How to measure dwell

Although you have a single distributor, each set of points operates only one banks ignition system. You still have two coils, and two ignition systems. Thus, you have "two" four cylinder engine ignition systems. Try setting your dwell meters to read a four cylinder ignition.

Also, make sure of your connections. If you hook up your meter backwards it may read funny, though it shouldnt be fluctuating. You want to hook one lead to the - or negative side of the coil, and the other lead to chassis ground. Dont hook both leads to the two distributor terminals simultaneously, or it might read really funny. Each terminal is from each coils (-) terminal and thus, each set of points.

 

I didnt know Fiat ever had a points set you could adjust with a screw while running. Only ones I know that had that type were GM. Maybe someone adapted your distributor?

I know a few have tried adjusting points with the distributor installed, but I think its virtually impossible and will only suffer to make you angry. Three bolts and you can have the unit out of the car. You can spin it with a drill and set dwell if you can mock up some rudimetary wiring to mimic the ignition at the points. Better would be if you know someone with a distributor machine.

With both points in one distributor, the points really need to be phased as precisely to 90 degrees (45 distributor degrees) apart as possible, or bank to bank timing will be off. Lacking access to a machine, you really need to make it spin with something (a drill works), and attach a degree wheel and wire it up so you can put a strobe on it and read both dwell, as well as timing. You can probably get very close setting it with a test light and placed over a degree wheel, but I really think it needs to be set up dynamically for the best results. Once you get it set up right you shouldnt have to do it again for quite a while, so its worth spending the initial time trying to get it right.

 

The points are fastened to a plate that is slotted, so you can change the angular position of the entire points assembly. You would set the the rear bank set first, and phase them to break with the leading edge of the rotor aligned with the red index mark on the distributor body (IIRC).

Then set the front bank to break and simutaneosly have the correct dwell, 45 degrees (90 degrees crankshaft) after the rear bank.

If I understand what your saying, no, the points cam is not perfect, there is some discrepency from wear that will throw the points dwell off from lobe to lobe. By checking all four quadrants you can adjust for it by compromise. Little more gap on one may give a little less on the other. Try to "balance" it. Same goes for the phasing between the two points sets. You will be lucky to get within a degree or two all the way round between the two sets, so again, compromise.

All the distributors ive seen are spline drive, but from what I have heard here some of the early cars had some type of pin drive. I have never seen one or a pic of one, so I couldnt tell you anything. Maybe someone here can offer something.

As far as your meter readings for dwell, I dont think I can offer much more. I know if my leads are reversed on my meter I get a goofy reading, so you may want to try reversing your leads? One lead should go to a points terminal, other to ground. I do know my first car was missing the main ground leads from the engine to the frame, and someone had ran two heavy grounds from the distributors back to the chassis at the coils. Without them I had no spark! There should be two webbed and uninsulated (unshielded) ground straps from the gearbox to the frame. They are unshielded to disipate static energy, so make sure you replace them with the proper cabling. There should be another one for your main battery ground terminal. Make sure your engine is correctly grounded. I guess if you cant get a good reading off your points, pull the distributor and look it over.