1970 Cougar Eliminator (Competition Gold / Black Decor Interior)
1 of 70 with 428SCJ, Ram Air, 4-speed.

Offline 70cj428

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Re: Dual Points question...
« Reply #3 on: March 18, 2019, 08:09:36 pm »
If I'm not mistaken.....

You'll find the timing will change a few degrees when running with one set of points (The set that normally closes the circuit) than with both sets (or with the set that opens the circuit) That's probably whats changing the idle RPM. The way the dual point works is that the 2 sets of points are positioned on the breaker plate so one set opens (or closes) a little before the other set. The first set closes and energizes the coil then the second set closes (doing essentially nothing). When the first set opens nothing happens because the second set is still closed (normally this is when the spark would occur on a single point distributor) The second set then opens, breaking the circuit, and a spark is produced. Using 2 sets of points allows the coil to "charge" longer (increases the saturation time) resulting in a hotter spark.

Running off just the first set of points will cause the spark to occur sooner, in affect advancing the timing slightly.

Hope this helps, John

PS. If your really worried about it, just take out that old stock dual point and mail it to me, and I'll send you a nice fresh single point distributor that's easier to set up and will save you a ton of money at tune up time …..  ;D ;D ;D ;D   ( just kidding)
« Last Edit: March 18, 2019, 08:15:55 pm by 70cj428 »
70 "R" code Mach1, Wimbleton White / Vermillion, Auto, AC,
70 "Q" code Mach1, Grabber Green / White, Auto,AC
70 "M" code Mach1, Med. Gold / Ginger, Auto, AC
68 "S" code GT Fastback, Wimbleton White / Black deluxe int.,4 speed, AC, (getting a 427SO..)

Offline Mike_B_SVT

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Re: Dual Points question...
« Reply #4 on: March 18, 2019, 08:40:30 pm »
Oh yeah...  That makes perfect sense!  Duh.

Thanks John :-)


If I'm not mistaken.....

You'll find the timing will change a few degrees when running with one set of points (The set that normally closes the circuit) than with both sets (or with the set that opens the circuit) That's probably whats changing the idle RPM. The way the dual point works is that the 2 sets of points are positioned on the breaker plate so one set opens (or closes) a little before the other set. The first set closes and energizes the coil then the second set closes (doing essentially nothing). When the first set opens nothing happens because the second set is still closed (normally this is when the spark would occur on a single point distributor) The second set then opens, breaking the circuit, and a spark is produced. Using 2 sets of points allows the coil to "charge" longer (increases the saturation time) resulting in a hotter spark.

Running off just the first set of points will cause the spark to occur sooner, in affect advancing the timing slightly.

Hope this helps, John

PS. If your really worried about it, just take out that old stock dual point and mail it to me, and I'll send you a nice fresh single point distributor that's easier to set up and will save you a ton of money at tune up time …..  ;D ;D ;D ;D   ( just kidding)
~ Mike B.

2003 Mustang Cobra Convertible
Sonic Blue 1 of 354

1970 Cougar Eliminator (Competition Gold / Black Decor Interior)
1 of 70 with 428SCJ, Ram Air, 4-speed.

Offline geraldt52

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Re: Dual Points question...
« Reply #5 on: March 18, 2019, 09:52:27 pm »
...So what would cause the difference in idle, and what do I do about it (if anything)?

I never knew anyone to try that, but as a guess I'm thinking that the advance comes out different running on one set of points vs the other...which could cause a difference in idle speed.
...  I figured the idle was low enough to prevent much (if any) advance from happening, but maybe not?...

The advance that is being affected is the "initial"...which is not affected by rpm or vacuum.