Total dwell for either a single point or dual point distributor should be 54 to 60 degrees. To set a dual point distributor, place a small piece of paper in between one set of points and set them to 30 degrees of dwell and then match that to the second set of points. You want to have both sets of points phased equally, of course the best way to do that is on a distributor machine.

 

General rule of thumb on dwell angles for single point distributors;

4 cylinder engine - 60 degrees dwell
6 cylinder engine - 45 degrees dwell
8 cylinder engine - 30 degrees dwell

The dwell angle is the number of degrees of distributor shaft rotation while the points are closed. This is why the dwell of an 8 cyl engine is exactly half that of a 4 cyl engine; there are twice as many lobes on the 8 cyl dist shaft meaning that the points are closed for half the time as compared to a 4 cyl.

The purpose of most dual point dizzys is to increase the dwell angle (which increases ignition coil saturation time which can reduce the chance of an ignition misfire at high revs).

Unfortunately, none of the 2600 manuals I have lists the dwell angle, only the points gap. For the Marelli S 93 A dualpoint fitted to the Spider/Sprint, the gap is 0.42-0.48mm.

 

So deep in the cardisk 2000 & 2600 Service Bulletins pdf I found the answer!

Page 72.

For Marelli S93A distributor dwell is 42-48 degrees

 

This confirms the 45º mentioned above for 6-cyl engines (and adds +/-3º tolerance). The underlying logic is that the coil has a duty cycle of 75% (75% charge time vs. 25 discharge or “off” time) to prevent overheating of the coil. You arrive at the dwell angle value by dividing 360º by the number of cylinders and multiplying that value by ¾. The contact point gap created by the cam lobe directly affects the dwell angle (the bigger the gap, the smaller the dwell angle).

The problem with just using dwell angle as reference for setting up a 2-contact system is that it will show you only the average of both contacts (i.e. it will show you the ideal value of 45º if one contact is at 42º and the other at 48º (and it probably will show you the same value if one contact is at 39º and the other at 51º -- outside the specs). Not only does the dwell meter not tell you if the settings are wrong, it also won’t tell you which contact(s) to adjust.

As pointed out elsewhere, the two contacts Marelli distributor are in series and driven by a 3-lobe cam (whereas the Bosch distributor has a single contact driven by a 6-lobe cam). In order to set the Marelli distributor right, both contact gaps should be the same, the contacts should be exactly 180º opposed and the spring force on the contact should be the same. The issue is that, in a perfect world, both contacts should open at exactly the same crankshaft angle relative to the TDC of the cylinder that should fire and that the contacts should open at exactly the same relative speed so that the discharge function and energy delivered to the spark plug is the same for both contacts.

Another issue with the difference between the contact settings is how they affect ignition advance: Any difference in gap size (and where the gap opens on the cam lobe) and any difference arising from the angle not being exactly 180º opposed will affect the ignition timing for each set of the 3 cylinders served by the contacts. A trick I read about for figuring out such differences is measuring ignition advance both on cylinder 1 and cylinder 6 (and adjust it so the average to comply with the specs).

 

Looking at the info on dual points in the car disk dvd, it says that the points are 120 degrees apart, not 180 degrees?

 

I believe this information is incorrect. Here's what the Marelli distributor looks like:

 

OK, so this is how I timed my 2600 Spider with the Marelli twin point distributor.

I timed the car at 43.5 degrees BTDC at 5000 rpm (as many people said to retard the timing for lead-free gas), and didn't worry too much about the idle timing.

43.5 degrees is just left of the center of the letter M at the max advance mark (47 degrees).

Why did I pick 43.5 degrees? Well the standard static timing is 7 BTDC, and this is marked as F on the pulley. Several people had said to retard the timing between 3 and 4 degrees, so I decided to split the 7 degrees in half = 3.5 degrees. I measured the distance from TDC (marked as P) to the F mark with my calipers, divided by two and then subtracted this distance from the M mark, which is at 47 degrees.

I did get the dwell on the twin point distributor to be within spec (42-48 degrees). I checked the dwell on each set of points separately by placing a piece of cardboard between one set of points and then the other and keeping them equal as I adjusted them. I also adjusted the points in relation to each other, using the twin points info on the car disk DVD.

The car starts and runs well, but I think buying a 123 dizzy really is the way to go!

John